What does the brightness of a cloud mean on the TV weather shows?
All objects emit electromagnetic radiation. The infrared radiometers on satellites measure radiation with wavelengths of 10 to 12 mm. For easy interpretation, the radiant energy measured by infrared radiometers is converted to a temperature. Cold objects are white and hot surfaces appear black. Time sequences of infrared images are animated and shown on television news programs.
The image to the left is an example of an infrared (IR) image made from a radiometer flown on the GOES-8 satellite. 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.
The vertical scale changes the cloud 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.
Use this applet to answer the three questions below:
 | What happens to the image when you lower the cloud? |
| What happens to the image when you decrease the ground temperature? |
| Adjust the cloud altitude and ground temperature so that the cloud is no longer visible in the image. Can you think of a situation when this might occur? |