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2.The birth of storm clouds

You may be surprised to learn that the actual temperature at the earth’s surface is not crucial; in the UK thunderstorms occur in January as well as July. However, thunderstorms in winter are normally short-lived and produce, perhaps, one or two rumbles of thunder. Summer storms can last a long time and produce impressive lightning displays. This difference is caused by the surface temperature contrast between summer and winter. Warm air has the capacity to hold much more water in the form of vapour and a third requirement for the formation of thunderstorms is that the air be moist. Therefore, a summer thunder cell holding much greater quantities of water has the potential to produce more violent storms.

The final requirement is that the winds at upper levels should be light. Strong winds will tip the developing clouds sideways, encouraging their moisture to fall out as precipitation. Very strong winds may even tear the cloud top away from the lower portion. A summary of the atmospheric conditions necessary for the formation of thunderstorms is shown below:

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  Unstable air through great depth of atmosphere.
  

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  Air to be moist.
  
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  The cloud top to be colder than 0ºC.
  
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  Light upper winds.
  

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  Heat from the sun warming the ground which in turn warms the air in contact with it.
  
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  Air forced to rise over hills or a mountain range.
  
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  Cold air moving over a warm sea.
  
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  A mass of cold air undercutting a mass of warm air (cold front).
  
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  Air converging at a point (e.g. low pressure centre).
  

These clouds (base 2000-5000 feet) evaporate quickly, but their evaporation adds moisture to the air and succeeding clouds may last longer and grow taller than those which have dissipated. Several clouds may merge into a single, much larger cloud whose outer portion acts as insulation for the heated air still rising within (see Figure 2).

Figure 2: Formation of a towering cumulus cloud.

At this stage, some droplets may collide and become heavy enough to fall to the ground as a light shower. But eventually the updraughts within a cloud reach an altitude so cold that its water particles begin to freeze (a process known as glaciation) The weight of these particles becomes too great to sustain and they begin to descend. By their drag they create downdraughts which spread out at the earth’s surface in the form of strong gusts of wind that invariably precede a storm.

As the water droplets start to freeze, the appearance of the cloud begins to change. The top of the cloud loses its clear, sharp, domed shape and becomes fuzzy or fibrous (now called a cumulonimbus cloud). Its top also spreads out into a distinctive wedge or anvil shape when it reaches a part of the atmosphere where the temperature of the surrounding air is no longer colder than the cloud itself (see Figure 3). Within a single cumulonimbus cloud several separate updraughts and downdraughts may exist simultaneously, creating extremely dangerous conditions for aircraft flying through one.



Figure 3: Formation of a cumulonimbus cloud

3.Hail

Once the process of glaciation has begun within a cloud, water droplets freeze around the ice crystals creating a thin coating of ice (a process known as accretion). These little balls of ice (called hailstones) are then swept earthwards in a downdraught but may be swept back up in an updraught and given another coating of ice. In severe storms this process is repeated many times and the hailstone gathers layer upon layer of ice. Some are the size of golf balls and occasionally even tennis balls. The largest recorded hailstone in recent times fell in Bangladesh in 1986. It measured 6 inches across and weighed 2½ pounds. It has been calculated that a stone this size would require an updraught of at least 100 miles per hour to keep it in the air.

The hazards of hail have prompted attempts to reduce their size and hence the damage caused. In the grain growing regions of the former USSR and also in Greece, some communities have installed anti-aircraft guns in order to fire special shells containing silver iodide crystals into the clouds. The more particles within the cloud, the better, as the same amount of cloud moisture is then distributed between many more, but smaller, droplets or crystals, rather than fewer, potentially destructive, large hailstones. This cloud seeding process has also been attempted from aircraft flying through the clouds, but the effectiveness of such schemes have not been proved.

4.Lightning and thunder

Is this decrease in lightning fatalities due to fewer storms affecting the country? There is no evidence for this, but instead a large shift in the population from rural to urban areas. During thunderstorms, towns and cities are considered relatively “safe” places to be, as lightning will usually strike the tallest object in the vicinity e.g. buildings, electricity pylons etc. However, the recent rise in numbers of people pursuing outdoor hobbies such as golf, hill walking, boating etc. may start to reverse this downward trend. If caught out in an exposed area avoid isolated places of shelter such as a tree, throw away all metal objects (golf clubs, umbrellas etc.) and head for a nearby dry ditch or hollow. Keep both feet together and crouch as low as possible with hands on knees and head tucked in. Getting a soaking means that if you are struck by lightning the current should flow through your clothes, albeit possibly causing burns, but bypassing the heart, lungs and other vital internal organs and you should survive.

It is known that lightning begins with the separation and build-up of positive and negative electrical charges in a cumuliform cloud. Of great importance is the presence of both ice crystals and water droplets in the cloud. As they are swept up and down and collide with each other, a separation of charge results, similar to the frictional effect of a person walking across a carpet and becoming charged with static electricity. A large positive charge builds up in the frozen upper layers of the cloud and a large negative charge, along with a smaller positive area, forms in the lower portion of the cloud.

