How do weather forecasts get made?

1. Will tomorrow be sunny or rainy? Nobody can see the future. But forecasters can work it out surprisingly well, and it all starts with measuring the weather right now.
2. Weather stations all over the world measure the air: how warm it is, how damp, how hard the wind blows and from where, and how much rain has fallen. Balloons carry instruments high into the sky.
3. A weather radar sends out pulses of radio waves. Raindrops bounce some of them back. The radar listens for these echoes, so it can map where rain is falling and which way it is moving.
4. Satellites look down from space and photograph the clouds over whole oceans and countries. They can spot storms days before they arrive, even where there are no weather stations at all.
5. A powerful computer divides the air into boxes and puts the measurements in. Then it uses the rules of how air moves to work out each box a few minutes later, again and again, far into tomorrow.
6. Forecasters check the computer's answer and tell everyone. A 70 percent chance of rain means rain is likely, but not certain. Tomorrow's forecast is usually very good. Next week's is much less sure.

Every forecast needs these tools working together. Tap each one to see what it adds.

• Weather station. A weather station measures temperature, dampness, air pressure, wind and rain, right where people live. The white box keeps the sun off the thermometer.
• Weather balloon. A weather balloon lifts a small box of instruments high above the clouds. It radios back how warm, damp and windy the air is on the way up.
• Weather radar. Inside the white ball a dish spins round, sending out radio waves and listening for echoes from raindrops. It shows where rain is and how it is moving.
• Weather satellite. A satellite looks down from space and takes pictures of the clouds over huge areas, including oceans where nobody lives.
• Forecast computer. A supercomputer takes all the measurements and uses the rules of how air moves to work out the weather, one small step of time after another.
• Forecaster. A forecaster, called a meteorologist, compares the computer's answers with what is really happening and explains the forecast to everyone.

Switch the weather station, the radar and the satellite on and off. Watch the forecast for the town get wider or sharper. Then play the day to check it.

Experiment: What happens to the forecast when you add more kinds of measurements?

1. Tap just the weather station. How wide is the forecast?
2. Add the radar, then the satellite. Watch the forecast change.
3. Press Play the day and see where the rain really goes.

What we found: Each tool tells the computer something different: the station gives the wind, the radar finds the rain and the satellite shows the whole cloud. With all three, the computer starts from a truer picture, so its forecast is sharper.

Turn on JavaScript to try it yourself.

Scan with the radar. It shows where the rain was an hour ago and where it is now. Work out which town it will reach, tap that town and send the forecast.

You can make a simple forecast yourself. If dark clouds are in the west and the wind is blowing from the west, rain is probably on its way. You looked at where the weather is now and which way it is moving, like watching a rolling ball. Real forecasting uses the same idea, with far more measurements.

Weather stations measure the air near the ground. Balloons measure it high up. Radar finds where rain is falling and how it is moving. Satellites photograph the clouds from space, even over the oceans. All of these measurements, from all over the world, are sent to forecasting centers several times a day.

There, a very powerful computer divides the air around the whole Earth into boxes. It fills each box with the measurements, then uses the rules of how air moves, warms and cools to work out each box a few minutes later. It repeats this again and again until it reaches tomorrow and beyond.

The rolling ball comparison has a limit. A ball rolls in a simple way, but air swirls, and tiny differences now can grow into big differences later. So the computer runs many times, each with a slightly different start. If most runs make rain, the forecast is a high chance of rain. That is also why tomorrow's forecast is usually right and next week's is only a rough guide.

Key words

Forecast

A careful estimate of what the weather will do, worked out from measurements and science.

Meteorologist

A scientist who studies the weather and makes forecasts.

Radar

A tool that sends out radio waves and listens for the echoes from raindrops to find where rain is falling.

Model

A computer program that uses the rules of how air moves to work out future weather.

Ensemble

Many forecasts run with slightly different starts, to see how sure the answer is.

Check your thinking

1. What does every forecast start with?

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   Measurements of the weather right now. The computer needs to know what the air is doing now before it can work out what happens next.

2. How does a radar find rain?

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   It sends radio waves and listens for echoes from raindrops. Raindrops bounce the radio waves back, so the echoes show where the rain is.

3. The forecast says a 70 percent chance of rain. What does that mean?

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   Rain is likely, but not certain. On 10 days like this one, it would rain on about 7 of them. Taking an umbrella is a good idea.

4. Why is tomorrow's forecast better than a forecast for ten days from now?

   Show the answer

   Small mistakes at the start grow bigger the further ahead you go. No measurement is perfect. A tiny mistake today can become a big one after many days of swirling air.

5. A model's grid boxes are made half as wide. What is the trade-off?

   Show the answer

   More detail, but far more computing is needed. Smaller boxes show smaller weather features, but there are many more boxes and the time steps must be shorter too.