(3) Climate System

El Nino/La Nina Explainer (Video)


What are El Niño and La Niña?


Global vegetation

The vegetation zones of the world are controlled by the annual average and seasonality of both temperature and precipitation. Temperature follows a latitudinal gradient with warmest conditions in the tropics and coldest at the poles. As we have seen there are three main rainfall belts, the convection rainfall belt in the tropics and the convergent rainfall belt in the mid-latitudes of the Northern and Southern Hemispheres. The two main desert regions of the world lie between these rainfall belts.

Vegetation follows these climate zones. So rainforest is found in the tropics where there is a lot of rainfall all year round. Savannah is found in the tropics when rainfall is high seasonally, but there are also long dry seasons lasting over 4 months. The world’s largest deserts are found in the mid-latitudes. Here the seasonality of rainfall is critical, as while many deserts have the same rainfall as, say, London, this rain falls over a very small period of time, with the rest of the year being extremely arid. When the rainfall occurs only in the winter months followed by a very dry summer period, the unique Mediterranean flora is found, such as in California, South Africa, and of course around the Mediterranean. In high mid-latitudes are the temperate or boreal forests. In areas with low annual rainfall, steppe vegetation is found. In high latitudes where the temperature is the limiting factor, tundra is found. Other factors can influence where different vegetation can exist, for example we have seen that major ocean currents can allow temperate-weather vegetation to exist much further north than would usually be expected. Later we will see that mountain ranges and plateaus have a huge influence on where rainfall occurs and thus where deserts form.

Finally, it should be remembered that vegetation has its own influence on climate. First, vegetation changes the albedo of any area, so tropical rainforests absorb much more solar radiation than does tundra. Second, vegetation is very good at recycling water and maintaining a moist atmosphere. For example, 50 per cent of all the rainfall in the Amazon Basin comes from water recycled by the trees, evaporating and creating new clouds.

Weather versus climate

Many people get weather and climate confused. This confusion is exacerbated when scientists are asked to predict climate 50 years from now when everyone knows they cannot predict the weather a few weeks ahead. So climate is generally defined as ‘the average weather’. The original definition of climate was ‘the average weather over 30 years’, this has been changed because we now know that our climate is changing and significant changes have been seen every decade for the last 50 years. The chaotic nature of the weather can make it unpredictable beyond a few days, while understanding the climate and modelling climate change is much easier as you are dealing with long-term averages. A good comparison is that though it is impossible to predict at what age any particular person will die, we can say with a high degree of confidence that the average life expectancy of a person in a developed country is about 80. The other confusion is that people always remember extreme weather events and not the average weather. So for example everyone remembers the heat waves in the UK in 2003 and the USA in 2012, or the floods in Pakistan and Australia in 2010. So our perception of weather is skewed by these events rather than by an appreciation of the average weather or climate.

Chaos theory

The National Weather Service in the USA spends over $1 billion per year ensuring the country has the most accurate weather prediction possible. In other countries similar resources are poured into weather agencies, as predicting the weather is big business and getting a storm prediction right can save billions of dollars and many lives. Today three-four day forecasts are as accurate as the two-day forecasts were 20 years ago. Predictions of rain in three days’ time are as accurate as one-day forecasts were in the mid-1980s. The accuracy of flash flood forecasts has improved from 60 per cent correct to 86 per cent, moreover potential victims of these floods get nearly an hour’s warning instead of the 8 minutes they would have had in 1986. The lead times of advance warnings of tornadoes, in other words, the time that residents have to react, has increased from 5 minutes in 1986 to over 12 minutes. Severe local thunderstorms and similar cloudbursts are typically seen 18 minutes beforehand rather than 12 minutes over two decades ago. Seventy per cent of all hurricane paths can be predicted at least 24 hours in advance and the landfall of a hurricane can be predicted to within 100 miles (160 km).

