A closer look at weather satellite images
Wednesday, June 28th, 2006So what does one actually see when looking at live images from a weather satellite?
The answer may seem obvious, but a closer look reveals some facts that are not really common knowledge and that may be useful for flight planning! So here is a summary of what you need to know to get the most out of weather satellites:
The weather satellites that provide the highest resolution images for the United States are the GOES satellites. The GOES satellites are in orbit roughly 22 thousand miles away from earth. Since the orbit is geosynchronous the satellites will always look at the same part of the planet. The flyagogo.net high performance server continuously processes the full resolution images from both the GOES west satellite for the western half of the US and the GOES east satellite for the eastern half, both in the visible range with 1km resolution and in the infrared with 4km resolution.
Weather satellites basically measure sunlight reflected by the clouds and surface of the earth. Water absorbs a lot of sunlight - it reflects just a little - so it appears dark. The percent of sunlight reflected by the land or water is called the surface albedo. The albedo of land ranges from about 10-30%, except for snow covered surfaces where the albedo is much higher. The raw satellite images are impressive but they are pretty much useless for flight planning. That’s why flyagogo.net first subtracts an average surface albedo, and then overlays the resulting clouds-only image with a colorful aviation specific map showing airport positions, land and sea boundaries, rivers, and state boundaries.
A cloud’s albedo is generally high, but can vary with its thickness and composition. Thick clouds have high albedos and show up bright in the visible satellite image. Thin cirrus clouds have low albedos and are usually semi-transparent to sunlight. The structure of clouds in the satellite image can tell you a lot about what you will encounter on your flight. The image on the left shows the cumulonimbus clouds of a thunderstorm with cirrus clouds being blown of from the anvil top of the thunderstorm. The wind direction is from the southeast! Note that clouds are only visible during daytime, unlike the infrared images which are available around the clock.
The satellite also measures the temperature of the clouds and the surface of the Earth with an infrared sensor. Clouds are usually colder than the surface (land or water). The temperature of the clouds also indicates how tall they are since temperature is inversely proportional to height in the atmosphere. The colder the cloud the more likely it is to produce rain. The flyagogo website displays warm clouds semi-transparent, the cool clouds whiter, and the very cold clouds bright white and opaque. The raw images are processed continuously to remove the surface temperature and only show the temperature difference between the surface and the atmosphere. The biggest advantage of the infrared image is that it is available 24 hours, even at night time. If you are planning a night cross country you should definitely consult the infrared images. As we all know flying at night is more risky than flying during daytime. Never rely on an infrared image and always make sure to consult a weather briefer before embarking on a flight.
Credits and thanks go to the NWS and NASA for providing some of the information presented here.
button to see the forecasted weather conditions at night. The data is provided by the National Weather Service.
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