Although often used interchangeably in casual speech, weather and climate are distinct. Weather refers to short-term atmospheric conditions in a particular place at a particular time, while climate is the long-term average of weather patterns over decades. The Köppen climate classification is the most widely used system for grouping world climates, based on temperature and precipitation. Among major biomes, the tundra is a cold, treeless region with permafrost, ground that stays frozen for two or more years in a row. The taiga, or boreal forest, consists of vast coniferous forests across northern Eurasia and North America. Savannas are grasslands with scattered trees, typical of much of Africa. The Amazon rainforest is the largest rainforest on Earth, and major biome categories also include deserts, temperate forests, and tropical rainforests.
Several atmospheric and oceanic processes shape weather and climate across continents. Monsoons, especially prominent in South Asia, are seasonal reversals of wind driven by the differential heating of land and sea. The rain shadow effect occurs when moist air rises over a mountain range, drops its moisture on the windward side, and leaves the leeward side dry. The Coriolis effect, caused by Earth's rotation, deflects moving air and water to the right in the Northern Hemisphere and to the left in the Southern, which is why hurricanes rotate counterclockwise north of the equator and clockwise south of it. The Gulf Stream, also called the North Atlantic Drift, is an ocean current that carries warm water across the Atlantic and helps keep Western Europe milder than other regions at the same latitude.
Winds and ocean cycles redistribute heat around the planet. Trade winds are steady, prevailing easterly winds that blow toward the equator in the tropics. The doldrums are the calm equatorial regions where these trade winds converge and winds are light and variable. El Niño is a periodic warming of the eastern Pacific Ocean that disrupts global weather patterns, often bringing droughts to some regions and floods to others, while La Niña is its cooling counterpart with generally opposite effects. Human activities also shape the climate system: desertification is the process by which fertile land becomes desert, often through drought or unsustainable land use, while deforestation is the clearing of forests for agriculture, logging, or development.