Weather and climate

Climate

提示Not what the sky is doing today, but what it usually does.

为什么Weather is the state of the atmosphere right now and changes hour by hour; the long-term pattern is normally averaged over at least thirty years before anyone calls it settled.

An anemometer

提示Its cups spin faster the harder it blows.

为什么It counts how fast its cups turn and converts that to kilometres per hour. A wind vane gives the direction, a rain gauge collects rainfall in millimetres, and a thermometer records temperature.

A barometer

提示A falling reading warns that unsettled, stormy days are coming.

为什么It weighs the atmosphere above you, in millibars. Low pressure means rising air, cloud and rain; high pressure means sinking air and settled skies.

Relief rainfall

提示Named after the shape of the land that does the lifting.

为什么This is why the western hills of Britain are so wet and the land beyond them sits drier in the rain shadow. The other two types are convectional, from heating below, and frontal, where warm air rides over cold.

The south-west

提示They cross a warm ocean on their way up from the Azores.

为什么Air takes on the character of whatever it has travelled over, so a long run across the Atlantic arrives mild and wet - the commonest British weather of all. Polar continental air from the east brings the bitter dry cold instead.

The Hadley cell

提示Named after the eighteenth-century Englishman who first worked it out.

为什么Fierce equatorial heating lifts air, which dumps its rain and then sinks dry near 30° - exactly where the great deserts sit. The Ferrel and Polar loops complete the pattern towards the poles.

The tropical rainforest zone

提示The sun is overhead all year and rain falls almost every afternoon.

为什么Air rises where the heating is strongest, cools and rains daily. Travel north or south and you pass through savannah, desert, temperate and finally tundra, and the pattern repeats in both hemispheres.

heat island

提示Brick and tarmac store the sun's warmth, and traffic adds more.

为什么Dark surfaces soak up sunshine by day and release it after dark, buildings block the wind, and heating and engines add their own. The gap is widest on still, clear nights.

About 27 degrees Celsius

提示Warm enough that the ocean can pour huge amounts of vapour into the rising air.

为什么A tropical storm is a heat engine running on warm water, so it forms only over seas above roughly 27 °C and weakens once it crosses land or cooler ocean. Its spin comes from the Earth turning, which is why none form on the Equator itself.

Around 10 000 years ago

提示Long before farming or the first towns, but well after the dinosaurs.

为什么Ice sheets covered much of Britain until then, and the climate has swung through warmer and cooler spells ever since, including the medieval warm period and the Little Ice Age. Natural change is normal - the speed of the present change is not.

Tiny bubbles of old air

提示Each winter's snowfall traps a sample of the sky as it settles.

为什么Antarctic and Greenland cores hold samples hundreds of thousands of years old, so past carbon dioxide levels can be measured directly rather than guessed. Thermometer records, retreating glaciers and satellite data all agree with them.

Adaptation

提示It does nothing about the cause; it prepares for the consequence.

为什么Mitigation tackles the cause by cutting greenhouse gas emissions - renewables, insulation, less flying. The other lever adjusts to effects already on the way: flood defences, drought-resistant crops, shaded cities. Serious plans use both.