Earth's Atmosphere and Climate

UPDATED: 14 February 2022; ppm for CO2 updated from 380 to 413

POSTED: 10 June 2020

• Weather and Climate

• climate: average long-term condition; controlled by location; regional- to global-scale
• weather: short-term condition; controlled by atmospheric anomalies; local- to regional-scale
• saying: "climate is what you wish for; weather is what you get"

• The Two Principal Controlling Factors of Climate

1. how much solar radiation is received by Earth (insolation)
• sun's energy is distributed unevenly on Earth
• equator receives 2.4 more energy annually than poles

To understand this, we have to be aware of two things:
1. the Sun is so far away that the rays reaching Earth are parallel, i.e. if Earth was a disk facing the Sun, each area would get the same amount of energy
2. since Earth is a sphere, the same amount of solar energy is spread over a larger area near the poles than near the equator; so the poles receive less energy

The area on Earth receiving the highest insolation changes throughout the year! It is the area where the sun is overhead at noon. This is the equator only during spring and fall equinox. At other times, this moves between the Tropic of Cancer (23.5°N) on the date of the northern summer solstice (June 20 or 21) and the Tropic of Capricorn (23.5°S) on the northern winter solstice (i.e. southern summer solstice; December 21 or 22).



2. how much solar radiation is retained in an area
• albedo: fraction of Sun's energy radiated back to space
• albedo of fresh snow 80%; grass 25%, oceans 10%; thick clouds 60-90%; thin clouds 30-50%
• due to high heat capacity of water (Lecture 2), oceans absorb vast amounts of solar heat



• Earth's Energy Budget

• Earth's surface receives ~5000 times more energy from Sun than from within
• of the Sun's energy received
  • 30% gets reflected back to space
  • 51% gets absorbed at Earth's surface
  • 19% gets absorbed by atmosphere
• Sun's energy comes from electromagnetic waves (light) of different wavelengths ranging from ultra-violet (UV) over visible to infra-red (IR)
• different Earth materials are transparent to some wavelengths but absorb and re-emit others (e.g. atmosphere absorbs a lot of IR but is transparent to visible light)
• Earth's surface warms when it receives/absorbs Sun light; it emits more IR radiation (heat) than it receives from the Sun

Depending on its phase (liquid or gaseous form) H₂O in the atmosphere has dual functions.
• in liquid form (as a product of condensation in clouds) it reflects incoming sunlight back to space. Earth's surface below the clouds is thereby cooler than it would be without clouds.
• in gaseous form (water vapor), the molecules absorb and release IR sunlight (see also greenhouse effect below). Earth's atmosphere and thereby the air near Earth's surface warm.
Both need to be taken into account in climate research that deals with global warming.


• Earth's Tilted Rotation Axis and Seasons

• Earth rotates (spins) about its axis once a day
• Earth's rotation axis (spin axis) is tilted with respect to its orbital plane around the Sun by 23.5 deg
• seasons are caused by the fact that Earth orbits around Sun (once a year), thereby changing the relative position of the tilted spin axis with respect to the Sun; this changes the local rate of insolation throughout the year