When the negative charges in the cloud reach a critical amount, an invisible avalanche of negative electrons (called the leader stroke) zigzags towards the positive ground below at about 60 miles per second seeking the path of least resistance. As they near the ground positive electrons may well stretch up towards the approaching leader stroke. Having made contact, the circuit is complete and an immense surge of energy (the return stroke) rushes up from the ground and is seen as lightning. Therefore a lightning flash is a high-voltage electrical spark which may produce a current of millions of volts. This surging current superheats the pencil thin conducting path up to temperatures of 30,000ºC - five times hotter than the surface of the sun - and the air expands explosively, creating shock waves which produce the familiar crack or rumble of thunder (see Figure 4).

Although lightning is visible almost instantaneously, thunder (sound) travels at only 750 miles per hour or one mile every 5 seconds. The time delay between a lightning stroke and its resulting thunder can be used to estimate the storm’s distance. For instance a separation between them of 10 seconds indicates the storm to be about 2 miles away. If a lightning flash is more than 10 miles away, thunder is rarely heard. One other question often asked is why thunder rumbles on for a long time? This is due to the length of the lightning bolt across the sky causing the thunder to rumble for several seconds as the sound arrives from the increasing distance. The sound may also bounce off surrounding hills, thus creating an echo effect.

The most vivid lightning and damaging strokes are between cloud and ground but far more discharges never hit the ground at all; they flash between one part of the cloud and another. Sometimes the fork is completely obscured by clouds and the resultant flickering light is termed sheet lightning. The strangest and rarest phenomenon associated with thunderstorms is ball lightning; a glowing sphere, up to about one foot in diameter, which floats about with a slow and erratic motion. It is extremely unpredictable, sometimes fading quietly away but on other occasions disappearing in a violent explosion with damaging results.

5.Tornadoes

Some parts of the USA are more at risk from tornadoes than others. The area which experiences by far the most is the notorious “Tornado Alley” – a slice of land extending from northern Texas through Oklahoma, Kansas and Missouri (see Figure 5). So frequent are tornadoes in this part of the USA that many homes are built with underground bunkers where residents can shelter during severe storms.

It is the unique geography of the USA which makes it so vulnerable to these storms. When cold, dry air coming down off the slopes of the Rocky Mountains collides with very warm, humid air heading northwards from the Gulf of Mexico, creating exceptionally unstable conditions high into the atmosphere, lines of spectacular thunderstorms, often accompanied by hail, sometimes known as supercells, are formed.

Another ingredient crucial to a tornado’s formation is rotation. A tornado is a rapidly spinning column of air within which wind speeds of up to 300 miles per hour occur. As the impact of wind increases by the square, a wind of this speed is thus not 10 times but 100 times more damaging than one of 30 miles per hour. The exact processes which create such vicious conditions are still not fully understood, but it is thought that strong wind currents moving quickly through the cloud from opposite sides creates rapid changes in speed and direction over a short distance (called wind shear) which in turn causes the wind to begin to spiral around a point. (The first observable indication of a potential tornado is likely to be a circular motion of the wind-tattered cloud base). As the air spirals upwards, a low pressure centre is formed and more air is sucked in. The spinning starts slowly, then as the spiral tightens it picks up speed, much as a rotating figure skater brings her arms in and spins faster and faster. The result beneath a storm cloud is ever more violent winds as the spiral tightens.



Figure 6: Cross-section through a tornado.

Although tornadoes are most common in the USA, they are sometimes reported in the UK, although not on the same scale of intensity. One recent example of a tornado which caused some damage was in Gwynedd, Wales on January 21st 1995. Caravans were wrecked on two sites, trees were snapped and tiles lifted from the roofs of houses. Tornadoes can be graded according to the damage sustained: In 1974 the Tornado and Storm Research Organisation (TORRO) was formed and a tornado intensity scale was devised from TO (litter raised, marquees damaged etc.) to T1O (entire buildings lifted & destroyed etc.). The tornado in Gwynedd (described above) was ranked a T4. However, in the USA the Fujita scale is used which grades tornadoes from F0 to F5; named after the Japanese-born meteorologist who devised it in 1971.

6.Tropical Revolving Storms

Although thunderstorms, hail and tornadoes can and do inflict damage, nothing can compare with the sheer scale of disaster caused by a full- blown tropical storm hitting the coast of a heavily populated area. Surprisingly, it is not the wind that causes the greatest number of fatalities but the accompanying storm surges which sweep across low- lying areas, submerging everything in its path. In November 1970 a severe cyclone and its associated storm surge killed close to one million people as it swept into the heavily populated River Ganges delta from the Bay of Bengal.

