Dry line

A dry line, (also called dew point line, or Marfa front) is an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the High Plains states (and sometimes stretching into the Canadian Prairies) 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 south-western states (to the west).

The dry line is also sometimes important for severe convective storms in other smaller regions of the world, particularly Bangladesh and adjacent eastern India, and south-eastern South America.

A dry line is a boundary that separates a moist air mass from a dry air mass.

Vertical structure

Near the surface, warm moist air is denser than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the cooler, drier air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.

Daily progression

The dry line typically advances eastward during the afternoon and retreats westward at night, mainly due to the increased mixing down to the surface of drier air aloft, rather than the air mass' surface density contrast. 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 south-easterly to west or south-westerly. (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.

See also

• Surface weather analysis
• Weather front

References