# Gases/Gaseous objects/Earth

The freezing cold polar vortex moved southward from central Canada into the U.S. from January 20 through the 29th. Credit: NASA/JPL-Caltech AIRS Project.{{fairuse}}

"A polar vortex is a natural part of nature gone wrong: It starts off as a standard low-pressure system over the Arctic, but when the jet streams keeping these low-pressure systems in check weaken, their freezing cold winds can fall down right onto midwestern American. Beyond extreme discomfort and disruption of services, the freezing cold, which hit 32 below zero in parts of Illinois, is responsible for over 20 deaths."[1]

"NASA's Earth-observing Aqua satellite was able to capture [the] system's movement across the globe. Among Aqua's six instruments is its Atmospheric Infrared Sounder (AIRS), which measures temperature and water vapor as functions of height. Every temperature profile, also known as a sounding, is sensitive to temperature and water vapor. AIRS can measure these soundings using its 2,378 sensors-prior to its launch in 2002, [early] Earth-observing satellites only had 15 such sensors."[1]

"The coldest temperatures in this image are the purple and blue, which range from -40 degrees Fahrenheit (also -40 degrees Celsius) to -10 degrees Fahrenheit (-23 degrees Celsius)."[1]

## Absorptions

A spectrum is taken of blue sky clearly showing solar Fraunhofer lines and atmospheric water absorption band. Credit: Eric Bajart.

"[P]referential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[2]

## Clouds

This image shows a cumulus cloud above Lechtaler Alps, Austria. Credit: Glg.
Cumulus clouds in fair weather are white. Credit: Michael Jastremski.

Def. a "large white puffy cloud"[3] is called a cumulus cloud.

Def. "[a] visible mass of

1. water droplets suspended in the air ...
2. dust,
3. steam ...
4. smoke ...
5. a group or swarm"[4] is called a cloud.

Cumulus clouds look white because the water droplets reflect and scatter the sunlight without absorbing other colors.

"On any given day, about half of Earth is covered by clouds, which reflect more sunlight than land and water. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth."[5]

## Aerometeors

An atmospheric river forms over Hawai'i then heads toward California 10-11 April 2017. Credit: UW-CIMSS.{{fairuse}}

"Several times a year an atmospheric river [shown in the image on the right forming over Hawai'i]—a long, narrow conveyor belt of storms that stream in relentlessly from the Pacific Ocean—drops inches of rain or feet of snow on the U.S. west coast. Such a system triggered floods and mudslides in central and southern California this past weekend [2-3 February 2019]."[6]

"Atmospheric rivers flow through the sky about a mile above the ocean surface, and may extend across a thousand miles of ocean to the coast. Some bring routine rain but the more intense systems can carry as much water as 15 Mississippi Rivers. The series of storms striking land can arrive for days or, occasionally, weeks on end. They hit west-facing coastlines worldwide, although the U.S. experiences more than most other national coasts."[6]

The “atmospheric river scale” "ranks severity and impacts, from category 1 (weak) to category 5 (exceptional)."[6]

"Without a scale, we really had no way to objectively communicate what would be a strong storm or a weak one."[7]

"Scientists, the media and the public viewed atmospheric rivers as primarily a hazard, but the weaker ARs are quite beneficial. Water managers made it clear to us that a rating scale would be helpful."[7]

"The scale, published Tuesday in the Bulletin of the American Meteorological Society, ranks atmospheric rivers on five levels:"[6]

• Category 1: Weak—primarily beneficial
• Category 2: Moderate—mostly beneficial, but also somewhat hazardous
• Category 3: Strong—balance of beneficial and hazardous
• Category 4: Extreme—mostly hazardous, but also beneficial (if persistent drought)
• Category 5—Exceptional—primarily hazardous

## Neutrals

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."[8]

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."[9]

## Cosmic rays

The diagram shows a possible proton collision with an atmosphere molecule. Credit: Magnus Manske.

When cosmic rays enter the Earth’s atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of billions of lighter particles, a so-called air shower.

An air shower is an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray (i.e. one of extraterrestrial origin) enters the atmosphere.

There is "a decrease in thunderstorms at the time of high cosmic rays and an increase in thunderstorms 2-4 days later."[10]

It is believed that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.[11]

The PAMELA experiment detected orders of magnitude higher levels of antiprotons than are expected from normal particle decays while passing through the SAA. This suggests the van Allen belts confine a significant flux of antiprotons produced by the interaction of the Earth's upper atmosphere with cosmic rays.[12] The energy of the antiprotons has been measured in the range from 60 - 750 MeV.

Atmospheric neutrinos result from the interaction of cosmic rays with atomic nuclei in the Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from the Tata Institute of Fundamental Research (India), Osaka City University (Japan) and Durham University (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in Kolar Gold Fields in India in 1965.

"The major problems associated with the balloon borne positron measurements are (i) the unique identification against a vast background of protons, and (ii) corrections for the positrons produced in the residual atmosphere."[13]

"[T]o account for the atmospheric corrections ... first [use] the instrument to determine the negative muon spectrum at float altitude. ... [Use this] spectrum ... to normalize the analytically determined atmospheric electron-positron spectra. ... most of the atmospheric electrons and positrons at small atmospheric depths are produced from muon decay at [the energies from 0.85 to 14 GeV]."[13]

## Protons

The Bastille Day Flare or Bastille Day Event was a powerful solar flare on July 14, 2000, occurring near the peak of the solar maximum in solar cycle 23.[14][15] [NOAA] Active region 9077 produced an X5.7-class flare, which caused an S3 radiation storm on Earth fifteen minutes later as energetic protons bombarded the ionosphere.[14][16] It was the biggest solar radiation event since 1989.[16] The proton event was four times more intense than any previously recorded since the launches of SOHO in 1995 and ACE in 1997.[14] The flare was followed by a full-halo coronal mass ejection[14] and a geomagnetic super storm on July 15-16. The extreme level, G5, was peaked in late hours of July 15.