• 4 key positions:
• 1: northern summer, southern winter
• rotation axis is tilted toward Sun (North is up)
• area of highest insolation, where Sun at noon is directly above (zenith), is north of the equator
• on the northern summer solstice, the Sun is in the zenith (overhead) at the Tropic of Cancer (23.5 deg N); longest day/shortest night in the northern hemisphere; usually 20 or 21 June
• equator receives less insolation
• South Pole receives no insolation
• North Pole receives little insolation but has daylight for 24h a day
• 2: northern autumn, southern spring
• relative tilt of Earth's rotation axis to Earth-Sun line in the orbital plane is zero (because tilted axis is moving sideways)
• for one day (equinox), nights and days are equally long (12h), everywhere on the planet
• on the equinox, the Sun at noon is in the zenith on the equator; usually 22 or 23 September
• 3: northern winter, southern summer
• rotation axis is tilted away from Sun (North is up)
• the Sun at noon is in the zenith south of the equator
• on the northern winter solstice, the Sun is in the zenith at the Tropic of Capricorn (23.5 deg S); shortest day/longest night in the northern hemisphere; usually 21 or 22 December
• equator receives less insolation
• North Pole receives no insolation
• South Pole receives little insolation but has daylight for 24h a day
• 4: northern spring, southern autumn
• relative tilt of Earth's rotation axis to Earth-Sun line in the orbital plane is zero (because tilted axis is moving sideways), again, but Earth is now on the other side along its orbit around the Sun
• for one day (equinox), nights and days are equally long (12h), everywhere on the planet
• on the equinox, the Sun at noon is in the zenith on the equator; usually 20 March


• Earth's orbit is slightly elliptical and Earth is a little closer to the Sun during the southern summer (Earth in perihelion) than during the northern summer (Earth in aphelion)



• Earth's Climate Zones

If insolation were the only factor that controls climate, then we would have only 3 zones, depending on latitude:
• the frigid zone near the poles
• the torrid zone near the equator
• the temperate zone in between

The actual climate zones are much more complicated and depend on several geographical factors
• latitude: amount of insolation controls temperature
• altitude: T decreases with elevation; e.g. Mt. Kilimanjaro is near the equator but has an ice cap near its summit at 6000m (though the ice cap has decreased by >80% since about 1935)
• proximity to water: water has great heat capacity, warming and cooling more slowly than land (i.e. interior of continents has more extreme and drier weather than coastal areas)
• proximity to ocean currents: currents can cool (NE Pacific, California Current) or warm (NW Atlantic and Europe by Gulf Stream)
• proximity to orographic barriers: landforms such as mountain ranges (e.g. Sierras keep moist air from getting into Nevada, i.e. lee side)
• proximity to high- or low-pressure zones: high-pressure zones typically drier

Examples of climate zones:
• polar
• sub polar
• tundra
• temperate
• subtropical
• continental
• tropical
• dry land (desert, steppe, savanna)

NB: A climate can be extreme (hot summers, cold winters) yet a more moderate climate may lead to more glaciation (the growth of glaciers) when summers are not warm enough to melt ice that is accumulated during winter. Also, the amount of precipitation in the cold period determines whether ice can grow or not. If this period is too cold to produce snow, glaciers will not advance (grow).


• The Role of the Oceans - A Climate Moderator

• oceans: store more heat than atmosphere (due to higher heat capacity)
To understand just how much heat the oceans are capable of absorbing without raising its temperature, it is educational to compare its heat capacity to that of other materials:

Material	Heat Capacity in cal/cm3/deg C
Air	0.0003
Quartz Sand	0.31
Granite	0.52
Aluminum at 20°C	0.58
Copper at 20°C	0.82
Glass at 20°C	0.5
Human Body	0.8
Water	1.0

• serve as buffers; heat and cool slower than atmosphere and land: coastal areas experience cooler summers/milder winters than inland
• movement of ocean currents is slower than atmospheric winds, i.e. atmospheric changes have short-term effects, while ocean currents have longer-term effects
• much more than atmospheric winds, ocean currents get diverted by continents; more complicated circulation patterns
• oceans absorb heat AND atmospheric CO₂, so has slowed recent global warming
• conventional wisdom: and often used model to explain oceanic heat transport between the equator and the poles is the (oceanic heat conveyor). In this model, deep ocean currents are most important in the global heat exchange and are driven by density variations in the water (thermo-haline circulation)
• colder and saltier water sinks
• warmer and fresher water floats
however: scientists are still struggling to observe and/or measure deep-ocean currents. Recent research by atmospheric scientists indicates that:
• the heat transfer from the equator to the poles in the atmosphere is larger than in the oceans
• thermo-haline circulation is only relevant at shallow ocean depths
• -> popular but an urban myth: Gulf Stream causes mild climate in Europe
Some oceanographers contest this idea and this is a subject of ongoing research!



