Clouds like those in the picture to the right are very commonly observed. They are called ‘cumulus’ because they have a heaped-up form (from the Latin cumulus, meaning ‘heap’). When their vertical extent is small, they are called ‘cumulus humilis’ or ‘fair-weather cumulus’. When their vertical extent is such that cloud width appears to be about the same as cloud height, they are called ‘cumulus mediocris’. When they are taller than that, they are called ‘cumulus congestus’. The tops of cumulus clouds often resemble cauliflowers.
The towering clouds that produce showers of rain, snow and/or hail are called ‘cumulonimbus’ (from the Latin nimbus, meaning ‘rainy cloud’). The tops of these clouds are often anvil-shaped, as shown in the picture below.
Cumulus clouds are manifestations of convection. They form when bubbles of buoyant air rise.
These bubbles, called ‘thermals’, form over ‘thermal sources’ such as concrete, bare sand and rocky hillsides, which become warmer on sunny days than surrounding areas of grass or forest. As the bubbles rise, the air inside them cools, because of adiabatic expansion. The rate of cooling is 9.8°C per kilometre of ascent. If bubbles rise far enough, the air inside them cools sufficiently for saturation to occur, at a height called the ‘condensation level’. Above this level (i.e. inside the cloud), the latent heat that is released when condensation occurs reduces the rate of cooling to about 6°C per kilometre.
The height at which condensation occurs depends upon the temperature and dew-point of the surface air. If this air is moist, the condensation level is low. In the British Isles in winter, it is typically at a height of about 600 metres. On a summer afternoon, it is typically at a height of 1,200 m or more. Over deserts, surface air is often so dry that saturation does not occur at all, even when convection is vigorous enough to lift air to substantial heights.
Convection is controlled by buoyancy relative to a thermal’s surroundings. When ascending bubbles encounter an ‘inversion’, a layer where temperature increases with height, they tend to lose their relative buoyancy and spread out sideways, as shown in the picture to the right.. As cumulus clouds grow higher, their tops become colder. Eventually, when a temperature of about -10°C is reached, the water droplets of the cloud (which are by then supercooled) begin to freeze and become ice crystals. The anvils of cumulonimbus clouds are composed predominantly of ice crystals.
Convection Currents in a Cup of Tea
Instead of tea, we heat a beaker of water. First, we put a small crystal of potassium permanganate in a beaker of water. Then (right), we heat the beaker with a bunsen burner positioned under the crystal. Currents should form in the water. Can you explain these currents? How can you demonstrate convection currents in a cup of tea?
Convection Currents in the Field
Cumulus clouds sometimes occur over the cooling towers of power stations or downwind of them. How do they form? Clue: Large volumes of buoyant moist air rise from the cooling towers of power stations. Cumulus clouds may be seen above forest fires, factory chimneys and other sources of hot air (see picture on the right). Insects are lifted and birds and gliders can soar in the rising air beneath cumulus clouds. How fast does the air rise? Answer: About 1 m/s under a fair-weather cumulus, maybe 4 or 5 m/s under a cumulus congestus cloud. Air can rise at 10 m/s or more beneath and inside vigorous cumulonimbus clouds. What does it feel like to ascend at (a) 1 m/s (b) 10 m/s? Relate to the speed a lift ascends. To work out the speed, divide the height the lift rises when it goes from one floor to another by the time taken to travel that height. Question: Is it safe for birds, gliders or, indeed, any other aircraft to be caught in the upcurrents of cumulonimbus clouds?