Weather, climate and the atmosphere
Geography · Key Stage 3 · The School
Weather is not climate
WEATHER is the state of the atmosphere now or over days — today's rain, this week's cold snap. CLIMATE is the average pattern over decades, usually taken over thirty years. A cold week says nothing about climate, and a warming climate still has cold weeks. Confusing the two is the source of most bad arguments about both.
How rain forms
Air that rises cools, and cool air holds less water vapour. When it can hold no more, vapour condenses onto tiny particles to form cloud droplets, which merge until they are heavy enough to fall. Air is made to rise in three main ways: heated from below, forced over hills, or lifted where two air masses meet.
Why latitude matters
Near the Equator the Sun's rays arrive nearly overhead, concentrating energy on a small area. Near the poles the same beam strikes at a shallow angle and spreads over a much larger area, and passes through more atmosphere on the way. The difference is geometry, not distance from the Sun — which is worth saying, because distance is the usual wrong answer.
Britain sits in the mid-latitudes where the prevailing winds blow from the south-west, off the Atlantic. That air is moist, so it brings rain, and it arrives at the western hills first — Snowdonia and the Lake District get over 2,000 mm a year. By the time the air has crossed and descended it has lost its moisture, so Essex gets under 600 mm. One wind direction explains the whole rainfall map.