Cloud Formation and Precipitation — Page 217, Lesson 184

I want to walk you through the different ways clouds form and the types of precipitation they produce. We're looking at several specific mechanisms, starting with convection cloud. Let's begin with two particular cases of convection cloud. First, there's fair weather Cu — that's fair weather cumulus — which often forms early in the morning. Second, there's large Cu/Cb — large cumulus or cumulonimbus — which often occur later in the day as heating builds up. Now, convection cloud is a heap type cloud — either cumulus or cumulonimbus. It's isolated, meaning it forms as individual clouds rather than a continuous layer. A key characteristic: convection cloud often forms over a particular place, then gets blown away by the wind, and further clouds form over that same place again. So you get a repeating cycle of formation, drift, and re-formation. The critical temperature is the surface air temperature required for the air to be lifted to the condensation level and for cloud to form. In other words, the ground has to heat up enough to trigger the rising motion that produces cloud. The cloud base — the bottom of the cloud — will vary because the temperatures on the ground surfaces vary. Different surfaces heat at different rates. The cloud tops, however, are usually limited by mixing with and evaporating into a drier environment. So the tops end up lower than the limit of uplift — the air could rise further, but the cloud evaporates before it gets that high. If there is turbulence accompanying the convection, then Sc — stratocumulus — can form. The cumulus clouds get spread out by the turbulence to form a layer cloud. Pure convection cloud cannot form over the sea. But here's the important exception: where there is cold air moving over a warm surface, the air will become unstable and convection-type cloud can form. This movement is called advection — the horizontal transport of air. Convection cloud formed over land by surface heating soon dissipates at night because insolation stops — that's incoming solar radiation — and the cloud droplets evaporate. Convective cloud may progress through various cumulus types as it develops. Starting from humilis — shallow, flattened cumulus — then mediocris — medium-sized — then congestus — towering, swelling cumulus — and finally calvus — a cumulonimbus with a smooth, rounded top that hasn't yet formed an anvil. A fully developed cumulonimbus may appear as cumulonimbus capillatus — that's the mature stage with the fibrous, anvil-shaped top. Now let's move to widespread ascent, also called frontal uplift. At a front, there is widespread lifting of air as warm air comes into contact with colder air. Layer-type clouds form in the stable air at a warm front, and heap clouds form in the unstable air at a cold front. Next is convergence cloud. When there is low pressure, there is always convergence at the surface — air flowing inward — which leads to air being lifted. So in depressions and troughs, where there are no actual fronts, cloud formation still occurs. With strong convergence at a trough, lifting can cause instability to develop so that the cloud type is cumulus or cumulonimbus, with possible thunderstorms. This is particularly the case when saturation occurs early, with an average or high ELR — that's the Environmental Lapse Rate, the rate at which temperature decreases with height in the surrounding atmosphere. Note: with circular isobars at a non-frontal low, normally only St/Sc — stratus or stratocumulus — cloud will be formed by convergence. Finally, mountainous areas. We've already seen how orographic lifting — air being forced up over terrain — produces cloud. In mountainous areas, this may be very active and produce extensive cloud and vertical development due to Convective Instability. Additionally, this may increase the intensity of precipitation. So to summarise the cloud formation mechanisms we've covered: convection from surface heating, advection over warm surfaces, frontal uplift at warm and cold fronts, convergence in low-pressure systems, and orographic lifting in mountainous terrain. Each produces characteristic cloud types and precipitation patterns.