Convective Clouds

Thunderstorms are convective in nature. This means that they are formed by the vertical transport of heat and moisture in the atmosphere. However, just because there is convection doesn’t mean that a thunderstorm will result. In fact, the Pacific Northwest commonly experiences convection, especially in the wake of cold fronts, and thunderstorms are very rare here. First, we’ll learn about some basic convective clouds known as cumulus clouds, and then we’ll learn about cumulonimbus clouds and thunderstorms.

There are four types of cumulus clouds: Humilis, mediocris, congestus, and fractus. Humulis are small and harmless, mediocris are slightly larger, congestus (also known as ‘towering cumulus’) reach high into the sky, and fractus are cloud fragments that have broken off from other clouds. For now, let’s just talk about humilis and congestus.

Humilis are known as “fair-weather cumulus,” because they are pretty short and fluffy due to a stable atmosphere overhead preventing them from growing them much further.

Cumulus congestus, on the other hand, are the cumulus that have the potential to grow into cumulonimbus clouds that have the potential to produce vivid lightning, large hail, and even tornadoes. Cumulonimbus is a conglomerate of the Latin words cumulus (heap or pile) nimbus (water). Smack these two words together, and BAM!; you got a heapish cloud that rains. Some people will tell you that cumulonimbus and thunderhead are synonymous, but this is not true. Just because a cloud is a cumulonimbus does not mean that it is producing lightning.

The atmosphere above their LCL (lifting condensation level) is much more unstable, as as a result, the air parcel visibly denoted by the cumulus congestus cloud is a lot more buoyant.

Take a look at the two skew-T plots below. The one on the left is for humilis, and the one on the right is for congestus. Notice how much more unstable the congestus skew-T is.

Here are some photos of humilis and congestus. Can you guess which is which?

The top one is humilis, and the bottom is congestus.

I like to refer to these clouds as cotton-ball and cauliflower clouds, respectively. Cotton-ball cumulus are an indicator of fair weather. See how the tops do not rise very high? This is because the air is fairly stable and does not allow cloud-producing updrafts to rise very high into the atmosphere. With cauliflower clouds, the atmosphere is unstable and has a high environmental adiabatic lapse rate (it cools sharply with elevation), so parcels of air near the ground are quite buoyant and tend to rise. It’s these cauliflower clouds that lead to cumulonimbus clouds.

Below are the three stages of cumulonimbus development: a rising cumulus congestus cloud, a mature cumulonimbus with updrafts and downdrafts, and a dissipating storm with the updraft choked off. We’ve gone over the towering cumulus/cumulus congestus/cauliflower cumulus clouds, so let’s go over the mature cumulonimbus cloud.

If a cauliflower cumulus keeps getting bigger and bigger, it will turn into a cumulonimbus cloud. There are two types of cumulonimbus clouds: Cumulonimbus calvus and Cumulonimbus capillatus. Calvus have a puffy top and look similar to a towering cumulus cloud, but they are even larger. A mega-cauliflower cloud, if you will. Capillatus have wispy, cirrus-like tops.

Cumulonimbus capillatus have a subtype: Cumulonimbus capillatus incus. The incus means that the wispy tops of the clouds are spread out and bear resemblance to the classic ‘anvil’ shape that we associate with big thunderstorms. Compare this…

… to this…

This anvil marks the beginning of the tropopause – the region in which the troposphere is transitioning to the stratosphere. Whereas the temperature of air decreases in height in the troposphere, it increases with height in the stratosphere due to heat given off in ozone-forming chemical reactions. Because air becomes colder and thus denser when it rises, it cannot rise very far through the stratosphere as gravity simply pulls it down back to a place where it is just as dense as the atmosphere around it. Capillatus with especially strong updrafts often have what is called an “overshooting top,” which is a well-defined ‘dome’ cloud created by the momentum of the air parcel within the updraft thrusting the cloud into the stratosphere until it can go no further.

