I promise you won’t need to pull up a comfy chair and beverage for this one.  I’ll keep it fairly brief!  You hear the weather folks using this term all the time…the “atmospheric cap” or “capping inversion”.  So what is it?

The cap is a region of warmer air over a layer of cooler air that keeps a lid on convective lifting and can inhibit thunderstorm development.   Remember from Thunderstorm 101, warm air from the earth’s surface wants to rise and will continue to rise as long as it’s warmer than its surroundings. Thus, when it reaches a layer of air equal to or greater than it’s temperature at that height, it will no longer rise.  Convection, or cloud formation, is then thwarted and unable to develop any further without the aid of other lifting mechanisms such as an approaching frontal boundary or surface heating that exceeds that of the cap.

What causes caps to form?  There’s quite a few, but the loss of daytime heating from the earth’s surface is one primary cause.  The earth’s surface cools faster than the air above, so you will have warmer air aloft most nights and mornings.  Once the sun has a chance to heat the earth’s surface, that heat is reflected back up into the atmosphere heating it as well.  As a result, the cap is typically strongest during the night and early part of the day, and weakest in the afternoon during peak daytime heating.  That’s when it’s most likely that the cap will break and enable storms to form if there is enough moisture and instability in the atmosphere.   Another primary cause is warmer mid-level air masses moving over the cooler ground level air masses.  Winds can move across us from different directions at different levels in the atmosphere, and one prevailing wind we have in Texas each summer at the mid-levels is a southwest or westerly wind which brings in that warmer layer of air from the arid regions of Mexico and our southwestern states.

A cap is considered “weak” or “breakable” when there is only a small temperature increase between the air aloft and the air at ground level.  Conversely, a cap is strong when there is a large temperature increase in the air aloft over the air at ground level.  The cap can be the difference between quiet weather and a severe weather.  If the cap HOLDS, it will be a quiet weather day.  However, if the cap weakens or BREAKS, daytime convection could explode and produce damaging thunderstorms.