## Tauons

The "incoming neutrino flux [is disentangled] from the consequent τ air-shower physics. [To] establish the τ production rate we introduce an effective volume and mass for Earth-skimming τ’s, which is independent on any incoming neutrino flux [...]. This volume describes a strip within the Earth where neutrino/antineutrino-nucleon, ντ (${\displaystyle {\overline {\nu }}_{\tau }}$) − N, interactions may produce emerging τ+ leptons which then shower in air. [General] τ upward-going showers [can be compared] to detectors such as the ongoing photo-fluorescence ground-based observatory Auger".[17]

## Muons

This is an image obtained from muon radiography of Japan's Asama volcano. Credit: H T M Tanaka.
The image shows the principle of muon radiography. Credit: Nicola Graf.
This is an image of a muon telescope. Credit: Paolo Strolin.

"Muons are produced, along with other particles, when cosmic rays (high-energy particles originating in outer space) interact with atomic nuclei in Earth’s atmosphere to produce ‘showers’ of secondary particles. The muons inherit the high energy of the parent cosmic rays, which enables them to penetrate and pass through the rock of the volcano and to be detected on the other side of the mountain. Because denser materials absorb more muons (just as dense materials such as bone absorb more X-rays), this provides a basis for producing shadow images of the volcano’s interior."[18]

"Muon radiography was first used in 1971 – not for volcanoes, but for investigating the interior of the pyramid of Chefren at Giza, Egypt. The Nobel-prize winning physicist Louis Alvarez placed a muon detector inside the pyramid to pick up changes in muon flux (rate of muon flow) that could indicate the presence of a hidden burial chamber. However, none was found."[18]

"In 2007, Hiroyuki Tanaka and collaborators from the University of Tokyo were the first to apply this technique to volcanoes. They carried out radiography of the top part of the Asama volcano in Honshu, Japan, which revealed a region with rock of low density under the bottom of the crater. The presence of low-density regions can be used in computer simulations that predict how possible eruptions could develop, indicating the most dangerous areas around the volcano. Their observations showed that muon radiography could indeed produce useful images of the internal structure of volcanoes."[18]

"The really important advantages of muon radiography of volcanoes are two-fold. First, whereas current indirect methods can provide information to a spatial resolution of some 100 m, muon radiography can be up to ten times more specific, mapping internal structures to a resolution of some 10 m. Second, muon radiography offers the possibility of continuous monitoring, thus potentially revealing the evolution of structures over time. The time resolution depends on the thickness of the rock traversed by the muons: the thicker it is, the fainter the muon flux and the longer it takes to accumulate enough muons for a picture. The time needed can thus be weeks, months or years."[18]

Vesuvius "is a special challenge, not only because it represents the highest volcanic risk in Europe, but also because of the mountain’s unusual structure. Vesuvius is in fact situated within the remnants of a much larger volcano, Mount Somma. Moreover, inside the summit of Vesuvius is a crater that is 500 m wide and 300 m deep: this means that, to look below the bottom of the crater, muons have to penetrate deep into the mountain, through almost two kilometres of rock, to reach the detector on the opposite side of the volcano. Only muons of very high energy travelling in a near-horizontal direction are able to pass through all this rock, so their flux at the detector is very low, making imaging extremely difficult."[18]

## Beta particles

The Fermi Gamma-ray Space Telescope sits on its payload attachment fitting. Credit: NASA/Kim Shiflett.

"The Large Area Telescope (LAT) is a pair-conversion gamma-ray telescope onboard the Fermi Gamma-ray Space Telescope satellite. It has been used to measure the combined [cosmic-ray] CR electron and positron spectrum from 7 GeV to 1 TeV [20, 21]. The LAT does not have a magnet for charge separation. However, as pioneered by [22] and [23], the geomagnetic field can also be used to separate the two species without an onboard magnet. Müller and Tang [23] used the difference in geomagnetic cutoff for positrons and electrons from the east and west to determine the positron fraction between 10 GeV and 20 GeV. As reported below, we used the shadow imposed by the Earth and its offset direction for electrons and positrons due to the geomagnetic field, to separately measure the spectra of CR electrons and positrons from 20 GeV to 200 GeV. In this energy range, the 68% containment radius of the LAT point-spread function is 0.1° or better and the energy resolution is 8% or better."[19]

"The Large Area Telescope (LAT) detects individual gamma rays using technology similar to that used in terrestrial particle accelerators. Photons hit thin metal sheets, converting to electron-positron pairs, via a process known as pair production. These charged particles pass through interleaved layers of silicon microstrip detectors, causing ionization which produce detectable tiny pulses of electric charge. Researchers can combine information from several layers of this tracker to determine the path of the particles. After passing through the tracker, the particles enter the calorimeter, which consists of a stack of caesium iodide scintillator crystals to measure the total energy of the particles. The LAT's field of view is large, about 20% of the sky. The resolution of its images is modest by astronomical standards, a few arc minutes for the highest-energy photons and about 3 degrees at 100 MeV. The LAT is a bigger and better successor to the EGRET instrument on NASA's Compton Gamma Ray Observatory satellite in the 1990s. {{clear}]

## Positrons

In 2009, the Fermi Gamma Ray Telescope in Earth orbit observed [an] intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation. Scientists wouldn't have been surprised to see a few positrons accompanying any intense gamma ray burst, but the lightning flash detected by Fermi appeared to have produced about 100 trillion positrons. This has been reported by media in January 2011, it is an effect, never considered to happen before.[20]

"The Gamma-ray Burst Monitor (GBM) detects sudden flares of gamma-rays produced by gamma ray bursts and solar flares. Its scintillators are on the sides of the spacecraft to view all of the sky which is not blocked by the earth. The design is optimized for good resolution in time and photon energy. The Gamma-ray Burst Monitor has detected gamma rays from positrons generated in powerful thunderstorms.[21]

## Electrons

The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: Abestrobi.
Auroras are mostly caused by energetic electrons precipitating into the atmosphere.[22] Credit: Samuel Blanc[2].
This graph shows the relationship of the atmosphere and ionosphere to electron density. Credit: Bhamer.

With respect to the rocky-object Earth, between the surface and various altitudes there is an electric field induced by the ionosphere. It changes with altitude from about 150 volts per meter at the suface to lower values at higher altitude. In fair weather, it is relatively constant, in turbulent weather it is accompanied by ions. At greater altitude these chemical species continue to increase in concentration. To dissipate the accumulation of greater charge differential between the surface and the ionosphere, the gases between suffer breakdown (ionization) that permits lightning to be either a draw of negative charge, usually electrons, upward from the surface or a transfer of positive charge to the ground.

