A photograph of the Earth over South America taken from the Space Shuttle (Mission STS 43) on 8 August 1991, showing the double layer of Pinatubo aerosol cloud (dark streaks) above high cumulonimbus tops. Image: Wikimedia Commons
The eruption of Mount Pinatubo on 12 June 1991. Image: Wikimedia Commons
is how will global warming affect both cloud cover and type, since this will have a considerable impact on Earth’s radiation balance? The answer is a very uncertain one. As oceans warm, you would rightly expect increased evaporation to increase the amount of atmospheric water vapour, which condenses to form clouds. This ought to add to albedo and therefore cool Earth, and so it would do if low-level stratus clouds were produced. However, some early climate change modelling experiments produced totally unexpected answers – the models produced a warming effect; a positive rather than a negative feedback. The reason was that increased temperatures produced high-altitude moisture convection, resulting in cirrus cloud formation with low albedo and a strong greenhouse effect. But it is unlikely that only high-altitude cirrus clouds will form, and more likely that low-altitude stratus clouds will also form. New coupled land-ocean-atmosphere general circulation models (GCMs) are beginning to link cloud formation and type with the degree of water vapour saturation, cloud condensation nuclei and aerosols, including important, but very poorly understood, physical interactions at the scale of water droplets and aerosol particles. Early models did not include these interactions, which accounts for the many past errors and uncertainties. The newest models are improving, but they are exceptionally complex and require years of supercomputing time to produce results. So for the moment at least, clouds remain enigmatic and perhaps best summed up in the words of the 1980s singer and song-writer, Joni Mitchell: ‘I’ve looked at clouds from both sides now From up and down, and still somehow It’s cloud’s illusions I recall I really don’t know clouds at all.’ Q Associate Professor Mike Lucas is employed within the University of Cape Town’s Zoology Department. He is also an Honorary Research Associate at the National Oceanography Centre (NOC) in Southampton, UK. He conducts much of his research in the North and South Atlantic, as well as in the Southern Ocean and in the Benguela upwelling system.
A spectacular thunderstorm underway. Image: Wikimedia Commons
Thunderclouds and lightning The start of a thunder and lightning storm is characterised by the formation of tall, anvil-shaped cumulonimbus thunderclouds. Thunderstorm clouds result from rapidly rising warm, moist air, driven by the heat of the sunbaked earth. As the warm, moist air rises, it cools, condenses, and forms anvil-shaped cumulonimbus clouds that can reach heights of over 20 km – as high as passenger jets fly. As the rising air reaches its condensation (dew) point, water droplets and ice form and begin falling through the clouds towards Earth’s surface. As the droplets fall, they collide with other droplets and become larger, creating a downdraft of cold air and moisture that spreads out at the Earth’s surface, causing the strong winds commonly associated with thunderstorms. Thunderstorms can result in many severe weather phenomena, including high winds, large hailstones, and flash flooding caused by heavy rainfall. The lightning often associated with thunderstorms is an electrical discharge or bright lightning bolt emanating from the clouds. Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by super-cooled water droplets colliding with ice crystals. When a large enough charge is built up, a lightning discharge will occur. The temperature of a lightning bolt can be five times hotter than the surface of the sun. Although the lightning is extremely hot, the duration is short and 90% of strike victims survive. Contrary to popular opinion that lightning does not strike twice in the same place, some people have been struck by lightning more than once, and tall buildings and towers can be struck several times during the same storm. This occurs because lightning travels the shortest distance it can between its origin in the clouds and the ground. Consequently, this is the reason why it’s a bad idea to stand under a tree during a lightning storm. If caught out in a lightning storm, find the lowest ground you can and lie down flat, or preferably, find a building for shelter, but stay away from walls and windows. The loud thunderclap heard with lightning is due to the super-heated air around the lightning bolt expanding at the speed of sound. Because sound travels much more slowly than light, the flash is seen before the bang, although both occur at the same moment. The two most common types of lightning are firstly ‘sheet lightning’, where the lightning jumps from cloud to cloud and does not shoot down to the ground, and secondly so-called ‘forked lightning’, which strikes the ground.
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