Wind is generated by differences in air pressure, which arise from uneven heating of Earth's surface. Air naturally flows from high-pressure regions to low-pressure regions, setting the atmosphere in motion. As air moves across the rotating Earth, it is deflected by the Coriolis effect — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection organizes winds into three main global belts: the trade winds between 0° and 30° latitude, the westerlies between 30° and 60°, and the polar easterlies between 60° and 90°. Higher in the atmosphere, the jet stream, a narrow band of fast-moving air in the upper troposphere, steers weather systems and can bring unusual cold or warm spells when its path shifts.
Local weather is strongly shaped by air masses and fronts. An air mass is a large body of air with relatively uniform temperature and humidity, having acquired its characteristics from the region where it formed. When two air masses meet, they form a front. In a cold front, a cold air mass pushes underneath a warm one, causing rapid uplift that often brings thunderstorms, heavy rain, and a sharp temperature drop. Other phenomena include atmospheric rivers — long, narrow corridors of concentrated moisture that can deliver heavy precipitation when they reach land — and atmospheric inversion layers, in which warm air sits above cooler air and prevents vertical mixing, trapping pollutants near the ground.
Several related concepts help interpret everyday weather. The dew point is the temperature at which air becomes saturated and water vapor condenses into liquid. The heat index combines air temperature with relative humidity to express how hot it actually feels. In cities, the urban heat island effect makes urban areas warmer than surrounding countryside because of concrete, asphalt, reduced vegetation, and waste heat from human activities. Forecasting predicts specific atmospheric conditions days ahead, whereas climate projection models long-term trends over decades using emission scenarios — another illustration of the weather-versus-climate distinction.