"[L]ow-altitude regions of downward electric current on auroral magnetic field lines are sites of dramatic upward magnetic field-aligned electron acceleration that generates intense magnetic field-aligned electron beams within Earth’s equatorial middle magnetosphere."[23]

## Gamma rays

The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010. Credit: NASA/Goddard Space Flight Center.

The Earth's atmosphere is a relatively bright source of gamma rays produced in interactions of ordinary cosmic ray protons with air atoms. In gamma-ray and X-ray astronomy, Earth is a dwarf gaseous object.

A number of observations by space-based telescopes have revealed gamma ray emissions, specifically, terrestrial gamma-ray flashes (TGFs). These observations pose a challenge to current theories of lightning, especially with the discovery of the clear signatures of antimatter produced in lightning.[24]

A TGF has been linked to an individual lightning stroke occurring within 1.5 ms of the TGF event,[25] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft "has been observing TGFs at a much higher rate, indicating that these occur about 50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV. [Apparently], the gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds."[26]

"These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."[27]

In 2009, [the] Fermi Gamma Ray Telescope in Earth orbit observed [an] intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation. Scientists wouldn't have been surprised to see a few positrons accompanying any intense gamma ray burst, but the lightning flash detected by Fermi appeared to have produced about 100 trillion positrons. This has been reported by media in January 2011, it is an effect, never considered to happen before.[28]

Airborne gamma-ray spectrometry is now the accepted leading technique for uranium prospecting with worldwide applications for geological mapping, mineral exploration & environmental monitoring.

## X-rays

Bright X-ray arcs of low energy (0.1 - 10 keV) are generated during auroral activity. The images are superimposed on a simulated image of the Earth. The color code represents brightness, maximum in red. Distance from the North pole to the black circle is 3,340 km (2,080 mi). Observation dates: 10 pointings between December 16, 2003 and April 13, 2004. Instrument: HRC. Credit: NASA/MSFC/CXC/A.Bhardwaj & R.Elsner, et al.; Earth model: NASA/GSFC/L.Perkins & G.Shirah.
This image is a composite of the first picture of the Earth in X-rays over a diagram of the Earth below. Credit: NASA, Ruth Netting.

The Earth is a known astronomical object. It is usually not thought of as an X-ray source.

At left is a composite image which contains the first picture of the Earth in X-rays, taken in March, 1996, with the orbiting Polar satellite. The area of brightest X-ray emission is red.

Energetic charged particles from the Sun energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the ionosphere, causing the X-ray emission.

X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit.

"Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."[29]

"Auroras are produced by solar storms that eject clouds of energetic charged particles. These particles are deflected when they encounter the Earth’s magnetic field, but in the process large electric voltages are created. Electrons trapped in the Earth’s magnetic field are accelerated by these voltages and spiral along the magnetic field into the polar regions. There they collide with atoms high in the atmosphere and emit X-rays".[30]

## Ultraviolets

This image shows how the Earth glows in the ultraviolet. Credit: NASA.

"This unusual false-color image [at right] shows how the Earth glows in ultraviolet (UV) light. The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 captured this image. The part of the Earth facing the Sun reflects much UV light and bands of UV emission are also apparent on the side facing away from the Sun. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines."[31]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph.[32]

Archaeological "[a]erial survey ... employs ultraviolet, infrared, ground-penetrating radar wavelengths, LiDAR and thermography.[33]

## Opticals

Earth is seen on July 6, 2015 from a distance of one million miles by a NASA scientific camera. Credit: NASA.

"A NASA camera on the Deep Space Climate Observatory (DSCOVR) satellite has returned its first view of the entire sunlit side of Earth [on the right] from one million miles away."[34]

"The color images of Earth from NASA’s Earth Polychromatic Imaging Camera (EPIC) are generated by combining three separate images to create a photographic-quality image. The camera takes a series of 10 images using different narrowband filters -- from ultraviolet to near infrared -- to produce a variety of science products. The red, green and blue channel images are used in these Earth images."[34]

"This first DSCOVR image of our planet demonstrates the unique and important benefits of Earth observation from space. As a former astronaut who’s been privileged to view the Earth from orbit, I want everyone to be able to see and appreciate our planet as an integrated, interacting system. DSCOVR’s observations of Earth, as well as its measurements and early warnings of space weather events caused by the sun, will help every person to monitor the ever-changing Earth, and to understand how our planet fits into its neighborhood in the solar system.”[35]

"These initial Earth images show the effects of sunlight scattered by air molecules, giving the images a characteristic bluish tint."[34]

"The images clearly show [...] complex cloud patterns."[36]

"The primary objective of DSCOVR, a partnership between NASA, the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Air Force, is to maintain the nation’s real-time solar wind monitoring capabilities, which are critical to the accuracy and lead time of space weather alerts and forecasts from NOAA."[34]

NASA's Earth Polychromatic Imaging Camera (EPIC) "is a four megapixel CCD camera and telescope. The color Earth images are created by combining three separate single-color images to create a photographic-quality imageequivalent to a 12-megapixel camera. The camera takes a series of 10 images using different narrowband filters -- from ultraviolet to near infrared -- to produce a variety of science products. The red, green and blue channel images are used to create the color images. Each image is about 3 megabytes in size."[34]

"The effective resolution of the DSCOVR EPIC camera is somewhere between 6.2 and 9.4 miles (10 and 15 kilometers)."[36]

## Blues

The Earth has a blue halo when seen from space. Credit: NASA Earth Observatory.

Def. "[t]he gases surrounding the Earth or any astronomical body"[37] is called an atmosphere.

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space, as shown in the image at right.

## Greens

In this International Space Station image, you can see green and yellow airglow paralleling the Earth’s horizon line (or limb) before it is overwhelmed by the light of the rising Sun. Credit: NASA Earth Observatory.

In the International Space Station image at right, you can "see green and yellow airglow paralleling the Earth’s horizon line (or limb) before it is overwhelmed by the light of the rising Sun. Airglow is the emission of light by atoms and molecules in the upper atmosphere after they are excited by ultraviolet radiation. ... Astronaut photograph ISS030-E-015491 was acquired on December 22, 2011, with a Nikon digital camera, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center."[38]

Airglow (also called nightglow) is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark (even after the effects of starlight and diffused sunlight from the far side are removed).