These are great achievements but it does not explain why with all our technology and our understanding of the climate system we cannot predict the weather 10 days, a month, or a year in advance. Moreover, think of all those times that the weather report on television has said it will be sunny today and then it rains. So why is it so difficult to predict the weather? In the 1950s and 1960s it was thought that our weather prediction was limited by our lack of data and that if we could measure things more accurately and clearly understand the fundamental processes we would be able to achieve a much higher level of prediction. But in 1961 Edward Lorenz a meteorologist at Massachusetts Institute of Technology made a cup of coffee that radically changed the way we think about natural systems. In 1960 Lorenz had produced one of the first computer models of weather. One day in the winter of 1961, Lorenz’s computer model produced some very interesting patterns, which he wanted to look at in greater detail. So he took a short-cut and started mid-way through the run. Of course this was one of the earliest computers so he had to retype all the starting numbers. Instead of typing them into six decimal places (e.g., 0.506127) he only typed the first three to save time and space, and then went and made the famous coffee. When Lorenz came back he found that the weather patterns had diverged from the initial run so much that there was no recognizable similarity between them. It seems the model was very sensitive to the very small changes, that one part in a thousand instead of being inconsequential had had a huge effect on the outcome. This original work has lead to the development of chaos theory. Chaos theory shows us that very small variations in atmosphere temperature, pressure, and humidity can have a major and unpredictable or chaotic effect on large-scale weather patterns.

Nevertheless, chaos theory does not mean there is a complete lack of order within a system. Far from it, chaos theory tells us that we can predict within certain boundaries what the weather will be like: we all know, for example, that most tornadoes occur in May in the USA and that winters are wet in England. But when it comes to more detailed prediction everything breaks down due to what has become known as the ‘butterfly effect’. The idea is that small changes represented by the flapping of the wings of a butterfly can have a large effect on the weather, for example altering the strength and direction of a hurricane. As errors and uncertainties multiply and cascade upward through the chain of turbulent features from dust devils and squalls up to continental size eddies that only satellites can see. In effect, we will never know which of the small weather changes will combine to have these large effects. While Lorenz used 12 equations in his weather model, modern weather computers use 500,000. But even the best forecasts, which come from the European Centre for Medium Weather Forecasts based at Reading in England, suggest that weather predictions for more than four days are at best speculative and beyond a week worthless, all because of chaos. So chaos theory says that we can understand weather and we can predict general changes but it is very difficult to predict individual events such as rain storms and heat waves. The study of climate, however, has one great advantage over meteorology because it only examines averages and thus chaos theory does not affect it. Moreover when it comes to modelling future climate change we can now understand that an increase in the Earth’s average temperature will make some weather phenomena more frequent and intense for example heat waves and heavy rainfall events, while others will become less frequent and intense, for example extreme cold events and snow fall.

Decadal and quasi-periodic climate systems

The climate system contains many cycles and oscillations that complicate our ability to predict the weather. These include decadal cycles such as the North Atlantic oscillation (NAO), the Atlantic multi-decadal oscillation (AMO), Arctic oscillation (AO), and the Pacific decadal oscillation (PDO). So the first of these is the NAO, which was first described in the 1920s by Sir Gilbert Walker (14 June 1868-4 November 1958), a British physicist and statistician. The NAO is a climate phenomenon in the North Atlantic Ocean and is represented by the atmospheric pressure difference at sea level between Iceland and the Azores. The difference in Icelandic low-pressure and the Azores highpressure systems seems to control the strength and direction of westerly winds and storm tracks across the North Atlantic Ocean. This in turn controls where and when in Europe it rains. Unlike the El Niño–Southern Oscillation, the NAO is largely controlled by changes in the atmosphere. The NAO is closely related to the AO and though both seem to change on a decadal scale there seems to be no periodicity. The NAO should not, however, be confused with the AMO. The AMO is the decadal-scale variability in the sea-surface temperatures of the North Atlantic Ocean. Over the last 130 years, 1885-1900, 1927-1947, 1951-1961, 1998–present day, the North Atlantic Ocean temperatures have been warmer than average and the time in between colder. The AMO does affect air temperatures and rainfall over much of the Northern Hemisphere, in particular North America and Europe, for example the North Eastern Brazilian and African Sahel rainfall and North American and European summer climates. It is also associated with changes in the frequency of North American droughts and it may influence the frequency of severe Atlantic hurricanes. There are also irregular or quasi-periodic cycles such as the Indian Ocean Dipole and El Niño-Southern Oscillation (ENSO). Of these ENSO is by far the best known.

