Climates are described by agreed-upon codes or by descriptive terms that are somewhat loosely defined but nevertheless useful. On a global scale, climate can be spoken of in terms of zones, or belts, that can be traced between the equator and the pole in each hemisphere. To understand them, the circulation of the upper atmosphere, or stratosphere, must be considered, as well as that of the lower atmosphere, or troposphere, where weather takes place. Upper atmospheric phenomena were little understood until the advent of such advanced technology as rocketry, high-altitude aircraft, and satellites.

Ideally, hot air can be thought of as rising by convection along the equator and sinking near the poles. Thus, the equatorial belt tends to be a region of low pressure and calms, interrupted by thunderstorms associated with towering cumulus clouds. Because of the calms, this belt is known as the doldrums. It shifts somewhat north of the equator in the northern summer and south in the southern summer. By contrast, air sinks in the polar regions. This leads to high atmospheric pressure, and dry, icy winds that tend to radiate outward from the poles.

Complicating this simplistic picture is the earth's rotation, which deflects the northerly and southerly components of the atmosphere's circulation. Thus, the tropical and polar winds both tend to be easterlies (winds from the east), and two intermediate belts develop in each hemisphere. Around latitude 30° North and South is a zone of high pressure, where the upper air sinks and divides, sending air streams toward the equator. Steady northeast trade winds blow in the northern hemisphere, and southeast trade winds in the southern hemisphere. These high-pressure areas lead to arid areas on the continents but to moist air over the oceans, because of evaporation. If these trade winds meet an island or mainland coast, moist air is pushed up into cooler elevations, and heavy rainfall might occur.

Around latitude 50° to 60° North and South is a belt of low pressure characterized by the prevailing westerlies, which are deflected to the southwest in the northern hemisphere and to the northwest in the southern hemisphere. These are relatively mild, moist winds that tend to bring frequent cyclonic precipitation to all elevations along the west-facing side of continents. The precipitation is characterized by polar fronts, where cold air from the polar easterlies drives in under the warm, moist air of the westerlies, which, on cooling, drop their moisture. In winter this is the cause of most snowfall on continents.