• Composition of Earth's Atmosphere

The components of dry air:
• 78% N₂
• 21% O₂
• 1% trace gases:
  • 0.93% Ar
  • CO₂ (280 ppm pre-industrial; now 413 ppm as of 2019) (greenhouse gas)
  • Ne (18 ppm)
  • He (5 ppm)
  • CH₄ (greenhouse gas) (2 ppm)
  • Kr (1 ppm)
  • also: O₃ (ozone) (greenhouse gas), Rn (from natural decay of U)
• aerosols: (diameter < 1µm) water, acid, sea salt, volcanic ash, clay, soot, pollen
• atmospheric pollutants:
  primarily through burning of fossil fuels
  • sulfates (SO₄^-2; also from volcanoes; causes acid rain)
  • nitrates (NOx; interaction with sunlight produces ozone; causes acid rain)
  • CO (an odorless toxin from combustion engines)
  
  
  • CFCs (destroys stratospheric ozone layer; from leaking cooling systems; not naturally occurring)
  NB: ppm is parts per million.



• Some Relevant Elements and Compounds

• H: Hydrogen, most basic element
• He: Helium, second most simple element
• C: Carbon
• O: Oxygen
• S: Sulfur
• N: Nitrogen
• Ar: Argon (noble gas)
• Ne: Neon (noble gas)
• Kr: Krypton (noble gas)
• Rn: Radon, decay element of U
• U: Uranium (last element in periodic table that is naturally occurring)
• CO: Carbon monoxide, toxin
• O₃: Ozone; greenhouse gas; pollutant in lower atmosphere; protects against UV radiation in higher atmosphere; naturally occurring but also anthropogenic through combustion engines
• CO₂: Carbon dioxide; greenhouse gas; emitted during volcanic eruptions; but also through fossil fuel burning
• CH₄: Methane; greenhouse gas; emitted during decay of organic matter and some geologic processes; but also through agriculture
• N₂O: greenhouse gas; released by bacteria growing in soil; but also through agriculture, combustion engines
• CFC: chloro-fluoro-carbons; greenhouse gas; destructor of stratospheric ozone; exclusively man-made/not naturally occurring

• Greenhouse Gases and The Greenhouse Effect

• atmosphere contains greenhouse gases that are transparent to visible light but trap infrared radiation
• atmosphere acts like glass of a greenhouse, hence keeping the planet warm
• greenhouse effect keeps Earth's surface at 16 deg C (61 deg F) on average
• without atmosphere, Earth would be 34 deg C colder (i.e. -18 deg C)
• air contains 0-4% water vapor (H₂O)
• natural greenhouse gases: H₂O and CO₂ are Earth's most important naturally occurring greenhouse gases; then CH₄ as number 3; other greenhouse gases include O₃ and N₂O
• anthropogenic (human-released) greenhouse gases: the top two anthropogenic greenhouse gases are CO₂ through burning of fossil fuels and CH₄ through agriculture; other anthropogenic greenhouse gases include CFCs, O₃ and N₂O



• Pressure and Density as Function of Altitude

• air pressure: the push that the air exerts on its surroundings due to the weight of the overlying atmosphere.
• pressure and density are highest near sea level (1 atm, 14.7 PSI, 1035 mbar or g/cm²)
• pressure and density decrease exponentially with altitude (50% of air molecules are below 5.6km, 90% below 16km and 99.99997% below 100km).



