The Atmosphere, Global Circulation, Monsoons and El Nino

Layers of the atmosphere

Composition by volume of dry air: about $78\%$ nitrogen, $21\%$ oxygen, $0{,}93\%$ argon and roughly $0{,}04\%$ carbon dioxide. The layers are defined by whether temperature rises or falls with height:

• Troposphere (0–about 16 km) — temperature falls about $6{,}5^{\circ}\text{C}$ per km. Warm air below cold air is unstable, so this layer mixes vigorously: nearly all weather happens here.
• Stratosphere — temperature rises with height because the ozone layer absorbs ultraviolet radiation. Warm air on top of cold is stable, so there is little vertical mixing, which is why pollutants that reach it stay for years.
• Mesosphere — temperature falls again; the coldest part of the atmosphere.
• Thermosphere — temperature rises as scarce molecules absorb high-energy radiation.

Three cells, and why deserts sit at 30 degrees

Intense heating at the equator lifts moist air; it cools, rain falls, and the dry air aloft moves poleward and sinks near 30 degrees, warming and absorbing moisture. That descending limb of the Hadley cell is why most great deserts lie near $30^{\circ}$ N and S. The Ferrel and polar cells complete the pattern, with rising wet air near $60^{\circ}$ and sinking dry air at the poles. The Coriolis effect deflects moving air right in the Northern Hemisphere and left in the Southern, turning these flows into the trade winds and the westerlies.

Monsoons, El Nino and La Nina

A monsoon is a seasonal reversal of wind caused by land heating and cooling faster than ocean: summer winds blow from sea to land bringing rain, winter winds from land to sea bringing dry air. In a normal year, Pacific trade winds pile warm water in the west and allow cold, nutrient-rich upwelling off Peru. During El Nino the trade winds weaken or reverse, warm water spreads east, upwelling stops, fisheries collapse, Peru gets floods and Australia and Indonesia get drought. La Nina is the exaggerated normal state: stronger trade winds, stronger upwelling, wetter western Pacific.