Weather forecasting

Weather forecasts are made by collecting data on our atmosphere’s current behaviour (particularly the temperature, humidity and wind) and predicting how it will evolve in the future.

Weather satellites

Weather satellites are used to monitor the weather and climate on Earth. They can be polar orbiting, cover the entire Earth asynchronously (at different times) or be geostationary (hovering over the same spot on the equator).

How do they work?

Weather satellites carry radiometers which scan the Earth to form images. These usually have a small telescope or antenna, a scanning mechanism, and detectors which record visible, infrared or microwave radiation. They monitor weather around the world: for example, the European Space Agency’s Meteosat satellite covers Europe and Africa.

Polar satellites

As polar orbiting satellites are closer to the Earth than geostationary satellites, it is easier for them to take sharper photographs. Their orbits are also nearly Sun-synchronous which means that their position relative to the Sun remains the same, as the Earth itself orbits the Sun. Some Sun-synchronous orbits are set up so that a satellite can remain in sunlight with its solar panels operational at all times. This type of orbit also means that the light conditions (the surface illumination angle from the Sun) will be the same every time they pass over the same part of the Earth, so that images can be more easily compared for changes. View this video for a visualisation of the orbit of ESA’s wind monitoring satellite, Aelus, in a Sun-synchronous orbit. It travels over sites on Earth at nightfall every time.

Geostationary satellites

Geostationary satellites have a very high altitude, around 35,880 km. That’s nearly 88 times higher than the International Space Station. It is more difficult for geostationary satellites to take high- resolution images than polar satellites, because they require elaborate telescopes and precise scanning mechanisms to compensate for being further away. However, they can view the same region of Earth throughout the course of a day. This is useful for real-time weather emergencies and helping to create weather models based on location.

Is this how they predict weather on TV?

The Met Office provides UK weather updates. It uses satellite data to build computer-based models which predict what the weather will do next. Sometimes weather forecasters on TV get it wrong. This happens when weather changes rapidly, especially in the UK. For example, rain during the summer is difficult to predict as it develops quickly and over small areas, with forecasters getting it wrong almost 40% of the time.