Airglow is caused by various processes in the upper atmosphere, such as the recombination of ions which were photoionized by the sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl ions at heights of a few hundred kilometers. It is not noticeable during the daytime because of the scattered light from the Sun.

## Oranges

An orange sunset in the Mahim Bay is shown around the Haji Ali Dargah in India. Credit: Humayunn Peerzaada.
The image shows a sunrise in Kodachadri. Credit: Chinmayahd.
The image shows an orange sun in Boracay, Philippines. Credit: Sarahr.

But truly, the now famous sun set at the peak where you will witness the panorama of rolling green hills and also a glimpse of Arabian sea and the bright orange sun going down the hills might force one to contemplate on the nature and its complex beauty.

## Reds

The Belt of Venus is imaged at 42,000 feet. Credit: George7378.
The Belt of Venus is shown at sunrise, over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Credit: Brocken Inaglory.
The Belt of Venus is shown at sunset, looking east from the Marin Headlands just north of San Francisco. (Note: there is a thin greyish cloud layer partially obscuring the horizon in this image.) Credit: Brocken Inaglory.
The Full moon is rising, as seen through the Belt of Venus. A very small part of the Earth's shadow (dark blue) is also visible in this image, but the horizon here is too high to see more of the Earth's shadow. Credit: Fir0002.

The Belt of Venus or Venus's Girdle is the Victorian-era name for an atmospheric phenomenon seen at sunrise and sunset. Shortly after sunset or shortly before sunrise, the observer is, or is very nearly, surrounded by a pinkish glow (or anti-twilight arch) that extends roughly 10°–20° above the horizon. Often, the glow is separated from the horizon by a dark layer, the Earth's shadow or "dark segment". The Arch's light rose (pink) color is due to backscattering of reddened light from the rising or setting Sun.

The first image at left shows the Belt of Venus from 42,000 feet altitude locally above the Earth's surface.

In the image at right is the Belt of Venus, a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky.

In the image at second left, the Belt of Venus is shown at sunset, looking east from the Marin Headlands just north of San Francisco. There is a thin greyish cloud layer partially obscuring the horizon in this image.

The second image at right shows the full Moon rising as seen through the Belt of Venus.

In the right viewing conditions, a pink (or orange or purple) band is visible in the twilight sky just above the dark blue band of the Earth's shadow. This pink band is called the "anti-twilight arch" or "Belt of Venus". The name "Belt of Venus" is not connected with the planet Venus; the Belt of Venus is part of Earth's upper atmosphere which is illuminated by the setting or rising sun. It is visible either after the sun ceases to be visible (at sunset) or before the sun becomes visible (at sunrise).[39][40]

When the sun is near the horizon at sunset or sunrise, the light from the sun is red; this is because the light is reaching the observer through an especially thick layer of the atmosphere, which works as a filter, scattering all but the red light.

From the viewpoint of the observer, the red sunlight directly illuminates small particles in the lower atmosphere on the other side of the sky from the sun. The red light is backscattered to the observer, and that is why the Belt of Venus appears pink.

The lower the sunset sun descends, the less clearly distinguished the boundary between the Earth's shadow and Belt of Venus becomes. This is because now the setting sun illuminates a thinner part of the upper atmosphere. The red light is not scattered there because there are fewer particles, and the eye only sees the "normal" (usual) blue sky, which is due to Rayleigh scattering from air molecules. Eventually, both the Earth's shadow and the Belt of Venus dissolve into the darkness of the night sky.[40]

## Infrareds

The two images allow comparison of an optical astronomy image of Earth with an infrared astronomy image. Credit: NASA.
This is the first full-disk thermal infrared (IR) image taken by GOES 14, showing radiation with a wavelength of 10.7 micrometers emanating from Earth. Credit: GOES science team.
This Galileo spacecraft image was taken through the infrared (9680 nm) filter, which enhances the visibility of the land masses. Credit: Galileo orbiter, NASA/JPL.

The images at top right are from the optical astronomy and infrared telescopes aboard the MESSENGER spacecraft.

The optical astronomy image delineates gaseous, liquid, and rocky portions of the Earth's atmosphere, oceans, and land masses. The infrared astronomy image detects primarily the rocky objects or land masses of Earth.

The second image at right is the "first full-disk thermal infrared (IR) image, [taken by GOES 14,] showing radiation with a wavelength of 10.7 micrometers emanating from Earth."[41]

"Infrared images are useful because they provide information about temperatures. A wavelength of 10.7 micrometers is 15 times longer than the longest wavelength of light (red) that people can see, but scientists can turn the data into a picture by having a computer display cold temperatures as bright white and hot temperatures as black. The hottest (blackest) features in the scene are land surfaces; the coldest (whitest) features in the scene are clouds."[41]

"In the heat of the midday sun, the exposed rock in sparsely vegetated mountain ranges and high-altitude deserts in western North and South America are dark. In North America, the temperatures cool (fade to lighter grey) along a gradient from west to east, as the semi-deserts of the West and South-west transition to the grasslands and crop-lands of the Great Plains, which transition to forests in the East."[41]

"A band of scattered storms across the equatorial Pacific shows the location of the Inter-tropical Convergence Zone, which is a belt of showers and thunderstorms that persists near the equator year round. Need help precisely locating the equator? Look for the dark (hot) spots in the Pacific Ocean west of South America: those are the Galapagos Islands, and the equator passes through the northern tip of the largest island."[41]

"Perhaps the most significant features related to U.S. weather appear in the upper right quadrant of the disk: the remnants of Tropical Storm Claudette drenching the eastern Gulf Coast, Tropical Depression Ana unwinding over Puerto Rico and the Dominican Republic, and Hurricane Bill approaching from the central Atlantic."[41]

At left is an image from the Galileo spacecraft. The "Galileo raw image of Earth [is] taken from 2.7 million km 3 days after the first flyby. Africa and the Middle East can be seen at the top and Antarctica at the bottom. Weather systems are visible in the clouds over the Atlantic and Indian oceans. The image was taken through the infrared (9680 nm) filter, which enhances the visibility of the land masses."[42]

This is a composite radar image of the June 2012 United States Derecho event. Credit: G. Carbin, NWS/Storm Prediction Center.

