Climate is the long term statistical description of atmospheric conditions and their variability in a particular place or across a broader region. It includes patterns of temperature, precipitation, humidity, atmospheric pressure, wind, cloudiness, radiation and other measurable environmental variables over periods long enough to distinguish persistent patterns from short term weather fluctuations. Weather describes atmospheric conditions at a particular time or over relatively short periods, whereas Climate concerns longer term averages, distributions, extremes and variability. Climate is influenced by latitude, elevation, proximity to oceans, ocean currents, prevailing winds, atmospheric circulation, topography, land cover and the composition of the atmosphere. It is therefore a spatial and temporal concept rather than simply an average temperature or precipitation value.
Climate is commonly characterised using long term averages and measures of variability. A climatic normal is often calculated over a standard thirty year period for comparison, although researchers may use other time intervals depending on the purpose of the analysis. Mean temperature alone is insufficient to describe a Climate because two regions can have similar annual averages but very different seasonal patterns, precipitation regimes or extremes.
Climate descriptions may include monthly temperature ranges, annual precipitation, frequency of frost, drought characteristics, snowfall, humidity, cloud cover and prevailing wind patterns. Statistical distributions are particularly important because changes in the frequency of extremes can represent substantial climatic change even when the average changes only modestly. Climate therefore incorporates both central tendencies and variability.
The physical controls of Climate operate at several spatial scales. Latitude determines the average angle and seasonal distribution of incoming solar radiation, making it a fundamental control on broad global climatic patterns. Elevation generally causes temperatures to decrease with height, although the exact rate varies with atmospheric conditions. Distance from the ocean affects temperature range because water heats and cools more slowly than land. Ocean currents transport heat and can warm or cool adjacent coastal regions.
Mountains influence precipitation through uplift of moist air and can create rain shadow regions on their leeward sides. Vegetation, soil moisture, snow cover and urban surfaces also influence local energy and water exchanges. These controls interact, so climate cannot usually be explained by a single factor.
Atmospheric circulation connects climates across continents and oceans. Unequal solar heating between equatorial and polar regions creates large scale temperature gradients that drive atmospheric circulation. Hadley cells dominate much of the tropical circulation, while Ferrel and polar circulation systems contribute to mid and high latitude patterns. Jet streams guide the movement of weather systems and influence the position of storm tracks. Seasonal shifts in atmospheric circulation contribute to monsoons and changing precipitation patterns.
Ocean circulation transports heat around the planet and interacts with atmospheric processes through evaporation, precipitation and surface heat exchange. Large scale phenomena such as El Nino and La Nina can temporarily alter rainfall and temperature patterns across extensive regions, demonstrating the difference between long term climate and shorter term climate variability.
Climate classification systems organise the enormous diversity of global climates into meaningful categories. The Koppen climate classification system uses temperature and precipitation patterns together with relationships to vegetation to identify major climate groups. Other systems use different thresholds or emphasise atmospheric processes, water balance or ecological characteristics. Classification is useful for comparison, mapping and education, but boundaries are simplified representations of continuous environmental gradients. A location near a classification boundary may experience conditions that resemble neighbouring categories. Climate classification should therefore be treated as a descriptive framework rather than an exact representation of every local environmental process. Modern climatology also uses statistical datasets and model based classifications that can represent climate characteristics at much finer spatial scales.
Climate changes naturally over many timescales. Orbital variations alter the distribution of solar radiation received by Earth, volcanic eruptions can temporarily influence global temperatures through atmospheric aerosols, and solar variability can produce smaller changes in incoming energy. Tectonic movements can alter the positions of continents and oceans and consequently affect atmospheric and oceanic circulation over geological timescales. Changes in ice sheets, vegetation, ocean circulation and atmospheric composition can also produce feedbacks that amplify or moderate climate changes. Human activities have become a major influence on recent climate through increased concentrations of greenhouse gases and other atmospheric changes. Distinguishing individual natural influences from human driven changes requires observations, physical understanding and climate modelling.
Climate has direct relationships with ecosystems, agriculture, water resources and human settlements. Plants and animals have physiological limits linked to temperature, moisture, seasonality and extreme events, so climatic conditions strongly influence biome distributions. Agriculture depends on temperature, rainfall, frost risk, growing season length and water availability. Water resource planning requires knowledge of precipitation, evaporation, snow accumulation and drought patterns. Buildings, transport systems and infrastructure must account for local climatic conditions such as heat, heavy rainfall, snow, wind and coastal exposure. Climate can also influence disease ecology, energy demand and wildfire risk. These relationships explain why climatic information is essential in environmental planning and resource management.
Climate is measured using networks of weather stations, ocean buoys, satellites, radiosondes, aircraft observations, paleoclimate proxies and other instruments. Historical observations are combined into datasets that allow researchers to calculate averages, trends and extremes. Climate models represent physical processes in the atmosphere, oceans, land surface and cryosphere and can be used to study past changes or explore possible future conditions under specified assumptions. Uncertainty can arise from measurement limitations, incomplete historical records, natural variability, model structure and future emissions or land use. A scientifically meaningful Climate assessment therefore distinguishes observed changes from model projections and distinguishes short term variability from long term trends.
