If you’ve ever wondered how weather forecasters work out what’s in store for tomorrow, look no further.

Meteorologists don’t just rely on the basic bits of kit you used in science lessons (think rain gauges, thermometers and wind socks)… it’s all about the nifty weather balloons.

What is a weather balloon?

National Weather Service hydro-meteorological technician Calvin Meadows prepares a weather balloon for launch in the Upper Air Inflation Building at the National Weather Service
(Chris Greenberg/AP)

Forecasters need to gather temperature, humidity and wind data from high above the ground to predict what will happen to the weather down on Earth. Since meteorologists can’t just put a thermometer on a very long stick, the Met Office uses weather (or sounding) balloons. The big balloons are filled with hydrogen or helium gas and grow as they float up into the stratosphere – expanding to around 35ft (10.7m) in diameter before popping.

How do they work?

French, British and American army meteorologists get together in plotting weather conditions for United Nations fliers

Inside each balloon is a small radio transmitter called a radiosonde which measures the various atmospheric parameters and sends the data back to the ground.

What do they measure?

A barometer hangs on a tent indicating dry weather as temperatures soar and festivalgoers try to find shade out of the sun during Glastonbury Festival 2010 at Worthy farm in Pilton, Somerset.
(Ben Birchall/PA)

The balloons gather data on three things: pressure, temperature and humidity. Temperature is measured with a tiny thermistor (a resistor whose resistance is dependent on temperature), and humidity by a small capacitive sensor – both of these sensors are exposed to the elements. A pressure sensor – an aneroid capsule, aka barometer – is enclosed in an insulated container.

There is usually a GPS or other navigation aid inside the radiosonde to measure wind information. As the balloon travels horizontally, the wind direction and speed can be deduced.

How often are they let off?

Balloons are let off around the world every day to collect forecasting data. The Met Office, for example, has daily launches from Camborne Observatory in Cornwall. But they are available to buy, if you fancy some DIY forecasting in your spare time.

How high do they fly?

Snappa graphic about the layers of the atmosphere.

Small radiosonde balloons usually climb to around 65,000 ft but larger ones can reach heights of 115,000 ft or more. While far higher than normal aeroplanes, this is nothing in comparison with the rest of the atmosphere.