On the right is a composite of hourly radar images. These wind gusts averaged ~75 mph over about 450 miles. This is referred to as the Derecho event.

## Gases

Def. matter "that can be contained only if it is fully surrounded by a solid (or in a bubble of liquid) (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly"[43] is called a gas.

"Turbines have been around for a long time—windmills and water wheels are early examples. The name comes from the Latin turbo, meaning vortex, and thus the defining property of a turbine is that a fluid or gas turns the blades of a rotor, which is attached to a shaft that can perform useful work."[44]

Def. relating "to, or existing as, gas"[45] is called gaseous.

## Gaseous objects

GOES-10 images a water vapor Earth. Credit: NCDC, NOAA.{{fairuse}}

"[T]he evolution of star accretion onto a supermassive gaseous object in the central region of an active galactic nucleus [may be addressed using] a gaseous model of relaxing dense stellar systems".[46]

Gaseous objects have at least one chemical element or compound present in the gaseous state. These gaseous components make up at least 50 % of the detectable portion of the gaseous object such as the image on the right which was produced by detecting only water vapor in the infrared by the GOES-10 satellite.

## Chemicals

The concentration of water vapor varies significantly from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).[47]

The remaining gases are often referred to as trace gases,[48] among which are principally carbon dioxide, methane, nitrous oxide, and ozone.

Major constituents of dry air, by volume[49]
Gas Volume(A)
Name Formula in ppmv(B) in %
Nitrogen N2 780,840 78.084
Oxygen O2 209,460 20.946
Argon Ar 9,340 0.9340
Carbon dioxide CO
2
400 0.04[50]
Neon Ne 18.18 0.001818
Helium He 5.24 0.000524
Methane CH4 1.79 0.000179
Not included in above dry atmosphere:
Water vapor(C) H2O 10–50,000(D) 0.001%–5%(D)
notes:

(A) volume fraction is equal to mole fraction for ideal gas only,

(B) ppmv: parts per million by volume
(C) Water vapor is about 0.25% by mass over full atmosphere
(D) Water vapor strongly varies locally[47]

The relative concentration of gases remains constant until about 10,000 m (33,000 ft).[51]

## Atmospheres

This is a graph of the global mean atmospheric water vapor superimposed on an outline of the Earth. Credit: NASA.
This image demonstrates obstacles to observation (the Singapore skyline) and one atmospheric object: haze. Credit: SpLoT.
This is a map of global long term monthly average surface air temperatures in Mollweide projection. Credit: PZmaps.
This map of Earth's solid and liquid surface is divided into climate zones, largely influenced by latitude. Credit: Waitak.
Temperature trends in two thick layers of the atmosphere were measured between January 1979 and December 2005. Credit: Jesse Allen, Carl Mears, Remote Sensing Systems, and Paul Newman and Joel Susskind NASA Goddard Space Flight Center.

These molecules in many instances are in turn made up of atoms of chemical elements. At your geographical location, specified in latitude and longitude, this gaseous envelope extends upward. The atmosphere of Earth changes with altitude. At high enough altitude the composition changes significantly, as does the temperature and pressure.

The atmospheric pressure at sea level averages 101.325 kPa (14.696 psi),[52] with a scale height of about 8.5 km (5.3 mi).[53]

A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.[52]

Water vapor content varies between 0.01% and 4%[52] but averages about 1%.[53]

The atmosphere transports water vapor.[54]

It causes small meteors to burn up before they strike the surface.[54]

Trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature: water vapor, carbon dioxide, methane, nitrous oxide, and ozone, where the average surface temperature would be −18 °C (0 °F), in contrast to the current +15 °C (59 °F).[55]

It moderates temperature.[54]

In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[56][57]

The ozone layer blocks ultraviolet solar radiation.[58]

The map at right shows the global long term monthly average surface air temperature for Earth.

Temperature trends in two thick layers of the atmosphere [image on the right] were measured between January 1979 and December 2005 by Microwave Sounding Units and Advanced Microwave Sounding Units on NOAA weather satellites. The instruments record microwaves emitted from oxygen molecules in the atmosphere.[59]

Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb).[60]

The zones of Earth's climates, going from the equator upward (and downward) are Tropical, Dry, Moderate, Continental and Polar. There are subzones within these zones.

### Climates

An updated Köppen–Geiger climate map is shown.
 tropical rainforest climate (Af)   tropical monsoon climate (Am)   tropical savanna climate (Aw/As) hot desert climate (BWh)   cold desert climate (BWk)   hot semi-arid climate (BSh)   cold semi-arid climate (BSk) hot-summer Mediterranean climate (Csa)   warm-summer Mediterranean climate (Csb)   cold-summer Mediterranean climate (Csc) humid subtropical climate (Cwa)   subtropical highland climate (Cwb)   subtropical highland climate (Cwc) humid subtropical climate (Cfa)   oceanic climate (Cfb)   subpolar oceanic climate (Cfc) humid continental climate (Dsa)   humid continental climate (Dsb)   subarctic climate (Dsc)   subarctic climate (Dsd) humid continental climate (Dwa)   humid continental climate (Dwb)   subarctic climate (Dwc)   subarctic climate (Dwd) humid continental climate   humid continental climate   subarctic climate (Dfc)   subarctic climate (Dfd) tundra climate (ET)   ice cap climate (EF)
Credit: Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F.

The image centered above contains the Köppen-Geiger climate classification maps at 1-km resolution.[61]

### Tropics

World map shows the intertropical zone highlighted in crimson. Credit: KVDP.
Areas of the world are shown that have tropical climates. Credit: Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F.
The graph shows the zonally averaged monthly precipitation. Credit: NASA.
Worldwide zones of Tropical rainforest climate (Af) are marked in blue for land areas. Credit: Peel, M. C., Finlayson, B. L., and McMahon, T. A. (University of Melbourne).
Worldwide zones of tropical monsoon climate (Am) are shown. Credit: Peel, M. C., Finlayson, B. L., and McMahon, T. A. (University of Melbourne).
Worldwide zones of Tropical savanna climate (Aw/As) are marked. Credit: Peel, M. C., Finlayson, B. L., and McMahon, T. A. (University of Melbourne).
This map is an excellent illustration of the Hadley cell. Credit: William M. Connolley.

