What Is Weather?

Weather is what is happening in the atmosphere right now: how hot or cold it is, whether it is raining or sunny, how strong the wind is, what clouds are in the sky. It is one of the things that makes Earth so different from any other planet we know. Weather is driven by energy from the Sun, which heats different parts of the Earth unevenly. Warm air rises, cool air sinks, water evaporates and condenses, and all of this creates the wind, clouds, rain and storms we experience every day.

  • Main elements6Temperature, pressure, wind, humidity, rain, clouds
  • Powered byThe SunPlus Earth's spin and water cycle
  • Hottest recorded56.7 degrees CDeath Valley, USA, 1913
  • Coldest recordedMinus 89.2 degrees CVostok, Antarctica, 1983
  • Strongest wind ever408 km/hTropical Cyclone Olivia, Australia, 1996
  • Most rainfall in a dayApproximately 1.8 mReunion Island, 1980

The main weather elements

  • Temperature: how hot or cold the air is.
  • Air pressure: how heavy the atmosphere is pressing down. High pressure usually means settled weather; low pressure usually means storms.
  • Wind: air moving from high to low pressure.
  • Humidity: how much water vapour is in the air.
  • Precipitation: rain, snow, sleet, hail.
  • Clouds: visible collections of water droplets or ice crystals in the air.

What causes weather

Almost all weather comes from one basic fact: the Sun heats the Earth unevenly. The equator gets more direct sunlight than the poles. Land heats and cools faster than water. Mountains, deserts and forests all absorb the Sun's heat differently. The result is constantly shifting patterns of warm and cool air, which create wind, clouds, rain and storms as the atmosphere tries to even things out.

The water cycle is the other key engine. Water evaporates from oceans, lakes and plants, rises as water vapour, condenses into clouds, and falls as precipitation. All this movement releases huge amounts of heat that drives most major storms.

How weather is measured

Weather scientists (called meteorologists) use many tools to measure the atmosphere.

  • Thermometer: measures temperature.
  • Barometer: measures air pressure.
  • Anemometer: measures wind speed.
  • Rain gauge: measures rainfall.
  • Hygrometer: measures humidity.
  • Weather balloons: carry instruments high into the atmosphere.
  • Weather satellites: photograph clouds and storms from space.
  • Doppler radar: tracks precipitation in real time.
Fact Modern weather forecasts use enormous supercomputers running complex mathematical models of the atmosphere. These models divide the world into millions of tiny grid boxes and calculate the temperature, pressure, wind and moisture in each one, recalculating every few minutes. The UK Met Office's newest computer can perform over 60 million billion calculations per second, making modern 5-day forecasts as accurate as 1-day forecasts were 30 years ago.

Weather versus climate

People often confuse weather and climate, but they are different.

  • Weather: what is happening in the atmosphere right now or in the next few days. Can change quickly.
  • Climate: the average pattern of weather in a particular place over many years (usually 30 years or more). Changes slowly.

The phrase often used is "climate is what you expect; weather is what you get". Britain has a temperate maritime climate, but today's weather in London might be sunny, rainy or snowy.

Did you know? The UK Met Office was founded in 1854 as a small office in the Board of Trade, to help sailors avoid bad weather at sea. Today it employs over 2,000 people, runs one of the world's most powerful supercomputers, and provides forecasts to governments and airlines around the world. The famous BBC Shipping Forecast, broadcast since 1924, is still used by ships and fishermen every day.
Deeper dive: why weather forecasts are limited

Modern weather forecasts are amazingly good, but they still have limits. A 1-day forecast is now around 95% accurate; a 5-day forecast is around 80% accurate. Beyond about 10 days, forecasts become little better than guessing.

The reason is something called the butterfly effect. The atmosphere is what mathematicians call a chaotic system: even tiny changes in starting conditions can produce huge differences in outcomes a few days later. The phrase "butterfly effect" comes from the idea that a butterfly flapping its wings in Brazil could, in theory, set off a chain of events that eventually produces a tornado in Texas.

Practically, this means weather forecasters can never measure the starting conditions of the atmosphere perfectly. Tiny errors in those measurements get amplified by the chaos of the system, until after a couple of weeks the forecast no longer matches the real weather. No matter how powerful our computers get, this limit will not go away.

One way around it is ensemble forecasting: running the forecast many times with slightly different starting conditions, and seeing how the results compare. If most of the forecasts agree, you can be confident in the prediction. If they diverge widely, you know the future is uncertain. Modern weather services use this approach to provide probability forecasts (like "70% chance of rain tomorrow") rather than yes/no predictions.

For more, see climate vs weather, clouds and wind.