Chapter summary

Geographic variation in the annual radiative balance at the top of the atmosphere drives the general circulation of the atmosphere and is a primary determinant of Earth's macroclimate. The latitudinal gradient in net radiation results in an equator-to-pole temperature gradient. However, if only radiative processes determined temperatures, the tropics would be tens of degrees warmer than they actually are, and polar regions would be much colder than they actually are. Instead, the uneven geographic distribution of radiation produces winds, set in motion by differences in air pressure, that reduce the poleward temperature gradient by carrying heat from the tropics to the poles. Winds are a balance of the pressure gradient force, Coriolis force, and friction acting simultaneously. These forces produce the general circulation of the atmosphere, which redistributes heat from the tropics to the poles and produces the major patterns of climate on Earth. The continents alter this idealized circulation because landmasses heat and cool faster than oceans. In winter, when the landmasses of the Northern Hemisphere are colder than oceans, high pressure systems form over land while low pressure systems are most pronounced over oceans. The opposite pattern occurs in summer when continents are warmer than oceans. Oceans also influence climate by transporting heat from the tropics to polar regions. Prominent features of this are wind-driven surface currents and the density-driven thermohaline circulation. The general circulation of the atmosphere varies over the course of a year in response to seasonal changes in solar radiation.