Def. from "or similar to a hot humid climate" occurring within or near to "the equatorial region between 23 degrees north and 23 degrees south"[62] as the "short term state of the atmosphere at a specific time and place, including the temperature, relative humidity, cloud cover, precipitation, wind, etc"[63] is called the tropics, or tropical weather.

The map on the right shows the approximate atmospheric locations per the geographic grid where the Köppen–Geiger climate classification map for Tropical occurs.

The map on the left indicates the geographic zone affected by the Sun being at the zenith for portions of an Earth year.

The graphs in the center image indicate water rainfall monthly averages per latitude. The tropics receive more precipitation than higher latitudes. The precipitation maximum, which follows the solar equator through the year, is under the rising branch of the Hadley circulation.

Under the Köppen climate classification, for tropical climates, a wet-season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more.[64]

In some tropical regions rainfall is equally distributed through the year.[65] Some areas with pronounced rainy seasons see a break in rainfall during mid-season when the intertropical convergence zone or monsoon trough moves poleward of their location during the middle of the warm season;[66]

In rainforest climates (Af) such as marked in blue in the second image down on the right has rainfall normally heavy throughout the year, where one day in an equatorial climate can be very similar to the next, while the change in temperature between day and night may be larger than the average change in temperature during the year.[67]

Tropical monsoon climate (Am) regions marked in the third image down on the right results when the monsoon winds change direction according to the seasons, where there is a short but distinct dry season which nearly always occurs at or soon after the "winter" solstice for that side of the equator with rainfall less than 60 mm, but more than 1/25 the total annual precipitation.[67]

Tropical savanna climate (As) or tropical wet and dry climate (Aw) shown in the fourth image down on the right is a type of climate that corresponds to the Köppen climate classification categories "Aw" and "As", with monthly mean temperatures above 18 °C (64 °F) in every month of the year and typically a pronounced dry season, with the driest month having less than 60 mm (2.36 inches) of precipitation and less than 100 – total annual precipitation mm/25 of precipitation.[67]

The map second down on the left is an excellent illustration of the Hadley cell where the vertical velocity is 500 hPa (Pa/sec) in July from ERA-40 reanalysis, 1979-2001 average; the negative (blue) values represent rising air; and positive (red) values indicate sinking air. It also illustrates that atmospheric regions extend across bodies of water between the land.

### Deserts

Regions with desert climates are marked
BWh (hot desert climates)
BWk (cold desert climates)
Credit: Peel, M. C., Finlayson, B. L., and McMahon, T. A. (University of Melbourne).

The desert climate (in the Köppen climate classification BWh and BWk) illustrated in the image on the right is a climate in which there is an excess of evaporation over precipitation, covering 14.2% of earth's land area, hot deserts may be the most common type of climate on earth.[68]

Most desert and arid climates receive between 25 and 200 mm (1 to 8 inches) of rainfall annually.[69] In the Köppen classification system, a climate is classed as arid if

1. its mean annual precipitation in millimeters is less than ten times its defined precipitation threshhold,
2. its mean annual precipitation is less than five times this threshold,
3. the precipitation threshold is twice its mean annual temperature in degrees Celsius, plus a constant to represent the distribution of its rainfall throughout the year
4. the constant is 28 for regions that receive 70% or more of their rainfall during the six winter months,
5. the constant is 0 for regions that receive 70% or more of their rainfall during the six summer months
6. it is 14 for any climates falling between these two extremes.[70]

There are deserts designated BWh (hot desert climates) and BWk (cold desert climates).

## Altitude regions

Comparison shows the 1962 US Standard Atmosphere graph of geometric altitude against air density, pressure, the speed of sound and temperature with approximate altitudes of various objects. Credit: Cmglee.{{free media}}
This image shows the Moon at the centre, with the limb of Earth near the bottom. Credit: ISS Expedition 28 crew.{{free media}}
Space Shuttle Endeavour is orbiting in the thermosphere. Credit: NASA/Crew of Expedition 22.
The volume fraction of the main constituents of the Earth's atmosphere are shown as a function of height according to the MSIS-E-90 atmospheric model. Credit: Amaurea.
Earth's atmosphere Lower 4 layers of the atmosphere in 3 dimensions is seen diagonally from above the exobase. Layers drawn to scale, objects within the layers are not to scale. Aurorae shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer. Credit: Kelvinsong.

On the right, the 1962 US Standard Atmosphere graph of geometric altitude is plotted against air density, pressure, the speed of sound and temperature with approximate altitudes of various objects.[71]

In the center image the Space Shuttle Endeavour is orbiting in the thermosphere. Because of the angle of the photo, it appears to straddle the stratosphere and mesosphere that actually lie more than 250 km below. The orange layer is the troposphere, which gives way to the whitish stratosphere and then the blue mesosphere.[72]

In summary, the mass of Earth's atmosphere is distributed approximately as follows:[73]

• 50% is below 5.6 km (18,000 ft).
• 90% is below 16 km (52,000 ft).
• 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line.

The image on the left shows the Moon at the centre, with the limb of Earth near the bottom transitioning into the orange-colored troposphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue-colored atmosphere. The silvery-blue noctilucent clouds extend far above Earth's troposphere.

Earth's atmosphere can be divided (called atmospheric stratification) into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere.[74] From highest to lowest, the five main layers are:

• Exosphere: 700 to 10,000 km (440 to 6,200 miles)
• Thermosphere: 80 to 700 km (50 to 440 miles)[75]
• Mesosphere: 50 to 80 km (31 to 50 miles)
• Stratosphere: 12 to 50 km (7 to 31 miles)
• Troposphere: 0 to 12 km (0 to 7 miles)[76]

The Earth's atmosphere is divided into altitude regions:[77]

• Troposphere — surface to 8,000 m at the poles – 18,000 m at the equator, ending at the Tropopause.
• Stratosphere — Troposphere to 50 km
• Mesosphere — Stratosphere to 85 km
• Thermosphere — Mesosphere to 675 km
• Exosphere — Thermosphere to 10,000 km.