El Niño-Southern Oscillation

One of the most important and mysterious elements in global climate is the periodic switching of direction and intensity of ocean currents and winds in the Pacific. Originally known as El Niño (‘Christ child’ in Spanish) as it usually appears at Christmas, and now more often referred to as ENSO (El Niño–Southern Oscillation), this phenomenon typically occurs every 3 to 7 years. It may last from several months to more than a year. ENSO is an oscillation between three climates: the ‘normal’ conditions, La Niña, and ‘El Niño’. ENSO has been linked to changes in the monsoon, storm patterns, and occurrence of droughts throughout the world. For example the prolonged ENSO event, in 1997 to 1998, caused severe climate changes all over the Earth including droughts in East Africa, northern India, north-east Brazil, Australia, Indonesia, and Southern USA; and heavy rains in California, parts of South America, the Pacific, Sri Lanka, and east central Africa. The state of the ENSO has also been linked into the position and occurrence of hurricanes in the Atlantic Ocean. For example, it is thought that the poor prediction of where Hurricane Mitch made landfall was because the ENSO conditions were not considered and the strong Trade Winds helped drag the storm south across central USA instead of west as predicted.

An El Niño event is when the warm surface water in the western Pacific moves eastward across to the centre of the Pacific Ocean. Hence the strong convection cell or warm column of rising air is much closer to South America. Consequently the Trade Winds are much weaker and the ocean currents crossing the Pacific Ocean are weakened. This reduces the amount of the cold, nutrient-rich upwelling off the coast of South America and without those nutrients the amount of life in the ocean is reduced and fish catches are dramatically reduced. This massive shift in ocean currents and the position of the rising warm air changes the direction of the jet streams that upset the weather in North America, Africa, and the rest of the world. However if you ask what causes El Niño, then the answer is of the chicken and egg variety. Does the westward ocean current across the Pacific reduce in strength, allowing the warm pool to spread eastward and moving with it the wind system? Or does the wind system relax in strength, reducing the ocean currents, and allowing the warm pool to move eastwards? Many scientists believe that long period waves in the Pacific Ocean that move between South America and Australia over time help shift the ocean currents which produce either an El Niño or a La Niña period.

La Niña is a more extreme version of the ‘normal’ conditions. Under normal conditions the Pacific warm pool is in the western Pacific and there are strong westerly winds and ocean currents keeping it there. This results in upwelling off the coast of South America, providing lots of nutrients and thus creating excellent conditions for fishing. During a La Niña period the temperature difference between the western and eastern Pacific becomes extreme and the westerly winds and ocean currents are enhanced. La Niña impacts on the world’s weather are less predictable than those of El Niño. This is because during an El Niño period the Pacific jet stream and storm tracks become strong and straighter and it is therefore easier to predict its effects. La Niña on the other hand weakens the jet stream and storm tracks, making them more loopy and irregular, meaning that the behaviour of the atmosphere and particularly of storms becomes more difficult to predict. In general where El Niño is warm, La Niña is cool, where El Niño is wet, La Niña is dry. La Niñas have occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995, 1999, 2008, and 2011, with the 2010-2011 La Niña being one of the strongest ever observed.

Predicting ENSO

Predicting an El Niño events is difficult but a lot of work has gone on for the last three decades to better understand the climate system. For example, there is now a large network of both ocean and satellite monitoring systems over the Pacific Ocean, primarily aimed at recording sea-surface temperature, which is the major indicator of the state of the ENSO. By using this climatic data in both computer circulation models and statistical models, predictions are made of the likelihood of an El Niño or La Niña event. We are really still in the infancy stage of developing our understanding and predictive capabilities of the ENSO phenomenon. There is also considerable debate over whether ENSO has been affected by global warming. The El Niño conditions generally occur every 3 to 7 years; however, in the last 20 years, they have behaved very strangely, returning for 3 years out of 4: 1991-1992, 1993-1994, and 1994-1995, then not returning until 1997-1998, and then not returning for 9 years, finally arriving in 2006-2007 and 20014-2015. Reconstruction of past climate using coral reefs in the western Pacific shows sea-surface temperature variations dating back 150 years, well beyond our historical records. The sea-surface temperature shows the shifts in ocean current, which accompany shifts in the ENSO and reveal that there have been two major changes in the frequency and intensity of El Niño events. First was a shift at the beginning of the 20th century from a 10-15-year cycle to a 3-5-year cycle. The second was a sharp threshold in 1976 when a marked shift to more intense and even more frequent El Niño events occurred. Moreover during the last few decades the number of El Niño events has increased, and the number of La Niña events has decreased. Even taking into account the effect of decadal cycles on ENSO the size of the ENSO variability in the observed data seems to have increased by 60 per cent in the last 50 years. However, as we have seen, to predict an El Niño event 6 months from now is hard enough, without trying to assess whether or not ENSO is going to become more extreme over the next 100 years. Most computer models of ENSO in the future are inconclusive; some have found an increase and others have found no change. This is, therefore, one part of the climate system that we do not know how global warming will affect. Not only does ENSO have a direct impact on global climate but it also affects the numbers, intensity, and pathways of hurricanes and cyclones, and the strength and timing of the Asian monsoon. Hence, when modelling the potential impacts of global warming, one of the largest unknowns is the variation of ENSO and its knock-on effects on the rest of the global climate system.

