![]() This outflow of air originating from within the downdraft undercuts and lifts the warm, moist air rising into the storm upwards. These form because cold air from the downdraft of a storm spreads horizontally when it hits the surface. Shelf clouds are ominous, wedge-shaped clouds that form on the leading edge of a thunderstorm. Shelf cloud over Enschede, The Netherlands. Often times, the damage in tornadic storms is higher from these straight-line winds than the tornado itself. After they hit the ground, they spread horizontally in all directions. 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. ![]() Downbursts are rapidly falling parcels of air that can cause massive destruction over an area they impact. In some cases, this characteristic can be deadly. 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.īecause this air is cool, it is more dense than the surrounding atmosphere. 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. 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. 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. The towering cumulus cloud was formed solely by updrafts. We’ve gone over the towering cumulus/ cumulus congestus/cauliflower cumulus clouds, so let’s go over the mature cumulonimbus cloud.ĭiagram of a typical thunderstorm. It’s these cauliflower clouds that lead to cumulonimbus clouds.īelow 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. 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. 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. Cotton-ball cumulus are an indicator of fair weather. I like to refer to these clouds as cotton-ball and cauliflower clouds, respectively. The top one is humilis, and the bottom is congestus. Taken at Cahokia Mounds Museum and Interpretive Center. Notice how much more unstable the congestus skew-T is. The one on the left is for humilis, and the one on the right is for congestus. Take a look at the two skew-T plots below. 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. Just because a cloud is a cumulonimbus does not mean that it is producing lightning. Some people will tell you that cumulonimbus and thunderhead are synonymous, but this is not true. Smack these two words together, and BAM! you got a heapish cloud that rains. ![]() 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.Ĭumulus 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). For now, let’s just talk about humilis and congestus. ![]() 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. There are four types of cumulus clouds: Humilis, mediocris, congestus, and fractus. Convective clouds are clouds that are formed by convection, which is simply the process of warmer air rising since it is less dense than the surrounding atmosphere. First, we’ll learn about some basic convective clouds known as cumulus clouds, and then we’ll learn about cumulonimbus clouds and thunderstorms.
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