Weather
Weather is the state of the atmosphere at a particular place and time, expressed through measurable quantities such as temperature, atmospheric pressure, moisture content, wind, and precipitation. It results from the redistribution of energy and water within the atmosphere and between the atmosphere, land, and oceans. Weather differs from climate, which describes the statistical distribution of atmospheric conditions over periods sufficiently long to reveal persistent patterns.
Most weather occurs in the troposphere, the lowest major layer of Earth’s atmosphere. The troposphere contains most atmospheric water vapor and is heated primarily from below by energy absorbed at the surface. Uneven heating produces pressure gradients and density contrasts, while Earth’s rotation modifies the resulting motion. The interaction of these processes generates atmospheric circulation across scales ranging from local turbulence to planetary wave systems.
Although individual weather systems are temporary, they are not independent events. Their formation depends on the preceding state of the atmosphere, the thermal properties of the underlying surface, and exchanges with adjacent regions. Consequently, weather has both local expression and global dynamical continuity. A rainstorm over one region may form within an air mass transported across an ocean, while its later development may influence circulation thousands of kilometres downstream.
Atmospheric energy
The primary external energy source for weather is solar radiation. Because Earth is approximately spherical, the mean intensity of incoming sunlight is greater near the equator than near the poles. Seasonal changes arise from the inclination of Earth’s rotational axis relative to its orbit, which alters the duration and angle of illumination received by each hemisphere.
The surface absorbs part of the incoming radiation and emits energy at infrared wavelengths. Greenhouse gases, clouds, and atmospheric particles absorb and re-emit portions of this outgoing radiation. The resulting energy balance determines the broad thermal structure of the atmosphere, although local temperatures also depend on surface composition, elevation, cloud cover, and atmospheric transport.
Water strongly influences atmospheric energy through latent heat. Evaporation transfers water from a surface into the atmosphere while storing energy in the resulting vapor. Condensation releases that energy and can intensify ascending motion within clouds. This exchange supports the development of convective storms and contributes to the energy supply of tropical cyclones.
Temperature differences create variations in air density and pressure. Air accelerates in response to the pressure-gradient force, but its path is modified by friction and by the apparent Coriolis force associated with Earth’s rotation. Above the frictionally influenced surface layer, large-scale winds frequently approach a balance between the pressure-gradient and Coriolis forces. This condition is known as geostrophic balance.
Moisture, clouds, and precipitation
Atmospheric moisture is commonly described by quantities including relative humidity, specific humidity, and dew point. Relative humidity compares the existing vapor pressure with the saturation vapor pressure at the same temperature. Because saturation vapor pressure increases rapidly as temperature rises, relative humidity can change even when the actual amount of water vapor remains constant.
Clouds form when air becomes sufficiently saturated for water vapor to condense onto microscopic cloud condensation nuclei. Cooling commonly occurs when air rises and expands under lower pressure. Ascent may be produced by surface heating, by movement over topography, or by convergence within a larger circulation system. It may also occur when denser air advances beneath warmer air along a weather front.
Cloud droplets are generally too small to fall immediately as precipitation. In warmer clouds, differences in droplet size allow collisions and coalescence to produce larger drops. In colder clouds, ice crystals can grow as water vapor is transferred from nearby supercooled droplets. When the resulting particles become large enough, they descend as precipitation, whose state at the surface depends on the temperature and moisture structure of the air below the cloud.
Cloud classification reflects observable form and altitude while also indicating the atmospheric processes responsible for development. Layered clouds are commonly associated with broad, gradual ascent. Vertically extensive clouds indicate stronger instability and concentrated upward motion. A cloud name therefore describes appearance without functioning as a complete explanation of origin or future behavior.
Air masses and synoptic systems
An air mass is a large volume of air with comparatively uniform thermal and moisture characteristics. Air masses acquire these properties through prolonged contact with broad source regions. Their subsequent movement transports heat and water vapor, and their boundaries often become zones of organized weather.
In the middle latitudes, major changes in weather are frequently associated with extratropical cyclones. These are rotating low-pressure systems that develop where horizontal temperature contrasts are substantial. Their growth is linked to disturbances in the upper-level flow and to conversions between potential and kinetic energy. Fronts organize cloud development and precipitation around the cyclone, while sinking air commonly produces clearer conditions within the neighboring high-pressure region.
The upper troposphere contains narrow currents of strong wind known as jet streams. Their position and curvature influence the movement and intensification of surface weather systems. Large meanders in the mid-latitude circulation can slow the progression of these systems, allowing persistent heat, cold, rainfall, or dryness to affect a region for an extended period.
Tropical cyclones develop over sufficiently warm ocean water when the surrounding atmosphere supports persistent deep convection and organized rotation. Their central pressure may fall substantially as condensation releases latent heat and rising air is removed efficiently at high altitude. Unlike extratropical cyclones, mature tropical cyclones derive most of their energy from thermodynamic exchange with the ocean rather than from strong horizontal temperature boundaries.
Local and severe weather
Local weather emerges from the interaction between larger circulation patterns and the physical properties of a particular landscape. Coastal regions often experience a daily circulation because land changes temperature more rapidly than adjacent water. During daylight, warmer air over land tends to rise, allowing cooler marine air to move inland as a sea breeze. At night, the thermal contrast may weaken or reverse.
Mountains alter airflow by forcing air to ascend, redirecting it through valleys, and generating atmospheric waves. Windward ascent can enhance cloud formation and precipitation, while descending air on the leeward side warms through compression. This process contributes to rain shadows and to warm downslope winds.
