Extreme weather

So far in our survey we've encountered nothing more unpleasant than a severe soaking, but what about when things turn really nasty? Step this way for all kinds of severe weather, starting with storms, cyclones and their even more extreme cousins the hurricanes and typhoons.


It you watch much news on the TV, you will be used to cyclones as the term for the tropical storms typically seen sweeping ashore and bending palm trees to the ground in a destructive manner. But if you live in temperate climes, a great deal of the weather you see is in the form of “mid-latitude” cyclones. Here, a region of low pressure around which winds are swirling inwards is a cyclone, and a high-pressure zone from which they are flowing outwards is an anticyclone.

Events here are determined by a simple balance of forces dominated by the Earths rotation and our old friend the Coriolis Effect. It dictates that a cyclone runs anticlockwise in the northern hemisphere and clockwise in the southern.

The vital importance of cyclones to weather in the temperate latitudes was first worked out by Jacob Bjerknes, son of Vilhelm whom we met a few pages back. He grasped that the collision between the warmer westerly air of the temperate zones and the chilly easterlies of the polar regions generates much of the weather of the middle latitudes. The zone in which they meet is the polar front and the science which Bjerknes Jr developed is called Polar Front Theory.

The mechanism is simple and dramatic. Remember that the poles are deserts. So the air that comes from them is dry as well as cold. By contrast, the temperate regions, both north and south, have plenty of ocean for the wind to blow across and are wet as well as warm. When these air masses meet, the first effect is a slight wave in the lower atmosphere. This soon grows under the influence of the Coriolis Force, with polar air being pushed towards the Equator and to the west of the centre of low pressure, while the temperate air is pulled poleward and to the east.

These cyclones can be hundreds of kilometres across and can travel hundreds of kilometres a day, usually from west to east with the prevailing wind. The collision between these air masses is a violent one, generating storms and rain on a continental scale.


If things were this simple, the weather forecast for the temperate zones would be a task for unskilled labour. It would involve endless rainy fronts hurtling around the planet until they ran out of energy and were replaced by the one behind. The only variation would be caused by the fact that the fronts themselves are not static. They move north in the northern summer and south in the southern summer.

However, nothing in the atmosphere is quite this smooth. These temperate-region cyclones are driven by temperature differences between air masses from different parts of the globe, and they vary in size and duration. There are also cyclones in the tropics, where there are no such heat differences to drive them. Here the secret is that a sea temperature of at least 27°C is needed to get one started. Indeed, you need water at that temperature that is at least 50m deep across a large area of ocean. When this happens, the evaporation of water vapour from the sea surface builds. As the vapour rises it condenses to cloud, releasing its “latent heat of evaporation”, the energy it took to vaporize it in the first place. This heat is now available to drive yet stronger winds.

The high temperature needed to start such a cyclone means that there is a season, in late summer and early autumn, when they are most likely to emerge. This is not the chapter of the book on climate change, but one does not have to be much of a scientist to work out that a warmer world might have more cyclones, spread over a longer part of the year.

Cyclone in the northern hemisphere, just off Iceland (visible top right). The combination of low pressure and the Coriolis Force brings air towards the centre in an anti-clockwise spiral.

Cyclones with wind speeds of over 17 metres per second are called tropical storms and are assigned a name, while those with winds of more than 33mps are so disturbing that like some iffy multinational corporation they trade under a variety of aliases. According to the US governments National Oceanic and Atmospheric Administration, they are called hurricanes in the north Atlantic, the north-east Pacific east of the International Date Line, and the south-east Pacific east of 160°E. In the north-west Pacific west of the dateline, they are typhoons. In the south-west Pacific west of 160°E, and of course the south-east Indian Ocean east of 90°E, they are severe tropical cyclones. In the north Indian Ocean they are severe cyclonic storms. And if you see one trading as a tropical cyclone, you are in the south-west Indian Ocean. Quite how this variety has lasted in the era of the Internet is a mystery, but harmonizing it would probably take more effort than bringing peace to the Middle East.

The monsoon

Nowhere are annual variations in weather more extreme than in the Indian Ocean, home of the nionsoon. In the northern hemisphere summer, the monsoon wind blows from the south-west, and is a wet wind whose rain is vital to life across India and adjoining countries and which deposits metres ot snow across the Himalayas. The severe weather that the monsoon brings is the ultimate orographic rain. If the Himalayas were not there, the weather would not be trapped and the wind would blow into central Asia instead ot depositing rain and snow en masse in and around India. As we have seen, the weather systems shift south with the Sun during the southern summer, and this wind is replaced by one from the north-east, which is dry as it has passed over the land. It brings with it the lengthy dry season which, along with the monsoon rains, characterizes the climate of India and the neighbouring region.