Modelling climate

The whole of human society operates on knowing the future weather. For example, a farmer in India knows when the monsoon rains will come next year and so they know when to plant the crops. While a farmer in Indonesia knows there are two monsoon rains each year so each year they can have two harvests. This is based on their knowledge of the past, as the monsoons have always come at about the same time each year in living memory. But weather prediction goes deeper than this as it influences every part of our lives. Our houses, roads, railways, airports, offices, cars, trains, and so on are all designed for our local climate. Predicting future climate is, therefore, essential as we know that global warming is changing the rules. This means that the past weather of an area cannot be relied upon to tell you what the weather in the future will hold. So we have to develop new ways of predicting and modelling the future, so that we can plan our lives and so that human society can continue to fully function.

There is a whole hierarchy of climate models, from relatively simple box models to the extremely complex three-dimensional general circulation models (GCMs). Each has a role in examining and furthering our understanding of the global climate system. However, it is the complex three-dimensional general circulation models which are used to predict future global climate. These comprehensive climate models are based on physical laws represented by mathematical equations that are solved using a three-dimensional grid over the globe. To obtain the most realistic simulations, all the major parts of the climate system must be represented in sub-models, including the atmosphere, ocean, land surface (topography), cryosphere, and biosphere, as well as the processes that go on within them and between them. Most global climate models have at least some representation of each of these components. Models that couple together both the ocean and atmosphere components are called atmosphere-ocean general circulation models (AOGCMs).

Over the last 25 years there has been a huge improvement in climate models. This has been due to our increased knowledge of the climate system but also because of the nearly exponential growth in computer power. There has been a massive improvement in spatial resolution of the models from the very first Intergovernmental Panel on Climate Change (IPCC) in 1990 to the latest in 2007. The current generation of AOGCMs has a resolution of one point every 110 km by 110 km, and this is set to get even finer when the next IPCC Science Report is published in late 2013. The very latest models or as some groups are now referring to them ‘climate simulators’ include much better representations of atmospheric chemistry, clouds, aerosol processes, and the carbon cycle including land vegetation feedbacks. But the biggest unknown or error in the models, is not the physics, it is the estimation of future global greenhouse emissions over the next 90 years. This includes many variables, such as the global economy, global and regional population growth, development of technology, energy use and intensity, political agreements, and personal lifestyles.

Over 20 completely independent AOGCMs have been run using selected future carbon dioxide emission scenarios for the IPCC 2007 report, producing global average temperature changes that may occur by 2100. This is a significant change from the IPCC 2001 report, in which only 7 of these models were used. Using the widest range of potential emission scenarios the climate models suggest that global mean surface temperature could rise by between 1.1°C and 6.4°C by 2100. Using the best estimates for the 6 most likely emission scenarios, then this range is 1.8°C to 4°C by 2100. Model experiments show that even if all radiation forcing agents were held at a year-2000 constant, there would still be an increase of 0.1°C per decade over the next 20 years. This is mainly due to the slow response of the ocean. Interestingly, the choice of emission scenario has little effect on the temperature rise to 2030, making this a very robust estimate. All models suggest twice the rate of temperature increase in the next two decades compared with that of the 20th century. What is significant is that the choices we make now in terms of global emissions will have a significant effect on global warming after 2030. The next IPCC report to published in late 2013, use  greatly improved emission scenarios, will have a very similar potential change of warming by the end of the century. What is amazing and very reassuring is that over the last 25 years the climate models have consistently given us the same answer, meaning we do understand the climate system and we can understand the consequences of our past and future actions.

Extreme climates

Humans can live, survive, and even flourish in the extreme climates ranging from that of the Arctic to that of the Sahara. We have populated every continent except Antarctica. We can deal with the average climate of each region through our adaptations of technology and lifestyle. The problems arise when the predictable boundaries of local climate are exceeded, for example by heat waves, storms, droughts, and/or floods. This means that what we define as extreme weather, such as a heat wave, in one region may be considered fairly normal weather in another. Each society has a coping range, a range of weather with which it can deal: what is seen as a heat wave in England would be normal summer conditions in Kenya. However, one of the most unpredictable and dangerous elements in our climate systems are storms. In this chapter we examine how and why storms are formed and their impact. Hurricanes, tornadoes, winter storms, and the monsoons will all be discussed.


