What are those cloud images on TV?

What does the brightness of a cloud mean on the TV weather shows?

The image to the left is an example of an infrared (IR) image made from a radiometer flown on the GOES satellites. This is very typical of the images you see on TV weather shows. A piece of this image has been extracted and appears as a square below the larger satellite image. This portion of the image contains a cloud and some clear sky areas. You can change the appearance of this extracted image by changing the cloud altitude (and its associated ambient air temperature) or the surface temperature. You do this by moving the sliding scales (scroll bars) accompanying the picture on the right.

Drag the cloud up and down to change its altitude and its temperature. The numbers on each side of figure show the altitude and corresponding temperature, assuming standard atmospheric conditions.

You can modify the surface temperature with the horizontal sliding scale.