APES 4.3-4.4: Earth's Atmosphere and Global Winds

Atmosphere of Earth

The atmosphere is a gravitationally retained mixture of gases surrounding Earth's surface. It also contains variable amounts of water vapor, aerosols, and particulates.

What are the major protective and life-supporting functions of Earth's atmosphere?

It absorbs much ultraviolet radiation, burns up or slows many meteoroids, moderates day–night temperature changes through heat retention, redistributes heat and moisture, and provides gases and conditions needed for respiration and photosynthesis.

What is the approximate dry-air composition of Earth's atmosphere by mole fraction?

Dry air is approximately $78.08\%$ $N_2$, $20.95\%$ $O_2$, $0.93\%$ $Ar$, and $0.04\%$ $CO_2$, with the remainder consisting of trace gases.

Why are atmospheric gas percentages commonly reported for dry air?

Water vapor varies substantially with location and temperature, so excluding it provides a more consistent basis for comparing the concentrations of other gases.

How does water vapor differ from the major permanent gases in Earth's atmosphere?

Water vapor is highly variable, ranging from roughly $10$ ppm in very cold air to as much as $5\%$ by mole fraction in hot, humid air. It averages about $0.25\%$ of the atmosphere by mass.

What are important trace gases in Earth's atmosphere?

Trace gases include $CO_2$, $CH_4$, $N_2O$, $O_3$, and noble gases such as neon, helium, krypton, and xenon. Their concentrations are small but some strongly affect climate or atmospheric chemistry.

What atmospheric substances can occur as aerosols or particulates?

Natural aerosols include mineral and organic dust, pollen, spores, sea spray, and volcanic ash. Human-produced pollutants may also occur as gaseous substances or aerosols.

What general trends occur in atmospheric pressure and density as altitude increases?

Both pressure and density generally decrease with altitude because less air lies above a given location and the gas is less compressed.

What are the five principal atmospheric layers in order from Earth's surface upward?

They are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

Troposphere

The troposphere extends from Earth's surface to roughly $9$–$17\ \mathrm{km}$, averaging about $12\ \mathrm{km}$. It contains about $80\%$ of atmospheric mass, nearly all atmospheric water vapor, and most weather.

Why does temperature usually decrease with altitude in the troposphere?

The troposphere is heated primarily from Earth's surface. As air rises and pressure decreases, it expands and cools, producing a general decrease in temperature with altitude.

Stratosphere

The stratosphere extends approximately from $12$ to $50\ \mathrm{km}$. Its temperature increases with altitude because ozone and molecular oxygen absorb solar ultraviolet radiation, and it is relatively stable with little weather-related turbulence.

How does the ozone layer affect the temperature profile of the stratosphere?

Ozone absorbs ultraviolet radiation from the Sun and converts that energy into thermal energy. This causes temperature to rise with altitude in much of the stratosphere.

Mesosphere

The mesosphere extends from about $50$ to $80$–$85\ \mathrm{km}$. Temperature decreases with altitude, making the mesopause the coldest major atmospheric region, with an average temperature near $190\ \mathrm{K}$.

What atmospheric phenomena are especially associated with the mesosphere?

Most meteors burn up there, and polar mesospheric noctilucent clouds can form from condensed water vapor. The mesosphere is also too high for ordinary aircraft and too low for stable orbital spacecraft.

Thermosphere

The thermosphere extends from roughly $80\ \mathrm{km}$ to a thermopause typically between $500$ and $1000\ \mathrm{km}$. Temperature rises with altitude because the sparse gas absorbs high-energy solar ultraviolet and X-ray radiation.

Why would a person not necessarily feel hot in the thermosphere despite temperatures that can exceed $1500\ ^\circ\mathrm{C}$?

Thermospheric particles may have high average kinetic energy, but the gas density is extremely low. Very few collisions occur, so little thermal energy would be transferred to a person by conduction.

