Weather and Climate

Nothing in daily life is discussed more and understood less than the weather. Every cloud, breeze and downpour comes from the same simple cause: the Sun heats the Earth unevenly, warming the tropics far more than the poles, and the atmosphere and oceans move that heat around. Everything else, from a summer shower to a hurricane, is detail.

The engine

Warm air is lighter than cold air, so at the equator heated air rises, spreads toward the poles high in the atmosphere, cools, sinks and flows back along the surface. Because the Earth spins, those flows are bent sideways, breaking the simple loop into bands: the trade winds of the tropics, the westerlies of the middle latitudes where most of the world's people live, and the polar easterlies. Where air masses of different temperatures meet, rivers of fast-moving air form at the altitude of airliners. These jet streams, blowing at 200 kilometres per hour or more, steer the storms below them.

Water is the other half of the engine. The Sun evaporates about 500,000 cubic kilometres of water from the oceans every year. Rising air cools, and cool air cannot hold as much vapour, so the water condenses into droplets and makes clouds. When droplets grow heavy enough they fall as rain, or as snow if the air is cold enough all the way down. Condensing water releases the heat it absorbed when it evaporated, and that released heat is what powers thunderstorms and hurricanes.

Storms

A thunderstorm is a chimney of rising warm, moist air. Inside it, updraughts can exceed 150 kilometres per hour, carrying droplets high enough to freeze into hail and tearing electrons from colliding ice particles until the charge discharges as lightning. A bolt heats the air around it to about 30,000 degrees in a fraction of a second; the resulting explosion is thunder. Lightning hits the ground somewhere on Earth about 44 times a second.

In the tropics, over water warmer than about 26 degrees, clusters of thunderstorms can organise into a spinning system fed by warm sea. When winds exceed 119 kilometres per hour it is a hurricane, a typhoon or a cyclone depending on where you are; they are the same thing. A mature hurricane releases heat energy equivalent to hundreds of times the world's electricity consumption. Its eye, a few tens of kilometres across, is calm and often cloudless, surrounded by a wall of the most violent weather on the planet. Over land, cut off from warm water, it dies within a day or two. Tornadoes are smaller but spin faster, with winds that can top 480 kilometres per hour, spawned by rotating thunderstorms, most commonly over the American plains.

Forecasting

Weather is chaotic: tiny differences in today's conditions grow into large differences within days, which is why forecasts beyond about ten days are little better than guessing the season's average. Within that window, though, forecasting has become remarkably good. Satellites, weather balloons, aircraft, ships and ground stations feed millions of observations into supercomputers that solve the physics of the atmosphere for the whole globe. A five-day forecast today is as accurate as a one-day forecast was in 1980.

Climate

Climate is weather averaged over decades. Britain has a wet, mild climate; the Sahara a hot, dry one. Climate is set by latitude, by distance from the sea, by mountains and by ocean currents, and over thousands of years it changes naturally as Earth's orbit slowly shifts, swinging the planet in and out of ice ages.

It is now changing quickly for another reason. Carbon dioxide and other gases let sunlight in but trap the heat radiating back out, the greenhouse effect that keeps Earth about 33 degrees warmer than it would otherwise be. Since the industrial revolution, burning coal, oil and gas has raised the concentration of carbon dioxide in the air by half, from 280 to over 420 parts per million, and the average surface temperature has risen about 1.2 degrees Celsius. That sounds small. It is enough to melt glaciers and Arctic sea ice, raise the sea by over 20 centimetres, make heatwaves hotter and more frequent, and load the atmosphere with more water, so that when it rains, it rains harder.

Why it matters

Weather decides the harvest, the flood, the water supply and the day's plans. Climate decides where crops can grow, where cities can stand and where people can live at all. Humans have flourished in a climate that has been remarkably stable for ten thousand years. Understanding the machinery of the sky is now a matter of understanding how fast we are changing it.