What is a trunk line?

The dry line, also known as the dew point front, is a discontinuous line of humidity in the horizontal direction. It is a narrow zone with a strong horizontal gradient of specific humidity (or dew point), mainly on the ground or 850hPa surface. The temperature stratification on one side of the air is close to the dry adiabatic lapse rate, and the other side is often warm air with low-altitude temperature inversion. The wet-dry transition zone has a vertical structure that is almost flat upward. When it converges with the low air-scale airflow, cumulonimbus cloud bands often appear on the main line, and some develop into convective storms. It is therefore a mesoscale weather system triggering mechanism. The dry line originally refers to the Great Plains region east of the Rocky Mountains in the United States. There is warm and dry air on one side and cold and moist air on the other side. When passing through the dry line, the horizontal dew point temperature changes drastically. The dew point temperature on both sides of the main line can differ by more than 14oC/500km.