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How do you see clouds at night on satellite images?

How do you see clouds at night on satellite images?
Satellite photos are a staple of TV weathercasts. But most of the time, what you're seeing is not simply a photograph from space. Otherwise, you'd only be able to see the clouds during the daylight hours (or else, that big flash needed to light up the ground would get really annoying at 2 a.m. :) )

Instead, to show clouds anytime, the most common satellite photos use infrared technology, which uses temperature instead of visible light to "see."

Since the atmosphere gets colder as you rise in altitude, the satellite can not only detect the cooler clouds from the warmer ocean and land areas day and night, but they can discern high clouds from low clouds. Tall, storm clouds have colder tops, so they usually come out white, while warmer, lower clouds like fog come out looking darker and grayer.

Here is an example of an infrared image:

However, we do also use a "visible" satellite image, which is, quite simply, a picture in space. That image is very helpful in finding fog and low clouds, since they don't show up as well in an infrared image as their temperature is close to the ground temperature. But as mentioned earlier, they only work during the day.

Here is a visible image -taken at the same time as the IR image above. (Note how the fog shows up better on the visible image):

Finally, there's a water vapor image. This shows light and dark areas based on water vapor content of the air. It's most useful in determining water content in the middle layers of the atmosphere and that can in turn help determine what energy is available for storm development.  

Dark areas are dry slots, while gray to white is wetter.

You can get the current version of all three at the University of Washington Atmospheric Sciences weather web site (Go Dawgs! :) )