A rapid change in moisture with height is called a hydrolapse. A hydrolapse is most well defined in a situation in which differential advection is taking place. Differential advection is the advection of different air masses in different levels of the troposphere. Convective instability exists when the air mass in the PBL is warm and humid while the mid-levels of the troposphere are dry. When air flows from a high elevation area, it tends to stay at the same height. High elevation air that was near the surface, is located higher above the surface once it moves into a region with a lower elevation. The Mexican plateau and high plains are good source regions for mid-level dry air. The dryness of the high elevation areas is important.
By research, it has been determined those years when the Mexican plateau and high plains are drier, the tornado and severe weather season in the spring is more active. If the air is very dry in high elevation regions, it will remain very dry as it advects to the east due to the upper level flow or forcing from a mid-latitude cyclone. A hydrolapse is created at the interface between the mid-level dry air and the PBL humid air.
On a Skew-T sounding, a huge decrease in dewpoint will be noticed in just a short vertical distance. Hydrolapses are very noticeable on days in which a strong dryline is found in the southern plains. To the west of the dryline the troposphere is dry at all levels; To the east of the dryline the PBL is humid and the mid-level air is dry. The dryline, mid-level dry air advection and the amount of PBL warmth and humidity are important to severe weather forecasting.
For more information on convective instability and the hydrolapse go to the Haby Hint below: