Satellite Observations — Page 462, Lesson 437

I want to walk you through satellite observations in meteorology — this is a key tool for us as pilots because it gives us a real-time view of the weather we're about to fly into. Let's start with the coverage. A single satellite can observe one-third of the Earth's surface, spanning from 70° West to 70° East. That's a huge swath — roughly 140 degrees of longitude. This satellite transmits a picture every 4 minutes. That's a rapid refresh rate, and a particularly useful feature is the time-lapse sequence you can create from those images. By playing successive frames in sequence, you can see the movement of weather over a period of time — clouds moving, fronts advancing, systems developing. That's incredibly valuable for forecasting en-route conditions. Now, there are two main types of satellite imagery we need to understand: visual images and infrared images. First, visual images. These are essentially photographs taken in visible light. They can be easy to interpret because what you see is what you get — clouds are white, land appears grey, and the sea appears black. However, visual images suffer a major disadvantage: they are not available continuously because they depend on sunlight. At night, you get nothing. So if you're planning a night departure or flying through polar darkness, visual imagery won't help you. That's where infrared, or IR, comes in. Infrared images have the advantage of being available 24 hours a day. The shading of the picture remains more or less the same by day and by night, because IR detects temperature, not reflected sunlight. Here's how to read it: cold cloud, which is high cloud, gives a white image. Lower cloud, which is warmer, gives a somewhat darker image. Warm land gives a dark image. There are 9 IR temperature bands used in the processing, and black normally denotes cloud-free areas — those are the warmest surfaces. But there's an important limitation for us as pilots: IR may not be able to distinguish between a sea surface and fog, because fog and the sea surface can have a similar temperature. In that case, a visual picture would be able to show the position of fog more precisely — because fog is visible as a white feature against the dark sea in visible light, whereas in IR it blends in with the sea temperature. So we need to use both types of imagery together to get the full picture. — this figure shows upper winds for January, which we'll relate to satellite imagery later. — this shows January surface weather and wind velocity, with upper wind velocity in yellow. — this shows July conditions, with the ITCZ having moved north with the sun. So to summarise: visual images are intuitive but only work in daylight; IR works around the clock but can miss fog over the sea. As a professional pilot, you'll use both to build a complete picture of the weather ahead.