A dry line is an imaginary line across a continental landmass separating moist air from an eastern body of water and dry air from the western interior. Dry lines occur in central North America, most notably in Texas, Oklahoma, and Kansas (especially in the spring and summer months), where moist air from the Gulf of Mexico contacts dry air from the desert southwest. A dry line typically moves eastward during the afternoon and retreats westward at night. In the dry west sector of a dry line, the air is typically clear, but if the associated winds are strong enough, dust storms may result. Cumulus and cumulonimbus clouds are common in the moist eastern sector of a dry line, but have their greatest development along the dry line itself. The passing of a strong dry line results in a sharp drop in humidity, clearing skies, and winds shifting from south or southeast to west or southwest.

Cumulonimbus clouds are dense, towering vertical clouds associated with atmospheric instability along a dry line, and are formed from water vapor carried by powerful upward air currents. The base of a cumulonimbus cloud may extend for several miles at altitudes of 700 -10,000 feet. The peaks of the cloud typically reach up to 20,000 feet, but can be as high as 75,000 feet. Well-developed cumulonimbus clouds are characterized by a flat, anvil-like dome caused by wind shear. The anvil may precede the main cloud’s vertical component for many miles. Cumulonimbus clouds can produce dangerous weather including intense rain, high winds, hail, lightning, and tornadoes. In the photograph, Midland County is on the western dry sector of a dry line, but Howard County is on the moist eastern sector.