The boundary between warm, humid air and hot, dry air. It is therefore distinct from a front, but has inherent instability, because humid air has a lower density than dry air (at the same temperature and pressure). The term is particularly used in North America for the boundary between humid air from the Gulf of Mexico on the east and dry desert air on the west. Normally lying approximately north–south over the Midwestern states, the dry line often advances eastwards in the day and retreats westward at night. Severe thunderstorms and tornado activity frequently develop along the dry line or immediately to the east of it.
Subjects: Meteorology and Climatology.