Weather satellites

On the morning of September 8th, 1900, the coastal city of Galveston in the United States is expecting a storm. The city is used to storms; no one’s especially worried. The Weather Bureau warns of some higher-than-usual winds and minor flooding. But by 8pm that night, 220 kilometer an hour winds and flood waters several storeys high have destroyed almost every building in the city. Between 6,000 and 12,000 people have died.

It will come to be known as the Great Galveston Hurricane. It remains the deadliest natural disaster in US history. In 1900, people had limited ways of predicting the weather. There was little information about the hurricane forming over the sea before it made landfall in Galveston. Today, weather experts, called meteorologists, can predict weather events much more accurately, giving people a better chance to prepare or evacuate.

One of the main tools meteorologists rely on are images taken from space showing activity in the Earth’s atmosphere. These images are made possible by weather satellites. You might have seen ‘satellite pictures’ when watching a weather forecast. These pictures aren’t actually photographs taken by satellites. Rather, they are computer-generated images formed from digital information collected by satellites.

Let’s see how the process works. Weather satellites carry a special instrument that measures the flow of electromagnetic energy between the sun and the Earth’s surface - the atmospheric radiation. This instrument is a radiometer. Atmospheric radiation is affected by factors like cloud droplets, humidity, and temperature, so by measuring atmospheric radiation, we can tell a lot about the weather. The radiation data are transmitted to Earth, to a receiving station.

Here, the data are processed by computers to form images. Markings like country borders are added to the images to help us identify which parts of the Earth we’re looking at. Finally, the images are sent to weather forecast centers around the world and made available online. The time from a satellite taking a measurement to an image showing online can be less than a minute. There are two main types of weather satellites.

Each orbits the Earth differently. The first type orbits very high, tens of thousands of metres above the Earth. It completes one full orbit in the same amount of time it takes the Earth to revolve once. This means that, from Earth, the satellite appears to stay still, always above the same area. This is a geostationary satellite.

Geostationary satellites allow meteorologists to watch for changes in the same region over time. This way, they can see the progress of weather systems such as hurricanes and other storms. The other kind of weather satellite orbits much lower, less than a thousand kilometres up. It travels above the imaginary lines that run north to south on the surface of the Earth — meridian lines. This is a polar orbiting satellite.

A polar orbiting satellite passes over the north and south poles once each revolution, monitoring a narrow north-south strip each time. As the Earth rotates to the east, the satellite measures a strip slightly further west with each pass. By piecing these strips together, side by side, meteorologists can look at a larger area than geostationary satellites cover. On the other hand, they cannot obtain continuous data for any one spot — the polar orbiting satellite passes the same location only once every 12 hours. By combining the continuous data provided by geostationary satellites with the global coverage provided by polar orbiting satellites, meteorologists can build a picture of what’s happening in the Earth’s atmosphere and spot weather events forming before they strike.