There are several pitfalls with using satellite images. This essay will explore many of these common pitfalls. 1. VIS (low sun angle and at night)- Visible imagery is of no use at night since it requires the sun to reflect its rays off objects in order to detect them. In the winter VIS is less useful since the days are so short and the sun angle is low. When the sun angle is low (near the horizon) there is not as much reflection off the clouds. Thus, around the time of sunrise and sunset VIS is not as useable. 2. Water Vapor (vapor vs. clouds)- There is an important difference between water vapor and clouds. Water vapor is in the gaseous state while cloud is in the liquid or solid state. On water vapor imagery it is important to distinguish between the vapor and the cloud. On black and white imagery the clouds will show up a fairly bright white. The water vapor will be wispy in appearance. Also remember the clouds and water vapor are mainly being detected in the upper levels of the troposphere, thus cloud and water vapor near the surface is not being detected as much. 3. Cloud location error / angle displacement- Since the earth is a spherical type shape and the satellite images you look at are flat there will be some error in the cloud location. Since the satellite is over the equatorial region, the cloud displacement error will be greater toward the pole. The displacement also increases when moving east or west of where the satellite is located. As the latitude increases or when looking further east or west of the satellite position the satellite is looking at the clouds at more of an angle. The angle eventually gets so large that it makes the image fairly useless for example in the polar regions. 4. Can't see clouds under cloud- The satellite can only see the highest cloud deck. If there is upper level cloud covering an area the satellite will be unable to detect clouds under this cloud deck. Because of this the satellite can be useless at times at detecting clouds that are influencing weather closer to the surface. Satellite can be used to sense surface features and the temperature profile of the troposphere. If clouds are present though this type of sounding can not be done. 5. Infrared is totally different from VIS- Infrared detects clouds be determining temperature by the wavelength of emitted radiation from the cloud. This is a totally different concept of detection from what VIS uses. VIS uses reflection to detect cloud. Clouds that do not look significant on IR can be very significant. For example, clouds near the surface will not show up well on IR since they are close to the earth surface temperature. These clouds may in fact though have a significant influence on the surface weather. 6. Satellite images are not pictures and not video recordings- VIS is closest to being a picture but of course it does not use a flash like a camera needs to sometimes. Satellite images don't require film and film developing. The satellite images are much more similar to digital pictures, except the entire picture is not taken all at once but rather is a line by line scan that when complete produces a complete image. It takes about 20 minutes to produce an image. Thus when looking at the satellite images in motion there are significant time gaps between each frame. This is far more time than the multiple frames per second that video recordings have. Because of this delay in data, satellite images are not near as close to being instantaneous as compared to radar images. |