With spring severe weather season fast approaching, I thought it would be a good time to review some related material over the coming days and weeks. One thing that forecasters and storm chasers have a "love / hate" relationship with is the Capping Inversion...
If you've been a "weather enthusiast" or lived in the Plains for any period of time, you've undoubtedly heard of the capping inversion (also known as a "cap" or "lid").
No...not that kind of cap or lid... the meteorological kind. This can be a fairly technical subject, but I'll try to keep it simple...
We're all familiar with the age old fact that "warm air rises and cold air falls..." The same concept comes into play when talking about the cap. As long as a parcel of air is warmer than its surroundings, it will continue rising.
Without getting too technical, parcels of air rise up into the atmosphere, condense, and form into clouds, which under the right circumstances can then form into a thunderstorm. The key here is that the air parcels need to be able to rise to the point where they'll condense and, for purposes of this discussion, form into a thunderstorm.
Under a scenario in the atmosphere where there is no cap in place, the air begins cooling as soon as you leave the surface level, and continues to cool steadily as you rise further and further up into the atmosphere. As a result, air parcels may rise steadily up into the atmosphere (remember - warm air rises), condense and form into clouds, which can eventually lead to thunderstorms (assuming that the other necessary ingredients for thunderstorm development are in place on a given day).
When a capping inversion is in place, the air begins to cool right above the surface layer as normal, but at some level (say 5,000 feet above ground in the illustration below) the air begins to warm again, forming the "cap" or "lid" (as shown by the orange and yellow layer in the illustration below):
Again using the above illustration, lets say that a parcel of air is rising away steadily through the first few thousand feet above ground level, minding its own business when suddenly it slams into the "cap" at 5,000 feet. Since the parcel of air is no longer warmer than its surroundings, the rising motion comes to a screeching halt.
Meteorologists use a tool called a sounding* (a plot of temperature, dew point and other variables that come about as a result of weather balloon launches around the world twice daily) in order to look at the atmosphere in a vertical representation:
In the above sounding, the solid red line shows how the temperature is changing with height, while the solid green line shows how the dew point is changing with height above the surface. The temperature lowers from right to left across the bottom of the image. With that in mind, you can see that the temperature initially falls on this sounding (as the solid red lines moves to the left as it goes upward), but at a certain level, begins to move back to the right again (which means the layer is warmer than the layer below it). This is another way to identify the strength and depth of the cap, as noted in the image below (blue circled area):
So, under the capping scenario, the air parcels have ceased to rise, meaning thunderstorms are unable to form. End of story, right? Well, maybe not...
There are a few scenarios under which the cap can be "broken", resulting in thunderstorm development. First, and the easiest (assuming the cap is weak) is that the lowest layer of the atmosphere can be warmed (via sunlight, downslope winds, and/or other local factors) to the point that it becomes warmer than the capping layer again. One way to examine whether or not this can realistically happen on a given day is to look at what's called the convective temperature. To do that, let's take a look back at the same sounding again:
Note the bright green encircled figure on the top of the sounding. "Tc" is the abbreviation for Convective Temperature on a sounding, and on this particular sounding the convective temperature is 91 degrees F. That means, theoretically, if the temperature were to rise to 91 degrees on that day, then convection (another word for thunderstorms) would be able to form.
What if the temperature is only forecast to rise to 85 degrees? No good, huh? Well, generally speaking, yes, unless something else can happen to allow the cap to be broken.
A second scenario for breaking the cap involves cooling the temperature of the capping layer itself (i.e., cooling it back down so that it is cooler than the layer below it again). This can happen as a result of "cold air advection" (i.e., colder air flowing into the capping layer from another direction). The arrival of a cold front would be a typical example of how this could happen. The term "cold air" is relative though, because as long as the air that is being advected into the region is cooler than the air originally in place at the capping layer, it will result in cooling (and therefore weakening) of the capping layer.
A third way that the cap can be broken is for a strong jet stream disturbance (which is often associated with a pool of colder air aloft and/or strong upward motion) to move across the capping layer, thereby weakening it to the point that convection is able to form.
On some severe weather days, a combination of the above scenarios all work together at varying degrees to break the cap and produce severe thunderstorms. On other days, nothing may change, the cap holds and thunderstorms don't form at all...
If you're a severe weather enthusiast, you know that the cap isn't always a bad thing. In fact, it can be very beneficial in preventing widespread rain and thunderstorm activity from going crazy in an area before other factors come into play for severe thunderstorm development later in the day. Kind of a "survival of the fittest" for thunderstorms, if you will...
Some of the biggest severe weather days in history have had a strong cap that held until the "magic moment" took place and all "heck" literally broke loose. Ask a storm chaser, and they'll also tell you that many a "bust" has also occurred where the cap held strong, nothing came along to break it, and not even so much as a puffy cumulus cloud formed in the sky at all that day. Just part of the fun that makes meteorology, and particularly severe weather forecasting, such a challenge!
*If you would like to examine weather balloon soundings for your area, there are hundreds of resources available on the web. The sounding plots that were pictured in this post came from the RAP/UCAR site, located here.
