BISMARCK, ND (KFYR) – A cap, or a layer of warm air a few thousand feet above our heads, can make or break a thunderstorm forecast in the summer.
A lot of things need to go right for thunderstorms to form through a convective process and some things can get in the way. The basics of how they form starts with the sun heating the ground during the day through radiation, the warm ground then heats the air through conduction, and this warmer air rises through convection. As the warmer air rises, the water vapor within it condenses as the air cools forming clouds. With strong enough rising air, or updrafts, strong thunderstorms can develop.
The more unstable the atmosphere is, the more quickly the air can rise to form taller cumulonimbus clouds and stronger thunderstorms. In this uncapped atmosphere scenario, where the temperature is decreasing quickly with height, warm air can rise freely to form these large updrafts and thunderstorms.
But a cap is a stable layer of the atmosphere several thousand feet up where the temperature increases with height, which can limit how far the warm air from the ground can rise. When a cap is present, it can act like a lid to prevent or delay the development of thunderstorms. When forecasting the possibility of thunderstorms, meteorologists need to determine if a cap is in place and how strong that cap is to create an accurate forecast.
Another way to think of the cap is like the lid on a pot of boiling water. The lid prevents the steam from rising out of the pot, but when you remove the lid the steam can rise freely, similar to how when a cap is overcome the air can rise freely throughout the atmosphere. Another example is a hot air balloon. The air inside the balloon has to be warmer than the air surrounding the balloon in order for the balloon to continue to rise, similar to how the air rising from the ground has to be warmer than its surroundings as it rises through the atmosphere in order to keep going higher up. And if the rising air encounters a layer of warmer air, like a cap, this can prevent it from rising further.
Another way to visualize the cap is with a temperature versus height plot. These types of plots are created when weather balloons are launched, but our computer models can also output these plots to help meteorologists determine the stability of the atmosphere at various heights. In the example below, the temperature decreases with height for the first few thousand feet of the atmosphere as the line on the plot moves to the left, allowing the air to continue to rise. However, when the updraft encounters a layer of warmer air aloft, it’s unable to continue upwards to form larger clouds or thunderstorms. On the plot, this cap is indicated by a jog to the right of the line, indicating a warmer layer of the atmosphere.
But where does the cap come from and why is it more common in the central part of the country? It all starts with the higher terrain of the Rocky Mountains. The sun heats up the higher terrain to the west and winds blow this now warmer air off the high terrain and into the Plains that gradually decrease in elevation from west to east. Due to this lower elevation, the warm air from the Rockies is now several thousand feet above the ground. This is our cap, or layer of warm air aloft! The air underneath the cap isn’t as warm, which is why rising air can’t overcome the cap. For instance, when warm, moist air from the Gulf of Mexico gets drawn into the Plains by southern winds, this spreads underneath the cap at ground level. But with even warmer air aloft, the air near the ground can’t rise far enough to form thunderstorms. We can break or erode the cap if the air near the ground becomes warm enough for updrafts to continue to rise.
An example of a cap preventing thunderstorm development in our area was on June 13, and a special weather balloon was launched at 3 pm to try to get a better idea of what the atmosphere looked like aloft to more accurately predict if thunderstorms would form. Circled on this sounding plot, you can see where the temperature red line moved to the right, indicating where temperatures were warming with height. This is our layer of warm air aloft, or our cap, that was preventing thunderstorm development! Throughout the rest of the atmosphere, the temperatures are cooling with height, as shown by the red line moving to the left as it goes from the bottom to the top of the plot (from the bottom to the top of our atmosphere). The faster this line moves to the left, the more instability there is, as is circled below. But air rising from the ground would have only been able to make it up a few hundred feet before encountering this cap and limiting its potential to rise further to produce thunderstorms and to tap into some of the more elevated instability.
A side-by-side example of a capped versus uncapped atmosphere shows the main differences between them. With a capped atmosphere, CIN, or convective inhibition, is present, which is the shaded blue area on the plot that does not promote rising air — that’s our cap! CAPE, or convective aavailable ppotential eenergy, is present above that cap, but since rising air near the ground can’t get past the cap to tap into that more unstable layer of the atmosphere, thunderstorms, or at least strong ones, are not likely in this environment. On the other hand, an uncapped atmosphere is displayed on a sounding plot with air cooling with height throughout the entire atmosphere. There’s no sudden increase in temperature with height to indicate a cap, so our air that originates near the ground can rise freely, and rapidly, high up into the atmosphere to form strong thunderstorms.
If a cap is in place and we need to overcome it to form thunderstorms, we can do that one of two ways. If our temperatures at the ground increase, this allows our parcels of air, or areas of air, near the ground to start off warmer and rise more rapidly, and therefore they would be closer to the temperature of the air at the height of the cap and can possibly break through it. The other way would be for some kind of lifting mechanism to arrive, like a cold front, and that can act like a nudge to our air near the ground in order to help push it higher up in the atmosphere above the cap. But if the cap remains too strong or too thick, then thunderstorms are not likely. That’s why forecasting thunderstorm development or severe weather is really tricky when a cap is in place. It’s an all-or-nothing bet most of the time because if the cap remains too strong or thick nothing might happen, whereas if the cap can be overcome, a severe weather outbreak is possible. The small subtleties matter!
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