From the devastating effects of tornadoes and typhoons to deciding the best day for a picnic, the weather impacts our lives on a daily basis. Despite new techniques and technologies that allow us to forecast the weather with increasing accuracy, most of us do not realise the vast global movements and forces which result in their day-to-day weather. Storm Dunlop tells us ten things we should know about weather in its most dramatic and ordinary forms.
Reports on TV, radio, and the newspapers often describe British ‘tornadoes’ that cause extensive damage. But these are not true tornadoes, which are much stronger and exceptionally rare in Britain. These events are more properly called ‘landspouts’, a term introduced because the phenomena are closely related to waterspouts – waterspouts become landspouts if they come ashore.
These whirls arise when there are powerful updraughts and downdraughts within moderately common clouds. True tornadoes form differently within vast thunderstorm systems that are organised into giant rotating columns of air.
2. Ozone hole
Ozone (O3) is a harmful gas, unwanted at ground level, but forms naturally in the high atmosphere, where it protects the surface from dangerous ultraviolet radiation from the Sun. Certain man-made chemicals destroy the ozone layer and produce ozone ‘holes’ Luckily, international agreement (the Montreal Protocol) governs the elimination of the harmful chemicals and there are signs that the ozone holes are decreasing.
3. Polar vortex
A polar vortex is a system of high-speed winds that surrounds the pole in each hemisphere, and isolates the polar region from air closer to the equator. The southern polar vortex is particularly strong, and is the main reason for the large size of the southern ozone hole. Warm air from middle latitudes cannot penetrate into the polar region. (In the north, the polar vortex is more variable, and any ozone hole is less pronounced.)
When the northern polar vortex is strong, cold air is confined to the polar region. When the winds weaken, the location of the northern polar jet stream becomes greatly variable, with waves extending to lower latitudes, so frigid arctic air spreads down to temperate regions.
4. Greenhouse effect
People sometimes regard the greenhouse effect as a ‘bad thing’, but without the greenhouse effect we would not be here. The atmosphere acts as a giant ‘blanket’, keeping the Earth warm with an average surface temperature of about 14°C. Without the atmosphere, the surface temperature would be around -18°C, much too cold for life.
5. Butterfly Effect
The Butterfly Effect is often quoted (wrongly) as illustrating how small events may have far-reaching consequences. The famous phrase ‘Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?’ was chosen by a conference organiser. The author, Edward Lorenz, actually discussed the impossibility of being absolutely certain of the outcome of any calculations about the weather: that there are limits to predictability. The technical term is ‘Sensitive-dependence on initial conditioning’ and the effect applies to many other areas of science. That original title could have been ‘Predictability: does the flap of a butterfly’s wings in Brazil prevent a tornado in Texas?’
6. Jet streams
Jet streams are narrow currents of high-speed winds in the upper atmosphere at altitudes of about 10 km, thousands of kilometres long, hundreds of kilometres wide, and a few kilometres deep. They have significant effects on depressions at the surface, which they may intensify or weaken. Waves in the jet streams greatly affect the weather at the surface and may occasionally lead to ‘blocking’ situations, where high- or low-pressure systems (and the accompanying weather) remain stationary over an area for a long time.
7. Atmospheric rivers
Streams of very warm, humid air, called ‘atmospheric rivers’ transport vast quantities of warmth and moisture from tropical regions towards temperate zones. When atmospheric rivers strike mountainous regions they may produce vast quantities of rain. It was an atmospheric river that caused the downpour and flooding in Cumbria in 2009, and another has caused the recent flooding in California.
The pressure in the centre of a depression occasionally drops dramatically. If it decreases by more than one hectopascal (one millibar) per hour for 24 hours, the system is known as a ‘bomb.’ One bomb occurred on 8 January 1993. By 10 January, it was the deepest depression ever recorded over the North Atlantic. The weather forecast that day remains unique, with the words: ‘Rockall, Malin, Hebrides, Bailey. South-west Hurricane Force 12 or more.’
The wind and waves from this storm completely ruptured the tanker Braer, driven ashore on Mainland, the largest island in the Shetlands, by an earlier storm. The oil caused an environmental disaster. The violent system is now known as the ‘Braer Storm.’
9. Centres of action
Nearly everyone is familiar with the terms ‘Azores High’ and ‘Icelandic Low.’ Technically, these are ‘centres of action’ in that they are semi-permanent areas of high or low pressure that are important features in the global circulation of air.
The Azores High is nearly always present, and often extends ridges high pressure outwards, usually bringing fine weather to Britain and western Europe. The Icelandic Low (rather like the corresponding North Pacific Low) is not so permanent, but gains its name from the fact that low-pressure systems (depressions) frequently pass across the area.
When Bernard Shaw’s play Pygmalion was turned into a film, a language exercise for Eliza Doolittle was ‘In Hampshire, Hereford, and Hertford, hurricanes hardly ever happen.’ But hurricanes never happen in Britain. The reason? Simple: the sea is too cold. For hurricanes to form, the sea-surface temperature needs to be at least 27°C. That only happens in the tropics. The technical term for hurricane, typhoon, and cyclone is ‘tropical cyclone.’
Featured image credit: Supercell Chaparral New Mexico by tpsdave. Public Domain via Pixabay.