Weather is the state of the atmosphere at a particular time and place. It includes temperature, humidity, precipitation, wind, and cloud cover. Weather changes hour by hour. A sunny morning can turn into a stormy afternoon. Forecasts try to predict these changes, with varying success.

Weather is driven by the Sun. Uneven heating of the Earth's surface creates pressure differences, which set air in motion. Water evaporates, forms clouds, and falls as rain or snow. Fronts mark the boundaries between air masses. Jet streams steer storms across continents. The system is chaotic, which limits how far ahead forecasts can be reliable. Beyond about ten days, predictions become probabilistic.

Weather affects everything. Farmers plant and harvest around it. Airlines route planes around storms. Cities prepare for heat waves and blizzards. Extreme weather kills thousands each year. Climate change is altering weather patterns, making some events more intense. The distinction between weather and climate matters. A cold snap does not disprove warming. The long-term trend is what counts.

Key elements

• Temperature
• Humidity
• Precipitation
• Wind
• Cloud cover and pressure