Putting a ‘cap’ on severe weather

(WRIC) — Sometimes all the right pieces can be in place for a severe weather event to thrive: temperatures in the mid 90s (hot air), dew point temperatures in the 70s (moisture), strong southwest winds (transferring more hot humid air into the region), high CAPE (Convective Available Potential Energy), abundant instability (buoyancy), and an approaching disturbance.

There are cases that despite the pieces falling into place, the impending severe weather doesn’t occur.

The warm moist air at the surface rises, trying to reach what we call the level of free convection. This would allow it to freely tap into the energy in the atmosphere, allowing thunderstorms to erupt. However, it then runs into nature’s version of a pressure cooker lid. The lid prevents the steam below to rise. If the lid were removed, the steam would rush out.

This lid separates the warm moist air below from the cool dry air above. If the two were to meet, severe weather would ensue.

This layer, a cap, is a layer of temperature inversion. The temperature within this layer is increasing with height and is warmer than the air below. This prevents the moist air at the surface to move past it and stifles severe weather.

Capping can occur when any of the following air sinks at higher levels of the atmosphere, if a shallow cold front moves into a region, surface temperatures cooling down overnight, or when warm air moves in at higher levels of the atmosphere. It creates a situation where there is cooler air at the surface than in this layer.

Strong caps completely nullify severe weather from forming. If that cap breaks, an explosive event of thunderstorms can occur, capable of producing hail and strong winds.

A good example of this was in the summer of 2010 when Central Virginia was scorched with the hottest summer on record. 3 weeks in a row, a strong cap would rupture around 5 p.m. and led to severe thunderstorms all over Central Virginia.

If the cap is initially weak, then instability wouldn’t grow below and this would lead to a reduced threat of severe weather.

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