What Is Climate?

Climate is the statistical description of weather over an extended period, expressed as the mean, variance, and extremes of quantities such as temperature, precipitation, wind, humidity, and pressure for a given place or region. Weather is the state of the atmosphere at a moment; climate is the distribution from which that state is drawn. The distinction is operational rather than philosophical, because meteorology treats climate as a set of computed statistics over a defined interval, and the choice of interval is a matter of international convention.

That convention is the climatological standard normal, an average computed over three consecutive decades. The World Meteorological Organization sets the current baseline at 1991 to 2020 and requires member services to recompute it every ten years using a common method. The WMO Climatological Standard Normals archived at NOAA's National Centers for Environmental Information hold monthly temperature and precipitation statistics for thousands of stations worldwide, and they serve two purposes at once: a reference against which a given month is judged unusual, and a baseline for engineering design values in construction, agriculture, and energy planning.

Climate Elements and Classification

The physical climate system comprises the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere, coupled through exchanges of energy, water, and carbon. Regional climate emerges from the interaction of incoming solar radiation, latitude, the general circulation, distance from oceans, ocean currents, and terrain. Classification schemes compress that complexity into discrete zones. The Koppen-Geiger system, first published in 1884 and revised repeatedly since, assigns categories from thresholds on monthly temperature and precipitation, which is why its boundaries shift measurably as those thresholds are crossed under warming. Alternative schemes based on potential evapotranspiration or on plant hardiness serve agricultural and ecological users whose thresholds differ.

Climate Variability

Climate variability refers to fluctuations of the climate statistics on timescales beyond individual weather events but shorter than long-term trends, and it arises both from internal dynamics and from external forcing. The El Nino-Southern Oscillation is the dominant mode on interannual scales, redistributing heat across the tropical Pacific on a two to seven year cycle and altering rainfall and temperature patterns on every continent through atmospheric teleconnections. Other recurring modes include the North Atlantic Oscillation, the Pacific Decadal Oscillation, and the Madden-Julian Oscillation. Separating this internal variability from a forced trend is one of the central problems in climate science: the IPCC Working Group I assessment of the physical science basis uses detection and attribution methods that compare observed patterns against model simulations run with and without anthropogenic forcing to make that separation quantitative.

Observation and Reconstruction

Instrumental climate records are assembled from surface stations, radiosondes, ships and drifting buoys, and satellite radiometers, then homogenized to remove artifacts from station moves, instrument changes, and time-of-observation shifts. Global coverage of that quality extends back roughly to the middle of the nineteenth century. Earlier climate is reconstructed from proxies, and the paleoclimatology archive maintained by NOAA collects tree ring, ice core, coral, speleothem, and sediment records that extend the picture back hundreds to millions of years. Reanalysis products combine observations with a fixed numerical weather prediction model to produce physically consistent gridded fields, and coupled general circulation models coordinated through intercomparison projects provide the forward simulations used for projection. The WMO climatological normals guidance defines the statistical procedures that make these national records comparable across borders.

Applications

Climate information has applications in a range of fields, including:

• Electric power planning, where load forecasts and renewable resource assessment depend on long-term temperature, wind, and irradiance statistics
• Civil and structural engineering, which draws design values for wind load, snow load, and rainfall intensity from climatological records
• Agriculture, for crop selection, planting calendars, and irrigation scheduling
• Insurance and reinsurance, where catastrophe models are calibrated on climate extremes
• Water resource management, including reservoir operation and drought planning
• Public health surveillance for heat stress and vector-borne disease