Overshooting tops are rare here in the Northwest, but I have seen them. The last one I can remember seeing was associated with a thunderstorm by Mt. Rainier on July 29, 2009 – the warmest day on record in Seattle. That storm spawned a flash flood warning from the NWS.

Here’s a picture with both cumulonimbus calvus and cumulonimbus capillatus. Can you make out the differences?

Anyway, let’s get back to the characteristics of a mature cumulonimbus cloud.

The towering cumulus cloud was formed solely by updrafts. A mature cumulonimbus has not only an updraft to continue supplying fresh air to the storm but a downdraft through which precipitation in the form of rain, hail, and even snow falls. One would think that the temperature under a downdraft would be warm due to adiabatic warming associated with sinking air, but the temperatures under downdrafts are quite cool, especially when the storm is strong. Remember, it takes an unstable atmosphere with a sharp decrease in temperature with height to form a cumulonimbus cloud, so the tops of these clouds are often exceptionally cold. To make matters even more frigid, the air can be cooled further by hail falling through the downdraft and the sublimation of ice crystals into water vapor.

Because this air is cool, it is more dense than the surrounding atmosphere. In some cases, this characteristic can be deadly. Downbursts are rapidly falling parcels of air that can cause massive destruction over an area they impact. They are very dangerous to airplanes because of the extreme turbulence, and since airplane pilots generally encounter them in the lower atmosphere where they are taking off or preparing to land, the stakes are even higher. After they hit the ground, they spread horizontally in all directions. Often times, the damage in tornadic storms is higher from these straight-line winds than the tornado itself.

Updrafts can also cause extreme winds. A massive thunderstorm producing prodigious amounts of rain and hail needs a lot of energy to support it, so air nearby won’t waste any time flowing into the updraft. These intense updrafts can cause tornadoes by shoving rotating air within the storm upright.

In addition to the overshooting top, thunderstorms have other characteristic features. Some have mammatus clouds, which form when ice crystals under the anvil sublimate and cool the air (the transition from ice to gas takes energy). This cold air sinks in little pockets and finally stops when all the ice crystals sublimate. These clouds are often indicative of a strong thunderstorm, as they can only be supported by a very moist updraft.

Two arcus clouds, shelf clouds and roll clouds, are also associated with thunderstorms. Arcus clouds are long, low-lying, and horizontal clouds which often form on the leading edge of thunderstorms, although roll clouds can be found detached from any sort of separate cloud or weather system.

Shelf clouds are ominous, wedge-shaped clouds that form on the leading edge of a thunderstorm. These form because cold air from the downdraft of a storm spreads horizontally when it hits the surface. This outflow of air originating from within the downdraft undercuts and lifts the warm, moist air rising into the storm upwards.

Shelf clouds form ahead of the “gust front” of a storm; the boundary between the warm, moist air from within the boundary layer and the denser, evaporatively-cooled downdraft from the storm itself. This can be thought of as the “leading edge” of the storm and an advancing gust front is a great example of a “gravity current”, which is where a mass of high-density fluid flowing along a horizontal bottom displaces a fluid of lower density.

If there is enough wind shear due to differences in wind speed with elevation, the opposite directions of the air going out of and into the thunderstorm will “spin up” a roll cloud just behind the gust front.

Here is a more technical diagram of how gust fronts (and associated shelf clouds) and arcus clouds work.

Put these all together, and you’ve got a typical thunderstorm! Not all thunderstorms have overshooting tops, mammatus clouds, and arcus clouds… those only occur in strong storms or supercells. Here’s an average thunderstorm below.

Cumulonimbus clouds dissipate when the updraft is choked off. If no new air is coming into the storm, the storm cannot support itself. This is usually because the downdraft has overtaken the updraft. The cloud will take on a fuzzy appearance, the wind and rain will lighten, and the skies will clear.

We’ve talked about the different types of clouds. Now, let’s have a very brief review of the environmental controls for the formation of deep convection and thunderstorms.