### Tropospheres

A view of Earth's troposphere is from an airplane. Credit: Nick Juhasz.
This image shows the temperature trend in the Middle Troposphere. Credit: Jesse Allen, Carl Mears, and Paul Newman, Remote Sensing Systems and Joel Susskind NASA Goddard Space Flight Center.
A zonal flow regime is illustrated by the dominant west-to-east flow as shown in the 500 hPa height pattern. Credit: Climate Prediction Center.
In the Meridional Flow pattern of October 23, 2003, note the amplified troughs and ridges in this 500 hPa height pattern. Credit: NOAA/NCEP.
Atmospheric circulation is modeled with three large cells. Credit: DWindrim.

Def. the "lower levels of the atmosphere extending from [a solid or liquid] surface up to the tropopause ... characterized by convective air movements and a large vertical temperature change"[78] is called the troposphere.

The troposphere, the lowest layer of Earth's atmosphere, extends from Earth's surface to an average height of about 12 km (7.5 mi; 39,000 ft), although this altitude varies from about 9 km (5.6 mi; 30,000 ft) at the geographic poles to 17 km (11 mi; 56,000 ft) at the Equator,[76] with some variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion (i.e. a layer of relatively warm air above a colder one), and in others by a zone which is isothermal with height.[79][80]

The troposphere "contains about four-fifths [80 %] of the mass of the whole atmosphere and 99% of the total mass of water vapor and aerosols."[81]

Rotational turbulent mixing plays an important role in the troposphere's structure and behaviour and most of the phenomena associated with day-to-day weather occur in the troposphere.[82]

The pressure of the atmosphere is maximum at sea level and decreases with altitude, because the atmosphere is very nearly in hydrostatic equilibrium so that the pressure is equal to the weight of air above a given point; thus, the change in pressure with altitude can be equated to the density with the hydrostatic equation[83]

${\displaystyle {\frac {dP}{dz}}=-\rho g_{n}=-{\frac {mPg_{n}}{RT}}}$

where:

• gn is the standard gravity
• ρ is the density
• z is the altitude
• P is the pressure
• R is the gas constant
• T is the thermodynamic (absolute) temperature
• m is the molar mass

The second image down on the right shows the temperature trend in the Middle Troposphere as measured by a series of satellite-based instruments between January 1979 and December 2005. The middle troposphere is centered around 5 kilometers above the surface. Oranges and yellows dominate the troposphere image, indicating that the air nearest the Earth's surface warmed during the period.[84]

The temperature of the troposphere generally decreases as altitude increases. The rate at which the temperature decreases, ${\displaystyle -dT/dz}$, is called the environmental lapse rate (ELR).

Environmental Lapse Rate (ELR)
Altitude Region Lapse rate Lapse Rate
(m) (Kelvin/km) (°F/1000 feet)
0 – 11,000 6.5 3.57
11,000 – 20,000 0.0 0.0
20,000 – 32,000 -1.0 -0.55
32,000 – 47,000 -2.8 -1.54
47,000 – 51,000 0.0 0.0
51,000 – 71,000 2.8 1.54
71,000 – 85,000 2.0 1.09

For dry air, which is approximately an ideal gas, the adiabatic equation for an ideal gas is[85]

${\displaystyle p(z)T(z)^{-{\frac {\gamma }{\gamma -1}}}={\text{constant}}}$

where ${\displaystyle \gamma }$ is the heat capacity ratio (${\displaystyle \gamma }$=7/5, for air).

Combining with the equation for the pressure, produces the dry adiabatic lapse rate.[86]

${\displaystyle {\frac {dT}{dz}}=-{\frac {mg}{R}}{\frac {\gamma -1}{\gamma }}=-9.8^{\circ }\mathrm {C} /\mathrm {km} }$

In the troposphere, the average environmental lapse rate is a drop of about 6.5 °C for every 1 km (1,000 meters) in increased height.[82]

The average thickness of the tropical troposphere is roughly 7 kilometers greater than the average tropospheric thickness at the poles.[87]

A zonal flow regime as illustrated in the third diagram down on the right is the meteorological term meaning that the general flow pattern is west to east along the Earth's latitude lines, with weak shortwaves embedded in the flow.[88]

When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional [in the fourth image down on the right]) direction, and thus the term "meridional flow" arises, which features strong, amplified troughs of low pressure and ridges of high pressure, with more north-south flow in the general pattern than west-to-east flow.[89]

The three cells model (fifth image down on the right) of the atmosphere attempts to describe the actual flow of the Earth's atmosphere as a whole using the:

1. tropical (Hadley cell),
2. mid latitude (Ferrel cell), and
3. polar (polar cell) regions to describe energy flow and global atmospheric circulation (mass flow) – the energy that the Earth absorbs from the Sun each year is equal to that which it loses to space by radiation,where the result is a circulation of the atmosphere that transports warm air poleward from the tropics and cold air equatorward from the poles so the effect of the three cells is the tendency to even out the heat and moisture in the Earth's atmosphere around the planet.[90]

The troposphere contains the planetary boundary layer, and ranges in height from an average of 9 km (5.6 mi; 30,000 ft) at the poles, to 17 km (11 mi; 56,000 ft) at the Equator.[91][92]

### Tropopauses

Def. the "zone of transition between the troposphere and the stratosphere"[93] is called the tropopause.

The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.[82]

"The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. It is defined as the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km."[94]

The tropopause as defined above renders as a first-order discontinuity surface, that is, temperature as a function of height varies continuously through the atmosphere but the temperature gradient does not.[95]

### Stratospheres

Def. the "region ... where temperature increases along with the altitude ... [extending] from the tropopause ... to ... the mesosphere"[96] is called the stratosphere.

### Mesospheres

Def. a "layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere"[97] is called a mesosphere.

Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere, the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).[98][99]

### Thermospheres

Nomenclature of atmospheric regions is based on the profiles of electric conductivity (left), temperature (middle), and electron number density in m−3(right). Credit: Bnland.

Def. a "layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere"[100] is called a thermosphere.

