Dry Lines Associated with large horizontal gradients in moisture, but not necessarily temperature. Results from the interaction of cyclones and fronts with large-scale terrain. Found over the U.S. Midwest, northern India, China, central West Africa and other locations. Acts as a focus for convection, and particularly severe convection. Most prevalent during spring/early summer in U.S.
Dry Line Surface boundary between warm, moist air and hot, dry air. Surface dry line Inversion or cap Well-mixed warm air
Trajectories Fundamentally the dry line represents a trajectory discontinuity between moist southerly flow and flow descending from higher elevations. Can only happen relatively close to the upstream barrier (no more than 1000 km) since otherwise air would swing southward behind the low system and thus would be cool and somewhat moist.
Dry Line: Tends to Move Eastward During the Day and Westward At Night After sunrise, the sun will warm the surface which will warm the air near the ground. This air will mix with the air above the ground. Since the air above the moist layer is dry, the mixed air will dry out. The dry line boundary will progress toward the deeper moisture.
Dry Line Warm, Moist Air Hot, Dry Air—Usually Well Mixed Top of moist layer before mixing Boundary after mixing Initial Position of the Dry Line Position of the Dry Line after mixing
Dry Line After sunset, a nocturnal inversion forms and the winds in the moist air respond to surface pressure features. The dry line may progress back toward the west.
WestEast Note weak inversion or “cap” over low-level moist layer east of the surface dry line
Sounding West of the Dryline NCAR Very Dry West Winds Albuquerque, NM 12Z -- 26 June 1998
Sounding East of the Dryline NCAR Moist South Winds Oklahoma City, OK 12Z -- 26 June 1998