Why is there precipitation at fronts

Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms. Terms for using data resources. CD-ROM available. The weather on the surface of the earth is controlled significantly by what occurs in the atmosphere above.

Therefore, meteorologists always analyze "upper air charts" for weather disturbances, moisture, temperatures, etc. The pictures above show what a very basic upper air weather map might look like in this example, at mb or about 18, feet.

At left, we see a " trough " of low pressure, which may be associated with a low pressure system red "L" , fronts, and precipitation at the surface ahead of the upper trough location.

At right, we see a " ridge " of high pressure aloft. A surface high pressure system and often fair weather typically are located ahead east of an upper ridge axis with lower surface pressure behind west of the ridge. Troughs and ridges can be weak or quite strong. In looking at upper air data, we also closely examine the " jet stream " which is a relatively high speed ribbon of air between 25, and 40, feet above the ground. The jet stream is strongest in the winter when the greatest temperature differences exist across the Northern Hemisphere.

In a general sense, warmest air masses are located south of the jet with cold air to its north left picture. Changes in the jet stream pattern and different wind speeds within the jet play a major role in surface temperature and precipitation patterns. At right is a typical upper air chart or mb, i.

In addition, wind vectors and colored isotachs lines of equal wind speed show the location of the jet stream green colors. Yellow, red, and pink colored isotachs represent a ribbon of higher speed winds within the jet. Notice that within the jet stream, wind speeds usually differ.

It is these differences in wind speeds within the jet stream that can cause significant weather. In this example, the highest winds are located along the East Coast.

This is called a " jet streak " within the overall jet stream. This picture is a satellite image showing clouds associated with a major low pressure system. Satellite pictures are taken by weather satellites located about 22, miles above the earth. The surface low center in this case likely would be located in western Iowa with a cold front extending south through Missouri into eastern Texas along the cloud band. Rain showers may be occurring within this band. Meanwhile, across Nebraska, South Dakata, Minnesota, and northern Wisconsin, steadier rain or snow may be occurring to the north and west of the surface low and warm front which probably extends northeast from the low across central Wisconsin.

Now we will look at different precipitation types. A warm mass of air will generally hold moisture picked up as it passes over large areas of water through the process of evaporation. As this warm moist air rises it cools and the water vapor condenses into rain. So a warm air mass tends to bring with it plenty of rain and drizzle.