Weather is the state of the atmosphere at a particular place and time, or over a relatively short period. It includes measurable conditions such as air temperature, atmospheric pressure, humidity, wind speed and direction, cloud cover, precipitation and visibility. It can also include phenomena such as thunderstorms, fog, hail, snow, frost, heatwaves, cold spells and strong winds. Weather is fundamentally concerned with short term atmospheric conditions and how they change.
The word has Old English and Germanic roots associated with atmospheric conditions and wind, and its everyday meaning developed around the observable state of the air. Meteorology is the scientific discipline that studies weather and atmospheric processes responsible for it. Weather is not a single variable but the combined result of many interacting physical processes involving radiation, pressure gradients, moisture, convection, atmospheric circulation, land surfaces, oceans and the topography of Earth.
Weather develops because Earths atmosphere is continuously receiving energy from the Sun while exchanging energy and moisture with the land and oceans. Solar radiation heats the surface unevenly because of differences in latitude, season, surface properties, cloud cover and time of day. The warmed surface transfers energy to the atmosphere through conduction, convection, turbulence and infrared radiation. Water evaporates from oceans, lakes, soils and vegetation, adding moisture to the atmosphere.
When moist air rises and cools, water vapour can condense into clouds if suitable conditions exist. Condensation releases latent heat, which can further influence atmospheric motion. Pressure differences cause air to accelerate from regions of relatively high pressure towards regions of relatively low pressure, although Earths rotation modifies this movement through the Coriolis effect. Weather therefore emerges from the interaction of energy, mass, moisture and motion within the atmosphere.
Temperature is one of the most basic components of weather, but temperature alone does not define weather. Air pressure describes the force exerted by the weight of the atmosphere and varies across locations and times. Pressure differences help generate wind. Humidity describes the amount of water vapour in the air, with relative humidity expressing the amount present compared with the maximum amount possible at a particular temperature.
Clouds form when air becomes sufficiently saturated and condensation occurs around suitable microscopic particles called cloud condensation nuclei. Precipitation occurs when water or ice particles in clouds grow sufficiently large to fall under gravity. Different atmospheric conditions produce rain, snow, sleet, freezing rain or hail. Wind, temperature, humidity, pressure, clouds and precipitation interact continuously, so a change in one variable can influence several others.
Weather systems can occur across many spatial scales. Small scale phenomena include turbulent gusts, dust devils and individual convective cells. Mesoscale systems include thunderstorms, sea breezes and organised convective systems extending over tens to hundreds of kilometres. Synoptic scale systems include extratropical cyclones, anticyclones and large frontal structures covering much larger regions. Tropical cyclones are large rotating storm systems that develop over sufficiently warm tropical or subtropical oceans under suitable atmospheric conditions.
Fronts form where air masses with different temperature and moisture characteristics meet, and they can be associated with clouds, precipitation and changing winds. Jet streams, large scale pressure patterns and ocean conditions influence the movement and development of these systems. Weather at a particular location is therefore often connected to atmospheric events occurring hundreds or thousands of kilometres away.
Weather forecasting involves observing the atmosphere and using physical principles and numerical models to estimate how conditions will evolve. Observations come from weather stations, radiosondes, aircraft, ships, buoys, radar, satellites and other instruments. Numerical weather prediction models divide the atmosphere into a three dimensional computational grid and solve mathematical equations representing fluid motion, thermodynamics, radiation, moisture processes and other phenomena. Because the atmosphere is chaotic and observations are incomplete, forecasts become less certain as the forecast horizon increases.
Modern forecasting therefore uses ensembles, in which many model simulations are run with slightly different initial conditions or model configurations. Comparing these simulations provides information about forecast uncertainty. Forecasts can be highly useful several days ahead for large scale patterns, while precise prediction of individual thunderstorms or local rainfall can remain difficult even at short ranges.
Weather forecasting has a long history. Early weather prediction relied heavily on visual observations, traditional knowledge and recognition of recurring atmospheric patterns. The development of barometers, thermometers and systematic weather observations made quantitative meteorology possible. Telegraph networks allowed observations from distant locations to be shared rapidly, enabling the creation of weather maps and the identification of moving pressure systems. Radiosondes later provided measurements through the vertical atmosphere.
Radar transformed precipitation monitoring, while satellites provided global observations over oceans and remote regions. Computers made numerical weather prediction practical during the twentieth century, and modern supercomputers now process enormous datasets from observations and models. The development of these technologies transformed weather forecasting from primarily descriptive practice into a highly quantitative science based on observations, mathematical models and probability.
Weather has major practical consequences for society because atmospheric conditions affect agriculture, transport, energy demand, construction, public health, water management and emergency planning. Heavy rainfall can cause flooding, strong winds can damage infrastructure, extreme heat can increase health risks, and snow or ice can disrupt transport. Farmers use weather forecasts to plan irrigation, planting, harvesting and protection against frost or storms. Electricity networks use forecasts to anticipate demand and the availability of weather dependent generation.
Aviation depends heavily on information about wind, visibility, turbulence, icing, thunderstorms and volcanic ash. Weather warnings communicate the potential for hazardous conditions and support protective action. These applications require not only accurate observations but also communication of uncertainty, because a forecast represents an estimate of future atmospheric conditions rather than a guarantee.
Weather is distinct from climate even though the two are closely connected. Weather describes short term atmospheric conditions and individual events, while climate describes statistical patterns and distributions of weather over much longer periods. A cold day, a thunderstorm or a week of heavy rainfall is a weather event. The frequency of such events, their typical intensity and the long term distribution of temperature and precipitation are climate characteristics.
Weather can vary substantially from one day to another because the atmosphere is dynamic and chaotic. Climate statistics smooth over much of this short term variability and reveal longer term patterns. Climate also influences the background conditions in which weather occurs, meaning that the two concepts are not independent. A single weather event cannot by itself define a climate trend, while climate analysis requires many observations of weather collected over sufficiently long periods.