These storms form in latitudes 5º - 20º north and south of the equator, initially travelling in a westerly direction following the prevailing wind. Gradually they curve away from the equator and slowly lose their fury as they head towards the poles, sometimes linking up with a mid-latitude depression to bring particularly stormy weather conditions to countries such as the UK. These storms are called different names in different parts of the world. (see Table 1). In this handout, they will generally be referred to as hurricanes.

Name	Location	Time of Year
Hurricane	Caribbean, SE USA, Gulf of Mexico	July to October
Typhoon	China Sea, S Japan, Philippines	July to October
Cyclone	Bay of Bengal, S India	April to December
Cyclone	S Indian Ocean, Madagascar	December to April
Cyclone	N Australia	December to April

Table 1: Locations and names for Tropical Revolving Storms

For hurricanes to form you need two basic ingredients – warmth and moisture. Therefore these storms only form over oceans which have a water surface temperature of 26ºC (80ºF) or more. The mechanism that sets them off is complicated but the birthplace of Atlantic hurricanes is between the coasts of Africa and the Caribbean where the north-east and south-east trade winds converge (see Figure 7). Given a disturbance in the trade wind flow, this can produce the initial circulation to trigger the formation of a hurricane.

The warm, moisture-laden air that has been heated by the sun rises from the water’s surface. As the air ascends it cools and the moisture condenses into cloud, a process which releases more heat energy into the surrounding atmosphere and causes the clouds to build further. Add in the rotation of the earth, which helps to intensify the spiralling motion of the air and before long a tropical storm is born with the lowest surface pressure at its centre.



Figure 7: Common tracks of Atlantic hurricanes

The typical surface pressure at the centre of these storms is about

960 mb, although the lowest recorded was 870 mb in a Pacific typhoon on 12^th October 1979. A typical mid-latitude depression can also produce very low pressure values (e.g. 912 mb south of Iceland on 10th January 1993) but one important difference between temperate and tropical depressions is the surface area over which they may extend. A large mid-latitude depression in the N. Atlantic may cover thousands of miles, whereas the diameter of a hurricane is measured in hundreds of miles. The result is that a similar change in pressure is concentrated in a much smaller area in a tropical storm, thus making the accompanying winds much more ferocious.

The most common time for their formation is in late summer when sea temperatures are at their highest. They do not form over land where there is insufficient moisture at the surface to fuel them. Indeed, once they cross land or cooler seas they quickly begin to dissipate. To reach the status of a full-blown hurricane, the mean wind speed must exceed 64 knots (73 mph). The strongest winds are found in a ring about 10-20 miles from the centre of the hurricane. The other notable feature is the copious amounts of rainfall, with the most torrential precipitation found in a ring just outside the ring of strongest winds.

Another notable characteristic of hurricanes is its eye - a calm, almost cloud free area on average about 20 miles across, right at the centre of the storm. This is caused by very warm, sinking air. Anybody unfortunate enough to be caught in such a storm will experience its full fury as it approaches, followed by a short period of eerie silence in the eye. Then as the eye wall crosses them, plunged back into the nightmare, but this time with the wind blowing in the opposite direction! The rate of movement of the storm centre (as distinct from the speed of the winds in its circulation) is generally less than 15 mph.

When, on two occasions in 1950, three hurricanes were being tracked at the same time, some confusion was caused between the various bulletins issued for each one. In 1953, it was decided that each storm whose winds reached mean speeds of 34 knots (39 mph) would be designated a woman’s name to distinguish it from others. In 1979 men’s names were added to the list. A six year cycle of names is produced by the World Met. Organisation, with suggestions for further names from those countries which are affected by them. Once a hurricane has caused great damage, its name is retired from the six year cycle. Recent names which have been withdrawn include Gilbert (1988), Hugo (1989), Bob (1991) and Andrew (1992)

A destructive hurricane of recent years was Hurricane Andrew which slammed into Florida in the early hours of 24 August 1992. It then marched on across the Gulf of Mexico, crossing the Louisiana coastline two days later, before heading northwards and dissipating. Lowest pressure was estimated to be 922 mb with maximum mean winds of 125 knots (144 mph) and gusts of at least 150 knots (172 mph). Damage ran into billions of dollars - in a few short hours Andrew became the most expensive natural disaster in US history.

In 1998 another hurricane, Mitch, swept over Honduras and Nicaragua dropping over 625mm of rain in 36 hours, over 250mm in 6 hours! Winds were recorded at over 160 knots. Landscapes were changed irrevocably and economies probably affected for decades. Over 10,000 were killed or lost in the floods and mudslides. Indeed the 1998 hurricane season, a ‘la Niña’ year, was the most severe on record, with 14 tropical storms producing 10 hurricanes. Indeed the 1995 – 1998 period was the most active ever recorded in the Atlantic, with 54 tropical storms producing 33 hurricanes.

 Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met Office College

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