Thunderstorms Flashcards
Thunderstorm definition
a rain bearing Cb cloud that produces one or more electrical discharges (lightning)
Requirements for TS development
Trigger mechanism - convection, widespread ascent (convergence, troughs, frontal lifting), or orography. Results in unstable saturated air being lifted.
Adequate supply of low level moisture - determines height of cloud base and provides fuel to drive the growth. Rising saturated air is subject to less pressure and cools/condenses, releasing latent heat into the atmosphere promoting further instability and vertical uplift driving more saturated air aloft. Becomes self feeding.
Conditionally unstable atmosphere - DALR>ELR>SALR, dry air is stable and saturated air is unstable, trigger only lifts saturated air providing the latent heat needed for development.
Entrainment
During the mature stage of TS development the downdraughts pull in colder, drier air from the atmosphere surrounding the cloud. Resulting in evaporation and cooling within the cloud, decreasing instability and buoyancy, increasing downdraughts. Eventually this process leads to the decay of the thunderstorm.
The dry air left behind however now has a higher relative humidity and subsequent vertical lifting in the area will come with entrainment which is less effective at decaying the TS than it originally was due to the increased moisture content. This is called “pumping” and results in a TCu/Cb growing much taller.
Latent heat
Latent heat is the key driver of TS size, severity, and extent after it is initiated. The condensation of WV as the air rises and is subject to lower atmospheric pressure is an exothermic diabatic phase change releasing latent heat into the atmosphere. This heat provides an internal trigger and instability source which accelerates the updraughts resulting in further condensation and latent heat and hence the process becomes self feeding.
