Drylines are a unique mesoscale weather phenomenon that occur mainly in the western Great Plains of the United States during the spring and summer. Basically, a dryline is a separation line between two different air mass boundaries. The air mass on the west side of the dryline is dry, continental air (usually from the Rockie Mountains or the desert southwest), while the air mass east of the dryline is maritime, tropical air (usually from the Gulf of Mexico).
Drylines are NOT considered fronts because they do not have one direction of motion. However, they can spark thunderstorms if there is enough moisture in the air, just like a front. Drylines act as a lifting mechanism to create showers and thunderstorms. During the daytime, drylines move eastward and during the evening, they retreat westward. Below is an example of what a dryline looks like on a weather map. The brown half circles (pips) indicate the direct of motion. Notice the dewpoint temperature west of the dryline vs. east of the dryline (stations circled in yellow). *CLICK ON IMAGE TO ENLARGE*