Watch a satellite animation of a tropical cyclone and two kinds of motion come into view. Cloud bands curl around the eye while the eye itself drifts across the ocean. Follow a single band and you can see the storm rotating. Follow the center against a familiar stretch of coastline and you can see where the whole storm is going.
A fixed view from space
Geostationary weather satellites orbit about 35,800 kilometers above the equator. Each completes an orbit in the time Earth takes to rotate once, keeping the same region in view. In a sequence of images, a coastal headland can therefore serve as a reference point. As the frames advance, forecasters can see whether the center is approaching the headland, moving parallel to the coast, or turning back out to sea.
That view continues after sunset. During daylight, satellite sensors detect sunlight reflected by the clouds, revealing the shape of the eye and the bands around it. Infrared sensors measure radiation emitted by the cloud tops and the surface below. Because those measurements work in darkness, forecasters can follow changes in the cloud pattern through the night.
Measuring how fast the storm travels
When a cyclone has a distinct eye, estimating its movement is straightforward. Suppose the eye appears 30 kilometers west of its earlier position in images taken two hours apart. Over that interval, the storm traveled west at an average of 15 kilometers per hour. That figure describes the movement of the center. Wind speed around the eye is a separate measurement.
From an observed track to a forecast
A sequence of center positions tells forecasters where the cyclone has traveled. The next question is whether it will keep that heading. To answer it, forecasters compare the observed track with guidance from weather models that simulate how the surrounding atmosphere will change. A shift in the winds steering the cyclone can send its center along a different course.
On a track map, the latest observed position is where those two parts of the story meet. Behind it lies a path reconstructed from observations. Ahead lies a forecast that meteorologists can check and revise as the next satellite images arrive.