Dry line
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A dry line or dew point line is a boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
Near the surface, warm moist air is less dense than dry air of greater temperature, and thus the dry air wedges under the moist air like a cold front. Severe weather is possible, especially when a triple point is formed with a cold front.
The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in dew point, clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime; see dry punch). These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along the slope reversal zone east of the surface dry line, especially when it begins moving eastward.