Within the thermosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions in the ionosphere beginning at about 80 km (50 mi) above sea level.[101]

The temperature of the thermosphere gradually increases with height, the inversion in the thermosphere occurs due to the extremely low density of its molecules, the temperature of this layer can rise as high as 1500 °C (2700 °F), though the gas molecules are so far apart, i.e., the air is so rarefied that an individual molecule (of oxygen, for example) travels an average of 1 kilometre (0.62 mi; 3300 ft) between collisions with other molecules.[102]

### Exospheres

Def. "the uppermost layer of a planet's atmosphere"[103] or "an extremely thin atmosphere""[104] is called an exosphere.

The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other.[105]

Mercury and several large moons, such as the Moon and the Galilean satellites of Jupiter, have exospheres without a denser atmosphere underneath,[106] referred to as a surface boundary exosphere.[107]

The lower boundary of the exosphere is called the exobase, exopause or 'critical altitude' as this is the altitude where barometric conditions no longer apply and atmospheric temperature becomes nearly a constant above this altitude.[108]

On Earth, the altitude of the exobase ranges from about 500 to 1,000 kilometres (310 to 620 mi) depending on solar activity.[109]

On 17 August 2015, based on studies with the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, NASA reported the detection of neon in the exosphere of the moon.[105]

## Ionospheres

In this diagram, the prominent features in the ionosphere-thermosphere system and their coupling to the different energy inputs show the complex temporal and spatial phenomena that are generated. Credit: NASA.

Def. the "part of the Earth's atmosphere beginning at an altitude of about 50 kilometers [31 miles] and extending outward 500 kilometers [310 miles] or more"[110] or the "similar region of the atmosphere of another planet"[111] is called the ionosphere.

Upon reaching the top of the mesosphere, the temperature starts to rise, but air pressure continues to fall. This is the beginning of the ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer. The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.

## Earth

This is the famous Blue Marble image of Earth taken by Apollo 17. Credit: NASA. Photo taken by either Harrison Schmitt or Ron Evans (of the Apollo 17 crew).

"Earth is a blue planet"[112].

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space. This is shown in the image at right.

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

The image on the left is the famous Blue Marble image of Earth taken by Apollo 17. The image shows the eastern Southern Atlantic Ocean, the South African portion of the Southern Ocean, and the Western Indian Ocean. The land consists of most of Africa, Madagascar, Saudi Arabia, and portions of Iran, Irag, Turkey, and southern Greece. The gaseous portion consists of water vapor clouds over the southern portion of this hemisphere. Antarctica is completely covered in snow (a water-ice rocky substance).

If this image is the one chosen to decide whether the Earth is a dwarf gaseous object, a dwarf liquid object, or a dwarf rocky object, the decision becomes difficult.

## Volcanoes

Mount Redoubt in Alaska erupted on April 21, 1990. The mushroom-shaped plume rose from avalanches of hot debris that cascaded down the north flank. Credit: R. Clucas, USGS.
This oblique astronaut photograph from the International Space Station (ISS) captures a white-to-grey volcanic ash and steam plume extending westwards from the Soufriere Hills volcano. Credit: NASA Expedition 21 crew.

Oblique images such as the one at right are taken by astronauts looking out from the ISS at an angle, rather than looking straight downward toward the Earth (a perspective called a nadir view), as is common with most remotely sensed data from satellites. An oblique view gives the scene a more three-dimension quality, and provides a look at the vertical structure of the volcanic plume. While much of the island is covered in green vegetation, grey deposits that include pyroclastic flows and volcanic mud-flows (lahars) are visible extending from the volcano toward the coastline. When compared to its extent in earlier views, the volcanic debris has filled in more of the eastern coastline. Urban areas are visible in the northern and western portions of the island; they are recognizable by linear street patterns and the presence of bright building rooftops. The silver-grey appearance of the Caribbean Sea surface is due to sun-glint, which is the mirror-like reflection of sunlight off the water surface back towards the hand-held camera on-board the ISS. The sun-glint highlights surface wave patterns around the island.

## Auroras

The Aurora Borealis, or Northern Lights, shines above Bear Lake, Alaska. Credit: Senior Airman Joshua Strang, United States Air Force.
Laboratory experiment produces aurora at the poles of a sphere. Credit: David Monniaux.

Computer simulations are usually used to represent auroras. The image at right shows a terrella in a laboratory experiment to produce auroras.

"Although auroras might first appear to be moonlit clouds, they only add light to the sky and do not block background stars from view. Called "Northern Lights" in the Northern Hemisphere, auroras are caused by collisions between charged particles from the magnetosphere and air molecules high in the Earth's atmosphere. If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well. Predictable auroras might occur a few days after a powerful magnetic event has been seen on the sun."[113]

Most aurorae occur in a band known as the auroral zone,[114][115] which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone will expand to lower latitudes. The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurora are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night. Discrete aurorae are usually observed only in the night sky because they are as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches[116] or arcs (polar cap arcs[117]), which are generally invisible to the naked eye.

## Jet streams

Clouds are shown along a jet stream over Canada. Credit: NASA.

Def. any of the high-speed, high-altitude air currents that circle the Earth in a westerly direction is called a jet stream.

Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude).[118] The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7–12 km (23,000–39,000 ft) above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km (33,000–52,000 ft). The Northern Hemisphere and the Southern Hemisphere each have both a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.

## Earth sciences

Meteorology is represented by cyclone Catarina. Credit: NASA.

Def. the study of the atmosphere and its phenomena, especially with weather and weather forecasting is called meteorology.

Def. the study of the theoretical effects of astronomical bodies and forces on the Earth’s atmosphere and on the atmosphere of other astronomical objects is called astrometeorology.

Def. a system of winds rotating around a center of low atmospheric pressure, the more or less violent small-scale circulations such as tornadoes, waterspouts, and dust devils is called a cyclone.

## Moon

The extent of Earth's geocorona is diagrammed not to scale to show where Earth's atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona. Credit: ESA.{{fairuse}}

"Where Earth's atmosphere merges into outer space, there is a cloud of hydrogen atoms called the geocorona."[119]

"A recent discovery based on observations by the ESA/NASA Solar and Heliospheric Observatory, SOHO, shows that the geocorona extends well beyond the orbit of the Moon, reaching up to 630 000 km above Earth's surface, or 50 times the diameter of our planet."[119]

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