The monsoon is a form of extreme weather wrhose predictability makes it a valued part of the Earths normal variation. As we shall see in Chapter 8, there are suggestions that the whole climate system that drives it is being altered by human behaviour. Certainly the possibility of the monsoon failing amounts to a mortal threat to life in western Asia.

Deadly weather

Despite the ordered pattern which the weather tends to follow, its extremes do not exist merely to provide footage for those late-night “when storms attack” programmes on the higher-numbered TV channels. Indeed, bad weather is the biggest killer after earthquakes among natural disasters ranging from volcanoes to meteorite strike. (People have been injured by meteorites but there are no reliable records of even a single fatality; see www.ilankelman.org/disasterdeaths.html for more on deaths from natural disasters.)

A single event, the 2005 flooding of New Orleans by Hurricane Katrina, makes the point. The economic damage was priced at about $200 billion, but the floods killed “only” 972 people in the official count. The decision to attempt to protect a major world city from flooding when it was sited below' sea level and located between a river, a lake and a sea noted for extreme storms was bound to come unstuck at some point. If a developing-world country had done the same thing, the financial cost would have been less because Americans have the most money. But the cost in lives would have been far greater. Indeed, anyone surprised at the fate of New Orleans has not looked at the lessons of history along the US Gulf Coast. In September 1900, a similar storm flooded Galveston in Texas, some 500km from New Orleans. At that time, both sea defences and warning systems were less developed. About 6000 people were killed.

Although too much water is the cause of many immediate disasters, having too little has killed more people over longer periods of time. In the US, the drought of 1988-89 was blamed for $40 billion of economic damage even though it killed few if any people directly.

In the developing world, NASA points to the drought of 1991-92 as one of the most serious, affecting 24 million people across 6.7 million square kilometres of Africa. In 1984-85, famine resulting from drought across the Sahel, the part of Africa at the southern boundary of the Sahara, killed about 800,000 people, perhaps three times the death toll of the worst earthquake on record.

There are many definitions of a drought. Some are very practical, relating to having too little water for crops. Others are more theoretical, and refer to how long rainfall has dropped by some percentage below normal. You cant have a drought in a place that is already a desert, because it is defined as a spell when the weather is drier than normal.

But droughts tend to have a simple set of causes. They are normal weather taken to extremes. One pattern of drought weather, often likened to the formation of an eddy in a flowing river, occurs when an air mass gets separated out from the general circulation of the atmosphere and sits in a particular spot for a long time. It may start out by dumping large amounts of rain. But once it is out of water, no more can fall and the drought will continue until such a “blocking system” drifts off or gradually rejoins the rest of the planets airflow.

As we will see in Chapter 8, there are suspicions that both floods and droughts are increasing in frequency as the Earth gets warmer. But much weather damage is caused by smaller-scale effects. For example, the tornadoes of which about 1000 a year sweep across the US are a highly specific weather phenomenon. They are defined as hanging from a cumuliform cloud at the top, and touching the ground at the bottom. They arise from rotating thunderstorms in areas where there are major temperature differences between adjacent bodies of air, but even the US government scientists who have devoted huge resources to finding out about them admit that they hold many secrets and often fail to appear when conditions seem promising.

Record breakers

Even places with dull weather can break records. In 1893 Mile End, then as now a bustling part of east London, and used to its fair share of wet, set the UK record by having no measurable rain for 73 days from 4 March to 16 May.

However, the real records are set nearer to the poles and the Equator. Arica in Chile has the least rain in the world, at about 0.76mm a year. This is under one ten-thousandth of the amount falling at Kauai in Hawaii, 1168cm a year.

Arica is in the Atacama Desert which, as we saw, is caused by descending hot, dry air that started its journey at the Equator. It turns out that most weather records, whether for heat or cold, or for dry or wet conditions, arise not from freak events but from normal weather that gets slightly more out of hand than usual. For example, extreme cold kills over 700 people a year in the US. It normally kills people who live in areas where there is extremely cold weather every winter. Extreme heat kills an average of 384 people a year in the US, again mainly in places where it is a predictable, and predicted, part of the annual weather pattern. Despite the attention paid to storms and hurricanes, it is cold and heat that cause the greatest loss of life. In the US, storms kill about 50 people a year, floods about 80 and hurricanes and tornadoes another 70.

The coldest place recorded on Earth is the Russian Vostok base in the Antarctic. It went down to -89.2°C on 21 July 1983, a month after mid-winters day. The heat record is held by Al Aziziyah in Libya, where it hit 58°C on 13 September 1922, a degree ahead of Death Valley in California on 10 July 1913.