On a windless day in January, the Valley of Mexico wakes up under a gray haze, while 11 km up, a plane flying from Cancún to Tijuana moves along without a single jolt. It is the same atmosphere: its layered structure and its dynamics explain both situations with the same ideas. At the end of the page we will return to this contrast.
Composition and Layers of the Atmosphere
People who have climbed Pico de Orizaba know that it is very cold at the top and that breathing takes effort. How do the composition, pressure, and temperature of the air change as we go up toward space?
That 1% seems small, but the greenhouse effect ($\mathrm{CO_2}$, water vapor), clouds and rain (water vapor), and protection against ultraviolet radiation (ozone) all depend on it. In addition, $\mathrm{CO_2}$ is the gas that goes into photosynthesis and comes out of respiration: it is the link between the atmosphere and the carbon cycle.
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Name the layers of the atmosphere in order from Earth's surface outwards into space and state a characteristic feature of each.
Name the layers of the atmosphere in order from Earth's surface outwards into space and state a characteristic feature of each.
State the main composition of air (the two dominant gases with their approximate proportions) and name two trace gases whose variation is of major environmental…
State the main composition of air (the two dominant gases with their approximate proportions) and name two trace gases whose variation is of major environmental importance.
Why does it cool? Air near the ground is heated by the surface and is less dense, so it tends to rise. As it rises it meets less pressure, expands, and cools. As long as the air below is warmer than the air above, vertical mixing is easy and clouds form.
On clear winter nights, the ground cools quickly and cools the air in contact with it, while the air higher up stays milder. A "lid" forms: exhaust and industrial gases cannot rise or disperse and build up near the ground. In a basin surrounded by mountains, like the Valley of Mexico, the effect is even greater.
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Explain what the lapse rate (vertical thermal gradient) of the troposphere is and what a thermal inversion is, relating the latter to episodes of trapped pollution over…
Explain what the lapse rate (vertical thermal gradient) of the troposphere is and what a thermal inversion is, relating the latter to episodes of trapped pollution over major cities in winter.
Based on the thermal profile recorded by a probe in the Earth's atmosphere (zones 1 to 4): a) Justify, using the concepts of air density and lapse rate, why zone 2 acts…
Based on the thermal profile recorded by a probe in the Earth's atmosphere (zones 1 to 4):
a) Justify, using the concepts of air density and lapse rate, why zone 2 acts as a dynamic lid that prevents the vertical development of storm clouds from zone 1.
b) Predict how the morphology of the thermal curve in zone 2 would change if the ozone layer were completely destroyed, and what direct consequence that change would have on the altitude of the upper boundary of zone 1.
c) Commercial flights frequently choose the transition level between zones 1 and 2, whereas auroras are only observed in zone 4. Justify the commercial flight altitude based on atmospheric stability and fuel consumption, and explain why auroras cannot occur at lower altitudes.
On a world map, the great deserts (Sonora, Chihuahuan, Sahara) line up near $30^\circ$ latitude, and the rainiest rainforests are concentrated at the equator. What moves air on a planetary scale, and why does it distribute rain so unevenly?
Now apply it to Mexico. The country is crossed by the Tropic of Cancer (near $23.5^\circ$ N), so the north lies under the influence of the subtropical anticyclones, while the south and southeast are closer to the ITCZ and to the humid winds from the Gulf and the Caribbean. That is why the north has very dry areas, like the Sonoran Desert, and the southeast, like the Lacandon Jungle, receives abundant rain.
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Outline in words the general circulation of the atmosphere (Hadley cell, Intertropical Convergence Zone, subtropical anticyclones) and use it to explain the climatic…
Outline in words the general circulation of the atmosphere (Hadley cell, Intertropical Convergence Zone, subtropical anticyclones) and use it to explain the climatic contrast between the humid north and the arid south-east of the Iberian Peninsula.
The atmosphere does not just move: it also adjusts the energy that enters and leaves the planet. If more comes in than goes out, the Earth warms.
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Consider Earth's radiation budget in initial equilibrium (average incident $340\ \text{W/m}^2$ at the top of the atmosphere). a) A climate model evaluates two…
Consider Earth's radiation budget in initial equilibrium (average incident $340\ \text{W/m}^2$ at the top of the atmosphere).
a) A climate model evaluates two independent anthropogenic disturbances: 1) a reduction in mean albedo from $30\ \%$ to $28\ \%$ due to Arctic ice melt and urbanization; 2) an increase in greenhouse gases that decreases the outgoing longwave radiation emitted to outer space by $3.5\ \text{W/m}^2$. Calculate the net radiative forcing introduced by each disturbance and interpret which one would cause a greater initial thermal imbalance.
b) A climate change denier argues: "If Earth absorbs more energy than it emits, the surface will immediately emit more infrared radiation into space and equilibrium will be restored without an increase in mean temperature." Reasonably refute this claim using the Stefan-Boltzmann law and the absorption/re-emission mechanism of the greenhouse effect.
c) Explain how radiometers aboard satellites (such as the CERES mission) manage to measure Earth's energy imbalance by distinguishing reflected shortwave energy from emitted thermal longwave radiation.
Why do the same car exhausts produce a haze near the ground on a cold morning and, on a sunny afternoon, an irritating air?
The Ozone Layer
The ozone layer, in the stratosphere, absorbs most of the Sun's ultraviolet (UV) radiation, which damages the DNA of living things. Chlorofluorocarbons (CFCs), once used in aerosol sprays and refrigerators, rose to the stratosphere and destroyed ozone through chain reactions. The Montreal Protocol (1987) banned their production almost worldwide. It is considered the greatest success of environmental diplomacy: almost every country signed and the agreement worked. The layer is recovering, though slowly, because CFCs last for decades in the atmosphere.
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Describe the function of the stratospheric ozone layer, what substances destroyed it (CFCs), and why the Montreal Protocol is considered the greatest success of…
Describe the function of the stratospheric ozone layer, what substances destroyed it (CFCs), and why the Montreal Protocol is considered the greatest success of environmental diplomacy.
Mexico City suffers from winter episodes of high pollution caused by $\text{NO}2$ and particulates. Analyze which meteorological conditions favor them, what emergency…
Mexico City suffers from winter episodes of high pollution caused by $\text{NO}_2$ and particulates. Analyze which meteorological conditions favor them, what emergency measures are activated, and what structural solutions would reduce their frequency.
Back to the beginning: the plane flies near the tropopause, where the stable layer above the troposphere prevents vertical currents and jolts. The haze over the Valley of Mexico is trapped when a temperature inversion keeps the cold, polluted air at the surface from rising and dispersing.
Summary
Let's practice! 3 medium exercises in this section