Earth’s Atmosphere

Amanda Stadermann1,‹

1Lunar and Planetary Laboratory, University of Arizona

Abstract. Summary of information about the Earth’s atmosphere. I will discuss the pressure-temperatureprofile of the atmosphere, the composition of the atmosphere, clouds and hazes on Earth, and wind speeds ofEarth’s atmosphere.

1 Introduction

Earth is the third planet from the Sun, but number one ineveryone’s hearts. It is the only planet to support life,including some unique creatures that like to learn aboutthe atmospheres of other planets. Some basics aboutthe planet are below in Table 1. Less interesting factsabout Earth include its 11.186 km/s escape velocity, its0.0167086 eccentricity, 23.4392811o axial tilt, and its 288K average surface temperature.

Table 1. Basic parameters for Earth

parameter value

mass 1 MC 5.972ˆ 1024 kgradius 1 RC 6371 km

orbital period 1 year 3.157ˆ 107 srotational period 1 day 86400 ssemi-major axis 1 AU 1.496ˆ 108 kmsurface pressure 1 atm 1.01325 barsurface gravity 1 g 9.8 m/s2

moons 1 the Moon

2 Temperature profile

The average temperature-pressure profile of the Earth’s at-mosphere is well modeled. Figure 1 shows the pressure-temperature profile of Earth’s atmosphere, as modeled inthe US Standard Atmosphere [1]. The surface temperatureis taken to be 288 K. This profile is the aggregate of manysources. These sources range from weather balloons, air-craft, rockets, and satellites, depending on the altitude inquestion [1]. These temperatures change somewhat sea-sonally and latitudinally, as shown in Figure 2 [2].

‹e-mail: acs@lpl.arizona.edu

Figure 1. Temperature of Earth’s atmosphere as a function ofaltitude and pressure [1].

3 Lapse rates

3.1 Dry lapse rate

For the Earth, the dry lapse rate can be calculated usingequations for hydrostatic equilibrium (equation 1) and adi-abatic motion (equation 2). This gives us an equation thatsolves dT{dZ as a function of gravity and specific heat.

αdP “ ´gdz (1)cpdT “ αdP (2)

Γ “dTdz“´g

cp(3)

Figure 2. Northern summer zonal averaged temperature [2].Temperatures are in degrees Celsius.

In equation 3, Γ is the dry lapse rate of the atmosphere, gis the gravitational acceleration, and cp is the specific heatat constant pressure.

The specific heat of Earth’s air is approximately 1.00kJ/(K kg), though this is a function of temperature. Thegravitational acceleration can be taken to be 9.8 m/s2 atEarth’s surface (sea level).

Γ “´9.81.00

“ ´9.8 K/km (4)

We can therefore see that the dry lapse rate on Earthapproaches 9.8 K/km.

3.2 Wet lapse rate

The wet lapse rate can be approximated by multiplyingequation 3 by a factor to include the effects of condensedphases in the atmosphere. In the case of Earth, the primarycondensible is water vapor, or H2O.

dTdz“´g

cp˚

11` pL{cpqpdw{dT q

(5)

In equation 5, L is the latent heat of vaporization,which is 2257 kJ/kg for water, and w is the mass of sat-urated water per mass of air. In this case, dw{dT is about4.25x10´4 for Earth-like temperatures.

dTdz“´9.81.00

˚1

1` p2257{1qp4.25x10´4q“ 5 K/km

(6)

Our calculated value for the wet lapse rate is approxi-mately correct, with values for the wet lapse rate rangingfrom 4 K/km to 7 K/km depending on altitude and temper-ature [2].

3.3 Environmental lapse rate

The actual lapse rate for Earth varies spatially and tempo-rally as humidity changes with the seasons, latitude, and

longitude. From the US Standard Atmosphere, the tropo-sphere has a lapse rate of approximately 6.5 K/km [1, 2].The value of the environmental lapse rate varies diurnally,seasonally, and spatially. This lapse rate accurately ex-plains the temperature profile below the tropopause.

4 CompositionThe composition of the Earth’s atmosphere is well docu-mented, and has been known to change with time. Thecurrent composition of the Earth’s atmosphere is given inTable 3.

Figure 3. Composition of the Earth’s atmosphere as a functionof geologic time [3].

Variation in the composition of Earth’s atmosphere isalso well documented [3]. The temporal variation in thecomposition of the Earth’s atmosphere with time is givenin Figure 3. This figure doesn’t take into account a primor-dial atmosphere, as it assumes that at 4.5 Ga, the Earth hadno atmosphere. The initial composition of the atmospherethus is that of volcanic outgassing. From here, the CO2was incorporated into rocks via the Urey reaction, shownin equation 7.

CaSiO3 ` CO2 Ø CaCO3 ` SiO2 (7)

This reaction helped the climate decrease in CO2 con-centration, as CH4 and other carbon compound increasedin abundance. At this point, small amounts of oxygenslowly destroyed the CH4 and NH3 in the atmosphere. Asthis took place, the amount of N2 rose steadily.

After 2.5 Ga, the atmosphere was largely removed ofreduced gases in the atmosphere. There was then a veryslow build up of oxygen in the atmosphere, in the form ofO2 and O3. At the time of the Cambrian explosion, oxygenhad built up enough in the atmosphere to block harmfulUV radiation from the sun, allowing life to prosper. Theincrease in biomass increased dramatically the amount ofoxygen in the air, creating the environment we see today.