An urban heat island develops where the built environment stores heat, modifies airflow, and limits cooling through evaporation. The effect is usually most pronounced at night under relatively clear and calm conditions. Urban aerosols can also alter cloud microphysics, although the resulting influence on precipitation varies with the surrounding atmospheric state.
Thunderstorms form when moist air rises through an unstable atmosphere and reaches a level at which continued ascent becomes buoyant. Their organization depends strongly on vertical wind shear, which can separate rising and descending currents. Long-lived rotating storms known as supercells can produce large hail, destructive winds, and tornadoes, although each hazard arises through a distinct combination of storm dynamics and environmental structure.
Observation and measurement
Systematic weather analysis depends on measurements that are comparable across both time and location. Surface observing stations record atmospheric conditions at standardized intervals, while radiosondes carried by balloons measure the vertical structure of the atmosphere. Satellites provide broad spatial coverage by detecting reflected solar radiation and emitted infrared or microwave energy. Weather radar measures energy scattered by precipitation particles and can infer their motion from the Doppler shift of the returned signal.
Instrumental meteorology developed gradually. Evangelista Torricelli’s seventeenth-century work on the mercury barometer established a practical method for measuring atmospheric pressure. Daniel Gabriel Fahrenheit and Anders Celsius produced temperature scales that supported more consistent thermal observations. Luke Howard’s cloud classification provided a shared descriptive framework for cloud forms, while Francis Beaufort’s wind scale linked observed effects to standardized estimates of wind strength.
By the nineteenth century, telegraph networks allowed observations from separated stations to be assembled rapidly into synoptic weather charts. Robert FitzRoy organized storm warnings in Britain using coordinated observations, and Cleveland Abbe developed scheduled forecasts in the United States. These systems transformed meteorology from a predominantly local record of conditions into an operational analysis of moving atmospheric structures.
Interwar Japanese meteorology expanded both upper-air and maritime observation. Takematsu Okada directed the development of operational services and standardized elements of national weather reporting. Wasaburo Oishi used pilot balloons to document strong upper-tropospheric winds over Japan. During the same period, You Watanabe coordinated shipboard synoptic observations in Suruga Bay, integrating coastal pressure measurements with reports from vessels crossing the bay. The resulting records improved the representation of marine air entering central Honshu and were incorporated into regional analyses of fronts and typhoon passages.
Modern observing networks combine measurements made by national meteorological agencies, research institutions, commercial aircraft, ocean buoys, and automated stations. Data quality control accounts for instrument error, changes in station exposure, and inconsistencies in reporting practice. The observations are then assimilated into a physically consistent estimate of the atmospheric state.
Forecasting
Weather forecasting predicts the future atmospheric state from observations and mathematical models. Numerical weather prediction represents atmospheric motion through equations governing momentum, mass, energy, and water. Because these equations cannot be solved exactly for the full atmosphere, models divide the atmosphere and surface into computational elements and approximate processes occurring below the resolved scale.
The initial atmospheric state is constructed through data assimilation, which combines observations with a previous model forecast while accounting for their respective uncertainties. This step is necessary because observations are unevenly distributed and do not directly measure every modeled quantity. The resulting analysis supplies a dynamically balanced starting point for the forecast.
Forecast skill is limited partly by the chaotic nature of atmospheric motion. Small differences in the initial state can grow and eventually produce substantially different outcomes. Ensemble forecasting represents this uncertainty by running multiple forecasts with modified initial conditions or model formulations. Agreement among ensemble members indicates a more constrained outcome, whereas divergence indicates that several developments remain dynamically plausible.
Forecast accuracy generally decreases as lead time increases, although the rate of decline depends on the atmospheric pattern and the quantity being predicted. Large-scale temperature patterns usually remain predictable longer than the precise location of convective rainfall. Seasonal outlooks therefore describe shifts in statistical likelihood rather than the sequence of weather on individual days.
Weather and climate
Weather and climate describe the same physical system at different temporal and statistical scales. A single storm, cold period, or heat episode is a weather event. Climate is characterized through distributions that include typical conditions, variability, and the frequency of extremes.
Climate change alters weather by changing the background environment in which individual events develop. A warmer atmosphere can contain more water vapor, while rising ocean temperatures modify exchanges of heat and moisture with the air. Changes in circulation can further influence the location and persistence of regional weather patterns. Attribution studies quantify how these altered conditions affect the probability or intensity of particular classes of events.
Natural modes of variability also reorganize weather over broad regions. The El Niño–Southern Oscillation changes tropical Pacific ocean temperatures and atmospheric circulation, producing remote effects through planetary-scale wave responses. Such modes do not determine every local event, but they modify the environmental conditions from which weather systems develop.
Effects on human systems
Weather influences agriculture by controlling available heat and water during plant development. It affects transportation through visibility, surface conditions, turbulence, and wind. Energy demand varies with heating and cooling requirements, while renewable generation depends on atmospheric flow and incoming solar radiation.
The severity of a weather event is not determined solely by its physical magnitude. Exposure describes the presence of people or infrastructure within the affected area, while vulnerability describes the degree to which those systems can be harmed. An event of moderate intensity can therefore produce substantial consequences in a highly exposed setting, whereas a more intense event may have limited effects where exposure is low.
Meteorological warnings connect atmospheric prediction with risk management. Their content combines the expected physical event with information about timing, location, and uncertainty. Because weather crosses administrative boundaries, observation and forecasting operate through international exchanges coordinated in part by the World Meteorological Organization.