• Temperature as Function of Altitude and Atmospheric Layers

• unlike pressure and density, temperature does not decrease continuously with altitude.
• variations in T approximately aligned with 4 atmospheric layers



• Atmospheric Layers

• troposphere: surface to ~ 10 km; T decreases from 18^oC to -55^oC; the "weather layer"; thicker in the tropics than near the poles; also seasonal variations;
• stratosphere: ~ 10km to 47km; very dry; T constant for about 10km, then rises to 0^oC at 47km; ozone layer that absorbs UV radiation; stably, stratified; no mixing with troposphere
• mesosphere: 47km to 82km; T decreases to -85^oC at 82km; less O₃; does not absorb much solar energy and thus is cooling with increasing elevation; meteors (shooting stars) occur in this layer by burning of meteorites;
• thermosphere: 82km to ~ 120km; contains very little gas (< 1%); T increases with elevation but contains little heat; ionizes O₂ by absorbing high-energy solar radiation



• ionosphere: (50-400km); layer related to Earth's magnetic field; short-wavelength (high-energy) solar radiation ionizes air by stripping N and O atoms of their electrons; acts like a mirror for radio waves sent from Earth, hence enabling radio transmission over great distances; hosts auroras (borealis and australis); charged particles ejected from the sun (especially from solar flares) interact with ions in the ionosphere making them release energy; this happens at high latitudes where particles get trapped by magnetic field;



• Moist and Dry Air; Latent Heat

• absolute humidity: water content in air; a variety of measures how to describe the content of water vapor in air; one is the content by volume, which is between 0.3% (hot desert) and 4% (rain forest)


• relative humidity: ratio between measured water content and the maximum possible amount of water that air can hold at its current temperature (saturated air)
• Warmer air can hold more water vapor than colder air. With no change in water vapor content and pressure, an increase in air temperature lowers the relative humidity, while a decrease in air temperature raises the relative humidity.


• saturated air: air is saturated when it can no longer absorb water vapor; relative humidity is 100%; if air cools, condensation starts to occur


• dew point: temperature at which point the air gets saturated as it cools; if air cools further, fog or clouds form
• In a broad sense, the dew point gives an idea about the minimum temperature air can achieve under current conditions (i.e. no change in pressure and moisture content). For example, the temperature is likely not going much below the dew point because condensation starts to occur, which releases heat. This is important to farmers who may need to protect their frost-sensitive crop (e.g. citrus)
• The difference between air temperature and dew point also gives an idea about the current relative humidity of the air. If the difference is great, then the air is relatively dry. If the difference is small, then the relative humidity is high.
• The dew point is also a good indicator of the air's actual water vapor content. High dew points indicate high water vapor content, while low dew points indicate low water vapor content. For example, compare polar air, with an air temperature of -2 deg C and a dew point of -2 deg C to desert air, with an air temperature of 35 deg C and a dew point of 10 deg C. The relative humidity of the polar air is 100%, while that of desert air only 21%. Nevertheless, desert air holds more water vapor (not to be confused with moist, tropical air, which holds even more moisture).


• frost: when dew point T lies below freezing


• clouds: as rising, moist air cools adiabatically, moisture condenses to form clouds. Once condensation occurs, energy is released and changes to the air are no longer adiabatic (a change with no release or uptake of energy; see Lecture 16).


• latent heat (also see Lecture 2): When a medium undergoes a phase change (e.g. H₂O goes from liquid to gaseous) heat is either released or absorbed. Evaporating water absorbs heat. Condensating water vapor releases heat. This heat is hidden and is called the latent heat of condensation. The heat released during condensation drives the movement of air (e.g. winds). When great amount of latent heat is available (e.g. tropical air can hold large amounts of water vapor), then the release of it through condensation can fuel great storms (e.g. hurricanes).

1. Sun's output changes throughout the year
2. Earth-Sun distance changes throughout the year
3. Earth's tilted rotation axis changes local insolation throughout the year
4. Moon shadow changes insolation

1. high electrical conductivity of sea water
2. high heat capacity of water
3. high Ekman transport
4. high refraction index

1. true
2. false

1. true
2. false

1. latent heat is absorbed
2. latent heat is release
3. latent heat remains constant