Climate is what you expect; weather is what you actually get
A single rainy week tells you about weather. Climate is the pattern underneath, conventionally averaged over 30 years, a span chosen because it is long enough to smooth out quirks like El Niño yet short enough to reveal real trends. The reference periods themselves date back to a 1934 meeting in Wiesbaden.
In the simplest terms, climate is a region's long-term weather: the averages and the spread of temperature, humidity, pressure, wind and rainfall. More broadly it describes the whole climate system, meaning air, oceans, ice, rock and living things, and how they interact. The word comes from the Greek for inclination. A place's climate depends on its latitude, altitude, terrain, land use and nearby water and currents.
The 30-year standard has an institutional history. The International Meteorological Organization, forerunner of today's World Meteorological Organization, set up a climatology commission in 1929, and in 1934 it fixed 1901 to 1930 as the first reference period. Later the WMO adopted 1961 to 1990 as the baseline, with 1991 to 2020 as the next set. Alongside the core measurements, observers also log things such as cloud cover, soil temperature, and days with thunder or hail.
Some influences barely budge over human history: latitude, altitude, the ratio of land to sea and the distance to oceans and mountains, which change only over millions of years through plate tectonics. Others shift faster. The ocean's thermohaline circulation keeps the northern Atlantic about 5 degrees Celsius warmer than other basins, vegetation affects how much heat and water a region holds, and changing levels of carbon dioxide and methane set how much solar energy the planet keeps.
Scientists sort climates in several ways. The Köppen scheme is the most widely used; Thornthwaite's, in use since 1948, adds evapotranspiration and helps study biodiversity. Paleoclimatologists reconstruct ancient climates from proxies such as lake sediments, ice cores, tree rings and coral. Climate scientist Lesley Ann Hughes has noted that a 3-degree shift in mean annual temperature moves temperature zones roughly 300 to 400 kilometres in latitude, or about 500 metres in elevation, so species are expected to move uphill or poleward as the climate warms.
Source: Climate