5 Clouds and hazesThe particulates in the atmosphere of Earth can be dividedinto two categories: direct emissions and in situ. Direct

emissions are aerosols that become entrained in the atmo-sphere due to emissions from the surface of the Earth. Insitu aerosols are ones created or condensed within atmo-sphere. Specific aerosols are shown in Table 2.

Table 2. Major particulates and aerosols in the Earth’satmosphere in Tg per year [2].

direct emissionsn. hemi. s. hemi.

carbonaceous aerosolsorganic matter (0-2 µm)biomass burning 28 26fossil fuel 28 0.4biogenic (> 1 µm) – –

black carbon (0-2 µm)biomass burning 2.9 2.7fossil fuel 6.5 0.1aircraft 0.005 0.0004

industrial dust (> 1 µm)sea salt

< 1 µm 23 311-16 µm 1420 1870total 1440 1900

mineral (soil) dust< 1 µm 90 171-2 µm 240 502-20 µm 1470 282total 1800 349

in situn. hemi. s. hemi.

sulfates (as NH4HSO4) 145 55anthropogenic 106 15biogenic 25 32volcanic 14 7

nitrate (as NO´

3 )anthropogenic 12.4 1.8natural 2.2 1.7

organic compoundsanthropogenic 0.15 0.45biogenic 8.2 7.4

This table seems to disregard liquid water as a conden-sible liquid in the Earth’s atmosphere. Water is the mainproducer of clouds on Earth. Clouds form primarily in thetroposphere (from 1 - 12 km altitude), and can be up toseveral km tall. Their shape is determined by how theyform, the altitude at which they form, their temperature,and their composition.

6 Atmosphere dynamics

The circulation, climate, and weather patterns of theEarth’s atmosphere have been observed by humans for ap-proximately 200,000 years. The general model for move-ment of air in the Earth’s atmosphere is shown in Figure4. The motion of air is broadly determined by differentialheating of the surface and the rotation of the Earth.

Figure 4. Schematic showing general circulation model of theEarth’s atmosphere.

6.1 Weather

Weather is very exciting on Earth. There are storms thatcan last from days to a couple weeks, depending on size.There is lightning that occurs during some of these stormsbecause of charge separation in the clouds. See Figure 8.

Tornadoes form when horizontal shear winds get up-lifted and and become vertical cyclones. It is visible be-cause air pressure is lowered causing condensation andbecause of the debris it picks up. Tornadoes may havemultiple vortexes (Figure 5).

Figure 5. Tornado with two vortexes in Tushka, OK on April 14,2011.

Lightning is also an interesting feature of weather onEarth. Charges become separated in a cloud due to fallinghailstones or graupel falling through the cloud, becomingnegatively charged. This negatively charged particles areconcentrated in a certain region of the cloud, between -10oC to -20oC (Figure 6). Other smaller particles aretherefore positively charged and become entrained in up-drafts and carried to the upper portion of the cloud. Thischarge separation increases the electric field until it’s morethan the air can sustain. This results in a lightning flash(Figure 7).

Figure 6. Distribution of charges in a cloud [2].

Figure 7. Time sequence of how lightning strikes occur [2].

6.2 Climate

Greenhouse gases are present in the Earth’s atmosphere.These gases include H2O, CO2, CH4, O3 and N2O. Thesegases warm the atmosphere by absorbing the infrared heat-ing emitted by the Earth’s surface, and keeping that heat inthe atmosphere instead of letting it radiate to space. Thesegreenhouse gases directly impact the temperature of theEarth and can spark several feedback mechanisms [4].

Positive feedback loops, such as melting of sea ice todecrease albedo, can cause increasingly rapid changes inglobal climate [4]. Hansen et al. (2007) suggest that theonly feasible way to slow global climate change is to ac-tively remove greenhouse gases from the atmosphere [4].

References

[1] NOAA, NASA, USAF, U.S. Standard Atmosphere(US Government Printing Office, Washington D.C.,1976)

[2] J.M. Wallace, P.V. Hobbs, Atmospheric Science: AnIntroductory Survey, Vol. 92 of International Geo-physics Series, 2nd edn. (Academic Press, 2006)

[3] M.H. Hart, Icarus 33, 23 (1978)[4] J. Hansen, M. Sato, P. Kharecha, G. Russell, D.W.

Lea, M. Siddall, Philosophical Transactions of theRoyal Society A 365, 1925 (2007)

[5] R.G. Prinn, J. Bruce Fegley, Annual Review of Earthand Planetary Sciences 15, 171 (1987)

Figure 8. Structure of a supercell storm with a tornado [2]

Table 3. Composition of Earth’s atmosphere with sources andsinks of each species [5]

species volume mixing ratio major source major sink

N2 0.781 biological biologicalO2 .209 biological biological

H2O ď 4ˆ 10´2 evaporation condensation40Ar 9.3ˆ 10´3 outgassing (40K) –CO2 3.4ˆ 10´4 combustion, biological biological

36,38Ar 3.7ˆ 10´5 outgassing (primordial) –20,22Ne 1.82ˆ 10´5 outgassing (primordial) –

4He 5.24ˆ 10´6 outgassing (U, Th) escapeCH4 1.7´ 3ˆ 10´6 biological photooxidation

80,82´84,86Kr 1.14ˆ 10´6 outgassing (235U) –H2 5ˆ 10´7 photochemistry (H2O) escape (as H)