METEOROLOGIST JEFF HABY
The cap is a layer of warm air aloft that resists the mixing between planetary boundary layer air and air in the middle and upper
troposphere. It is also called a lid and an elevated warm layer. The cap is a weather changer since when it breaks the weather
can go from warm and nice to very stormy. This transition can occur in the course of minutes.
Energy and moisture can build under the cap during the day. The cap helps contribute to clear or partly cloudy skies. This allows solar
energy to build in the planetary boundary layer and at the earth’s surface. Moisture can build up also as moisture advects in from a
moisture source. This energy helps to erode the cap during the day. If the cap is strong enough, it will not break. If just enough
energy can build up, the cap will explosively break like Mentos placed in a can of diet coke. The explosive convection can lead to
severe weather such as strong straight line winds, hail, torrential rain and tornadoes.
Triggers such as fronts, dry lines, convergence boundaries, upper level divergence, influx of moisture, and low level warm air advection
can combine with sunshine to help break the cap. The most likely time of the day for the cap to break is from about noon and into
the afternoon. A cap breaking in the afternoon can sometimes lead to stronger storms since the energy has had more time to build
up during the day. The cap, a huge potential weather changer, is monitored by weather forecasters closely since it has such a
dramatic influence on the forecast.