Atmospheric “cap” can prevent severe storms


Storm clouds at MoPac and 45 (KXAN/Todd Bailey)

It’s springtime in Central Texas, which means severe weather season. Flash flooding, hail, damaging winds and even tornadoes are all possible this time of year when all the right ingredients come together. But were you aware that even when all the ingredients are there for strong to severe storms, there is just one thing that can prevent them from forming?

Ingredients for a severe thunderstorm are quite lengthy. But generally speaking, you need warm/moist air at the surface, cold/dry air in the upper levels of the atmosphere, a lifting mechanism (cold front, dry line, higher elevation), and wind shear (change of wind with height).

Ingredients for a thunderstorm / Graphic courtesy of: NWS

The one caveat that puts an abrupt stop to storm development is called an “upper air temperature inversion”. You might hear the KXAN Weather Team refer to this as a “cap” on the atmosphere. This happens when high pressure aloft forces air to sink above the surface level of the earth. As air sinks, it warms and in this case causes the air to be warmer than the air at the surface.

Under normal conditions, as radiation from the sun hits the surface at short wavelengths it warms the surface of the earth. Some of it gets absorbed but most is radiated back into space at lower wavelengths causing it to cool the higher you go in the atmosphere. When this happens, storms that develop can grow in intensity incredibly fast as they have the entire troposphere (area of development for storms in the atmosphere) to develop in.

Supercell diagram / Graphic courtesy of: NWS

However, a temperature inversion will be sandwiched in the troposphere between the relatively cooler temperatures at the surface and the colder temperatures in the upper troposphere. Since storms need cooler and cooler air to grow and rise into, they are denied that with a temperature inversion, and thus storms can either develop but quickly dissipate, or not even form at all.

Temperature inversion / Graphic courtesy of: AgFax

As the relatively cooler air bounces off the bottom of the inversion, it is unable to mix out in the atmosphere. As a result, on days where there is an inversion, smog tends to form over cities or highly concentrated areas of pollution like power plants or oil refineries.

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