Cap Questions

[Rean Clover]

I'm going to sound stupid in asking, but what causes a cap to hold or break?

Looks like the cap is going to hold.

 

[Chris Hayes]

The cap is a warm layer of air. For warm air to rise it needs air that is cooler than it above it. If it runs a layer of air that is it's temp, or higher, it can no longer rise. Now, when it comes to the cap eroding, I'm not to familiar with that. Extreme forcing can cause the parcels of air to break through the cap. For best chances of supercells you want a strong but breakable cap. Strong enough to prevent a lot of storms from developing, but weak enough to break in spots. It's a fine line.

 

[Patrick Marsh]

I'm going to sound stupid in asking, but what causes a cap to hold or break?

I actually just answered this via email for someone, so I'll copy, paste, and modify what I said in my email. However, first let me say that forecasting if the cap will hold or break (in other words, thunderstorm initiation) is one of the most difficult things to forecast. I recommend that you read http://www.jondavies.net/700mbTcap/700mbTcap.htm as it contains some really good information.

What the cap comes down to is that in the absence of all other forces, warm air rises and cold air sinks. So, if you have a layer of warm air aloft (what we refer to as the cap), the air at the surface won't rise on it's own. You either need to heat the air at the surface so it becomes warmer than the air in the cap layer, cool the warm layer aloft, or forcibly lift the surface air through the warm air. In a nutshell you need one (or some combination of) the following: lift, low-level convergence, and/or strong surface heating.


Cooling the Cap (Lift)
It is often said that you need cold-air advection (CAA) to cool/weaken the cap so that you can get thunderstorm initiation. This is *not* the case. Without going into all the thermodynamics behind it, CAA tends to result in a sinking motion, which is exactly what we do *not* want, and warm-air advection (WAA) results in rising motion. (I should point out the subsidence we see in the wake of shortwave troughs often has a CAA component to it.) Thus we need another mechanism to cool the cap.

Again, without getting into the physics of it, when you lift an atmospheric layer, and you don't add heat, you actually cool the layer through a process known as adiabatic expansion. Thus, if you have strong vertical motion throughout the layer containing the cap, you lift the warm layer, which in turn cools it. This is why on severe weather days you can see an inversion (cap) at one layer in the morning and then it is at a higher layer in the evening...it was lifted during the day, and ultimately cooled.


Low-level Convergence
Another way to "break the cap" is to actually have something physically lift the surface air through the warm layer. The most obvious source of this kind of lift is a mountain. If you have wind blowing into a mountain, it will have to go up the side mountain (since it can't go into the ground!). If the surface wind is strong enough, it can force the surface air up along the mountain, through the cap. This is why you get a lot of thunderstorms along the Rocky mountains during the summer.

Another way you can force surface air through the cap is to have really, really strong low-level convergence (winds coming together). This is similar to the mountain in that if I have wind from different directions hitting each other, the air most go up because it can't go through each other nor can it go into the ground. Thus, if the low-level convergence is strong enough, the atmosphere can force surface air up through the cap. This is often the case along cold fronts and other surface boundaries.


Surface Heating
Another way to break the cap is to get the surface temperature so warm that it stays warmer than the cap even after lifting (and cooling) it. To get an approximation of this temperature, take a skew-t (sounding) and find the warmest temperature in the cap. Then move down toward the ground along a dry adiabat. When you intersect the surface pressure (the pressure at which the temperature line starts) this is approximately the temperature needed for the atmosphere to become absolutely unstable (and auto-convect). In other words, this is the temperature you have to reach in order for the surface air to make it through the cap on its own. Now, this doesn't guarantee a storm, because there is a chance that the surface air might dry sufficiently through warming that much that there isn't enough moisture for deep convection. If this is the case, the atmosphere might begin to "convect" (e.g., air rising), but there just isn't enough moisture for a thunderstorm to develop...or you might get extremely high based thunderstorms.


As for knowing if the cap will hold or break? It just takes experience monitoring all of this. Even then, it can be extremely difficult to predict correctly.