Satellite Observations — Page 473, Lesson 438

Let’s start with a technique called false colour pictures. In both visual and infrared satellite imagery, the raw picture is made up of shades of grey. To help you tell those shades apart more easily, a computer can convert them into different colours. This is used particularly with infrared systems, where the subtle temperature differences between cloud tops and the Earth’s surface can be hard to read in greyscale. By assigning colours to specific grey tones, you get a much clearer picture of what’s happening in the atmosphere. Now, a practical problem: when you look at a satellite image, it can be very difficult to pick out geographical features. If there’s thick cloud cover, you can’t see the land underneath. And over the oceans, there are no features at all — just a blank, featureless area. So how do you know where you’re looking? The answer is that satellite images are presented with a computer-produced graticule — that’s a grid of numbered parallels (lines of latitude) and meridians (lines of longitude) superimposed on the image. Coastlines may also be enhanced, meaning they’re made more visible, to help you orient yourself. Let me show you an example. shows a satellite visible image taken at 0909 GMT. And shows the surface weather map for the same time. You’ll see those side by side. Now, how do you actually interpret satellite photography? Some weather systems are so distinctive that they’re easy to spot — for example, a tropical revolving storm (that’s a hurricane or typhoon) produces a very clear, identifiable spiral pattern. But for normal, everyday weather pictures, the best approach is to use them in conjunction with synoptic charts — that is, the surface weather maps you’re already familiar with. The satellite image gives you the real-time cloud picture; the synoptic chart gives you the pressure patterns and fronts. Together, they tell you the full story. There’s also a powerful tool called time-lapse sequences. By looking at a series of satellite images taken over several hours, you can see how clouds are moving and developing. This can be used to confirm existing weather and forecast weather before you set off on a flight. So if the forecast says a front will pass through at a certain time, you can watch the satellite loop and see if that’s actually happening. Finally, shows a visual image with the surface analysis superimposed directly on top of it. That’s a very practical way to compare what the satellite sees with what the weather map says — you can literally see the fronts and isobars laid over the clouds.