What causes the thermosphere's temperature increase with altitude, whereas the mesosphere's temperature decreases with altitude?

The thermosphere absorbs high-energy ultraviolet and X-ray radiation, while the mesosphere lies above the ozone-heated stratosphere and has limited absorption of that energy. Thus their temperature gradients have opposite signs.

What is the atmospheric lapse rate, and why is it useful?

The lapse rate describes how temperature changes with altitude. Measured temperature profiles help identify atmospheric layers because each layer has a characteristic trend, such as cooling in the troposphere and mesosphere or warming in the stratosphere and thermosphere.

Why is the troposphere the main region for clouds and weather?

Nearly all atmospheric water vapor is concentrated in the troposphere, especially its lower portion. Surface heating and air circulation produce the vertical mixing and condensation processes that drive weather.

How have natural processes and human activities changed Earth's atmosphere over time?

Volcanism, outgassing, impacts, weathering, and the evolution of photosynthetic life altered the atmosphere over geologic time. Modern human activities, especially fossil-fuel use and deforestation, contribute to climate change, while other emissions contribute to ozone depletion and acid deposition.

What primarily drives global atmospheric circulation?

Unequal solar heating of Earth's surface drives global circulation. The equator receives more direct solar energy than the poles, creating temperature and pressure differences that move air and redistribute heat.

What are the three atmospheric convection cells in each hemisphere?

Each hemisphere contains a Hadley cell from approximately $0^\circ$ to $30^\circ$, a Ferrel cell from $30^\circ$ to $60^\circ$, and a Polar cell from $60^\circ$ to $90^\circ$ latitude.

How does a Hadley cell operate?

Strong solar heating causes warm, moist air to rise near the equator. The air moves poleward aloft, cools, and descends near $30^\circ$ latitude, then returns toward the equator near the surface.

How does a Ferrel cell operate?

The Ferrel cell lies between about $30^\circ$ and $60^\circ$ latitude. It is an indirect circulation cell driven by interactions between the Hadley and Polar cells, with surface air generally moving poleward.

How does a Polar cell operate?

Cold, dense air descends near the poles and moves toward lower latitudes at the surface. Near approximately $60^\circ$ latitude, it is forced upward where it meets warmer air.

What are the major global pressure belts associated with the three-cell circulation model?

They include equatorial low pressure near $0^\circ$, subtropical high pressure near $30^\circ$, subpolar low pressure near $60^\circ$, and polar high pressure near $90^\circ$ latitude.

What is the Intertropical Convergence Zone (ITCZ)?

The ITCZ is the region near the equator where the trade winds from the Northern and Southern Hemispheres converge. Warm, moist air rises there, producing low pressure, clouds, and frequent precipitation.

What are the prevailing surface wind belts produced by global atmospheric circulation?

They are the trade winds from approximately $0^\circ$ to $30^\circ$, the westerlies from $30^\circ$ to $60^\circ$, and the polar easterlies from $60^\circ$ to $90^\circ$ latitude.

What is the Coriolis effect?

The Coriolis effect is the apparent deflection of moving air caused by Earth's rotation. It causes motion to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

How does the strength of the Coriolis effect vary with latitude?

The Coriolis effect is weakest at the equator, where it is effectively zero, and becomes stronger toward the poles. It changes the direction of winds but does not initially cause the air to move.

How does the Coriolis effect produce the trade winds?

Air moving toward the equator from the subtropical high-pressure belts is deflected westward by the Coriolis effect. This creates northeast trade winds in the Northern Hemisphere and southeast trade winds in the Southern Hemisphere.

Why do the prevailing westerlies generally move from west to east?

Surface air moving poleward from the subtropical high-pressure belts is deflected by the Coriolis effect, producing winds that generally travel from the west toward the east in the mid-latitudes.

Why do the polar easterlies generally move from east to west?

Cold air flowing away from the polar high-pressure regions toward lower latitudes is deflected westward by the Coriolis effect, producing prevailing easterly winds near the poles.