ATPL · Meteorology · Meteorology Atpl Ground Training Series 2014 lesson — Page 199, Lesson 172

I want to walk you through the second half of the cloud chapter, focusing on two very important cloud types: cumulus and cumulonimbus. These are the clouds that really matter to us as pilots, especially when it comes to weather hazards. Let's start with cumulus clouds. I want you to picture a cloud that looks like the head of a cauliflower — that's the classic description. The parts of a cumulus cloud that are hit by sunlight appear brilliant white, but the bases are relatively dark. That dark base is a visual clue you'll learn to read. Now, here's the key distinction: cumulus clouds of small vertical development can look benign — harmless, almost friendly. But don't be fooled. They can grow rapidly when the atmosphere is unstable and there is no upper-air inversion to cap their growth. An inversion would normally act like a lid, stopping the cloud from growing upward. Without that lid, a cumulus cloud can develop into a cumulonimbus cloud, with its top reaching all the way up to the tropopause — the boundary between the troposphere and the stratosphere. For a typical cumulus cloud, the cloud base is usually between 1000 feet and 5000 feet above the ground. But here's an important relationship: that base height increases as the surface temperature increases. Warmer ground means the air has to rise higher before it cools enough to condense, so the cloud base lifts. Now let's move to cumulonimbus — abbreviated as Cb or CB. This is the cloud that the aviator should avoid. I cannot stress that enough. Cumulonimbus clouds consist of vigorous convective cloud cells of great vertical extent. That means they are powered by strong rising air currents, and they tower up through a deep layer of the atmosphere. The upper parts of a cumulonimbus cloud are made of supercooled water droplets and ice crystals. Supercooled water is liquid water that remains liquid even below freezing — and that's a major icing hazard. The base of a cumulonimbus cloud is often very dark, with ragged cloud appearing beneath the main cloud cell. The cloud base is usually between 2000 feet and 5000 feet. Now let's talk about the hazards, because this is where the professional pilot knowledge really matters. The risk of icing and turbulence associated with cumulonimbus is moderate to severe. Within the cloud, very strong upcurrents and downdraughts are continually at play. These vertical winds produce severe precipitation in the form of showers of rain and hail. But it doesn't stop there. Other hazards include lightning and static discharge. Static discharge can lead to airframe damage, erroneous instrument readings, and squally winds — those are sudden, violent gusts. What conditions are necessary for cumulonimbus to form? You need moist unstable air throughout a deep layer of the atmosphere. That instability is what fuels the vigorous convection. You also need a trigger mechanism to kick off the convection process, especially for isolated, heat-type cumulonimbus — those are the ones that form on a hot afternoon when the sun heats the ground enough to start the air rising. That figure shows you what a cumulonimbus cloud looks like — you'll see the towering shape, the dark base, and the anvil top that often spreads out at the tropopause. Keep that image in your mind, because recognizing a cumulonimbus from a distance is a survival skill in aviation.