AIR MASS THUNDERSTORMS AND FROZEN PRECIPITATION AIR MASS THUNDERSTORMS AND FROZEN PRECIPITATION

The air mass thunderstorm is a common and usually non-severe phenomenon that forms away from frontal systems or other synoptic-scale disturbances. They are produced where moist and unstable conditions exist in the atmosphere. Although several storm cells can develop, each individual cell lasts from 30 to 60 minutes and has three stages.

Cumulus Stage: Graph , Picture

- Starts with a warm plume of rising air.

- The updraft velocity increases with height.

- Entrainment pulls outside air into the cloud.

- Supercooled water droplets are carried far above freezing level.

Mature Stage: Graph , Picture

- Heaviest rains occur.

- Downdraft is initiated by frictional drag of the raindrops.

- Evaporative cooling leads to negative buoyancy.

- Top of cloud approaches tropopause and forms anvil top.

Dissipating Stage: Graph , Picture

- Downdraft takes over entire cloud.

- Storm deprives itself of supersaturated updraft air.

- Precipitation decreases.

- Cloud evaporates.

Air mass thunderstorms rarely produce destructive winds or hail because of the absence of vertical wind shear. Vertical wind shear is the change of velocity with respect to height.

Pics and Graphs from Steve Davis WSFO Milwaukee/Sullivan

TYPES OF FROZEN PRECIPITATION

Sleet is frozen raindrops. Sleet occurs when there is a layer of air that is above freezing on top of a sub-freezing layer.

Hail is coalesced frozen rain. It occurs only in thunderstorms.

Freezing rain is liquid water that freezes when it hits the ground. It usually occurs when the temperature is between -3°C and 0°C.

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