Climate is the long term pattern of atmospheric conditions in a particular region, including temperature, precipitation, humidity, pressure, wind and their seasonal and longer term variations. Many interacting factors affect climate, and no single factor determines every climate. The most fundamental controls include latitude, altitude, distance from oceans and other large water bodies, ocean currents, prevailing winds, mountain ranges, continentality, surface characteristics, vegetation, atmospheric composition and the distribution of land and water. Climate is therefore an emergent result of energy received from the Sun and the movement and redistribution of heat and moisture through the atmosphere, oceans and land surface.
Latitude strongly influences climate because it affects the angle at which solar radiation reaches the Earth and the length of daylight through the year. Near the Equator, sunlight is generally more direct and concentrated over a smaller surface area, while toward the poles it arrives at a lower angle and is spread over a larger area. High latitudes also experience greater seasonal differences in daylight duration. These differences influence average temperatures and seasonal cycles. Latitude is therefore a major reason why tropical regions tend to be warmer than polar regions, although local climates can differ substantially because other factors modify the basic latitudinal pattern.
Altitude affects climate because atmospheric pressure decreases with height and rising air generally cools. Higher locations are usually colder than nearby lowlands, all else being equal. Mountains can therefore contain different climatic zones at different elevations. Altitude also affects precipitation because rising air cools and may reach saturation, causing clouds and precipitation. At sufficiently high elevations, precipitation may fall as snow and accumulate as glaciers. The climatic effect of altitude can be especially strong in tropical and subtropical mountains where elevation produces large temperature differences over short horizontal distances.
Distance from the sea affects climate because oceans have high heat capacity and respond more slowly to temperature changes than land. Coastal locations are therefore often subject to smaller annual temperature ranges than inland locations at similar latitudes. Ocean water also supplies moisture to the atmosphere, influencing humidity, cloud formation and precipitation. Inland areas can become much hotter in summer and colder in winter because land heats and cools more rapidly. This continental effect becomes particularly pronounced in the interiors of large continents where oceanic influence is weak.
Ocean currents redistribute heat around the planet and can modify coastal climates substantially. Warm currents transport heat toward higher latitudes, while cold currents carry cooler water toward lower latitudes. Currents influence evaporation, atmospheric stability, cloud formation and precipitation. A warm ocean current can contribute to relatively mild and humid conditions along a nearby coast, while a cold current can cool the lower atmosphere and promote stable conditions, fog or coastal aridity. The climatic influence of a current depends on its temperature relative to the surrounding atmosphere, direction, seasonal behaviour and interaction with prevailing winds.
Mountains affect climate by forcing air to rise, changing precipitation and creating differences between windward and leeward sides. Moist air ascending a mountain slope cools and can produce precipitation. After crossing the summit, descending air warms and becomes relatively dry, producing a rain shadow on the leeward side. Mountains can also block or redirect air masses, creating climatic contrasts across relatively short distances. Large mountain ranges therefore influence regional circulation, precipitation distribution, snow accumulation, river systems and vegetation patterns.
Surface characteristics and atmospheric composition also affect climate. Forests, deserts, snow, ice, grasslands and urban areas have different albedos, heat capacities, roughness and moisture availability. Vegetation transfers water to the atmosphere through evapotranspiration and can influence humidity and cloud formation. Snow and ice reflect substantial solar radiation, while dark surfaces absorb more energy. Atmospheric greenhouse gases such as carbon dioxide, methane and water vapour absorb outgoing infrared radiation and influence the planetary energy balance. Human activities that alter greenhouse gas concentrations, land cover and aerosols can therefore modify climate on regional and global scales.
Climate is consequently controlled by a network of interacting factors rather than by one simple rule. Latitude establishes broad patterns of solar energy, while altitude modifies temperature and precipitation vertically. Oceans moderate temperature and provide moisture, currents redistribute heat, mountains alter atmospheric flow and precipitation, and surface properties influence energy and water exchanges. Atmospheric composition determines how efficiently the Earth system retains and redistributes energy.
Climate can also vary naturally over many timescales because of changes in orbital parameters, volcanic activity, solar variability, ocean circulation and other processes. Modern climate change adds another important factor because human activities have substantially altered atmospheric greenhouse gas concentrations and other components of the climate system.
