FRONTS FOUR

In Fronts Four, a description of satellite imaging will be presented. This description is needed in order that an understanding of how the use of satellite images to locate fronts is possible. There are three types of satellite images. One uses infra-red, the second uses visible light, and the third uses a special infrared band that illustrates water vapor concentrations in the upper troposphere. All substances that are warmer than absolute zero radiate infrared. The warmer the substance is, the greater the amount of infrared energy is radiated per unit time. Therefore, the rate infrared energy a satellite receives from a substance is a measure of its temperature. Some infrared satellite images are in black and white plus shades of gray. White indicates the coldest areas and black indicates the warmest. The lighter shades of gray indicate lower temperatures than the darker shades. If these white areas are in bands, a jet stream indicated along with a possible front. These cloud tops are usually higher than 20,000ft or are cirrus clouds. Thunderstorms appear to be white blobs that are sometimes arranged in rows or bands. Tops of thunderstorms are often 30,000ft or higher. Since middle clouds are found between 6,000ft and 20,000ft, they are warmer and are usually in shades of gray. Low clouds are almost black since their tops are less than 6,000ft and they are usually the warmest clouds. The ground in warm areas is also almost black. At the higher latitudes during the winter, the ground or low clouds may be white because of very low temperatures. Some infrared images are colorized. There is usually a scale at the bottom or side of the image indicating the range of temperatures each color segment represents. The two advantages of the infrared images are that they are available day or night and an estimate of cloud heights can be obtained. Visible images provide the best resolution. The resolution can be down to a few feet. Visible images are in black and white, and show clouds as through you were in a satellite taking black and white photographs of clouds and transmitting the images to receivers on the ground. Unlike the infrared images, visible images can indicate whether or not clouds with high tops are thin or not. Visible images can indicate features on the ground through high thin cirrus that the infrared image suggest as thick. Bands of low warm clouds below cirrus clouds could be missed by infrared imaging. However, visible images are available only during day light. Water vapor is a substance which will emit only certain bands of infrared unique to water vapor like all gasses. By using a filter, only the infrared bands emitted by water vapor can be used for water vapor imaging. Since jet streams and their associated upper fronts induce upward and downward vertical motions in the atmosphere, the resulting zones or belts of greater and lesser concentrations of water vapor in the upper troposphere can be depicted by water vapor imaging. Since sinking air is associated with upper fronts and is found along the cold air side of jet streams, the air will be depicted as dry. Below and on the warm air side of the jet, rising air will bring moisture to the upper troposphere. Exactly how to use these images will be used in finding fronts and jets will be presented in front five by using examples. Fredmet