Climate and weather
Climate is the long-term pattern of atmospheric conditions, including temperature, precipitation, humidity, wind and pressure, typically averaged over a minimum of 30 years for a given region.
The key distinction in this topic is between weather and climate:
- Weather describes short-term atmospheric conditions over hours, days or weeks: a rainy Tuesday, an unusually warm April.
- Climate describes what the atmosphere typically does in a place: the range of conditions, the seasonal rhythms, the extremes that can be expected.
Weather is what you wear today; climate is what hangs in your wardrobe.
Climate is not just about temperature. It covers the full range of atmospheric variables: precipitation patterns, prevailing winds, seasonal humidity cycles and the frequency of extreme events.
A region's climate is defined by all of these together, not by any single variable.
Climate sets the boundaries within which ecosystems function. It determines which biomes exist where, how productive soils are, when monsoons deliver freshwater and how much energy flows through food webs. Understanding climate as a system, not just an average temperature, is the foundation for understanding climate change.
Physical processes shaping regional climate
Several interacting physical processes combine to produce the characteristic climate of any location.
Solar radiation and latitude
The equator receives more concentrated solar energy than the poles because of Earth's curvature and axial tilt. This uneven energy input drives everything else: atmospheric circulation, ocean currents and the broad distribution of biomes from tropics to tundra.
Global atmospheric circulation
Three pairs of circulation cells (Hadley, Ferrel and Polar) transport heat and moisture poleward. Where warm, moist air rises, condensation brings heavy rainfall, as in equatorial tropics. Where dry air descends, arid zones form, as in subtropical deserts. These patterns explain why rainforests and deserts are where they are.
Seasonal variation
Earth's axial tilt means that as the planet orbits the Sun, solar intensity at any given latitude rises and falls. The result is seasonal change in temperature, day length, wind patterns and precipitation: monsoons in South and Southeast Asia, dry summers in Mediterranean climates, snowfall in boreal regions. Seasonal variation in temperature and precipitation is the primary driver of climate differences between regions and the main variable used to classify climate types.
The natural greenhouse effect
Greenhouse gases (water vapour, CO₂, methane and others) absorb outgoing longwave radiation and re-emit it in all directions, including back toward the surface. Without this process, Earth's average surface temperature would be roughly −18 °C rather than +15 °C. Life as we know it depends on it.
Evaporation, humidity and cloud formation
Solar energy drives evaporation from oceans, lakes and soils, loading the atmosphere with water vapour. This vapour rises, cools and condenses, forming clouds. Clouds both reflect incoming solar radiation and trap outgoing longwave radiation. Their net effect on temperature depends on cloud type and altitude, and is discussed further in the feedback loops section.
For any exam question asking you to explain why two locations at the same latitude have different climates, consider: proximity to ocean, altitude, prevailing winds, and position relative to a mountain range or desert.
Climate is never determined by a single factor.