A hurricane is a severe cyclonic tropical storm that starts in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, west coast of Mexico, or the northeast Pacific Ocean. They are called typhoons in the western Pacific and simply tropical cyclones in the Indian Ocean and Australasia. They are, however, all the exactly the same type of storm and here we will call all of them hurricanes. Hurricanes occur in the tropics between 30°N and 30°S, but not near the Equator as there is not enough atmospheric variation to generate them. For a storm to be classified as a hurricane, the sustained wind speed must exceed 120 km/hr. Of course in a fully developed hurricane, wind speeds can exceed 200 km/hr.

A hurricane is a tropical storm run amok, a rotating mass of thunderstorms that has become highly organized into circular cells, which are ventilated by bands of roaring winds. Hurricanes develop over the oceans and tend to lose their force once they move over land—this is because unlike temperate storms hurricanes are driven by the latent heat from the condensation of water. The sun is most intense close to the Equator where it heats the land, which in turn heats the air. This hot air rises and consequently sucks air from both the north and south producing the Trade Winds. As the seasons change so does the position of the clash of the Trade Winds, which is called the Inter-tropical Convergence Zone (ITCZ). To generate a hurricane the sea temperature must be above 26°C for at least 60 m below the surface and the air humidity must be at about 75–80 per cent. This combination provides the right amount of heat and water vapour to sustain the storm once it has started. For example these conditions occur during the summer in the North Hemisphere when the tropical North Atlantic Ocean heats up enough and its water starts to evaporate. Initially the warm ocean heats the air above it and causes that to rise. This produces a low-pressure area which sucks in air from the surrounding area. This rising air contains a lot of water vapour due to pronounced evaporation from the hot surface of the ocean. As the air rises it cools and can no longer hold as much water vapour; as a result some of it condenses to form water droplets and then clouds. This transformation from water vapour to water droplets releases energy called ‘latent heat’. This in turn causes further warming of the air and causes it to rise even higher. This feedback can make the air within a hurricane rise to over 10,000 m above the ocean. This becomes the eye of the storm and the spiraling rising air it produces creates a huge column of cumulo-nimbus clouds. You can see a mini version of this with steam coming out of a kettle. As the hot air rises from the kettle it hits the colder air and it forms steam, a mini-cloud. If you have ever put your hand near the steam you can feel it is very hot and this is because of all the energy being released as the water vapour changes from a gas back to a liquid. When the air inside the hurricane reaches its highest level it flows outwards from the eye producing a broad canopy of cirrus cloud. The air cools and falls back to sea level where it is sucked back into the centre of the storm. Because of the Coriolis force, the air that is sucked into the bottom of the hurricane spins into the storm in a clockwise direction, while the air escaping at the top spins out in a counter-clockwise direction. This pattern is the opposite in the Southern Hemisphere. Hurricanes form at least 345 miles or 5° of latitude away from the Equator, where the Coriolis Effect is strong enough to give the required twist to the storm. The size of hurricanes can vary from 100 km to over 1,500 km. A hurricane can form gradually over a few days or in the space of 6 to 12 hours and typically the hurricane stage will last 2–3 days and take about 4-5 days to die out. Scientists estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day, equivalent to about 1 PW (1015 Watt). This rate of energy release is equivalent to 70 times the human world energy consumption and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes. Hurricanes are measured in the Saffir-Simpson scale and go from a tropical storm through category 1 to the worst at category 5.

However, the formation of hurricanes is much rarer than might be expected given the opportunities for them to occur. Only 10 per cent of centres of falling pressure over the tropical oceans give rise to fully fledged hurricanes. In a year of high incidence, perhaps a maximum of 50 tropical storms will develop to hurricane levels. Predicting the level of a disaster is difficult as the number of hurricanes does not matter-it is whether they make landfall. For example, 1992 was a very quiet year for hurricanes in the North Atlantic Ocean. However, in August, one of the few hurricanes that year, Hurricane Andrew, hit the USA just south of Miami and caused damage estimated at $26 billion.

Hurricane Andrew also demonstrates that predicting where a storm will hit is equally important-if the hurricane had hit just 20 miles further north it would have hit the densely populated area of Miami City and the cost of the damage would have doubled.

In terms of where hurricanes hit in developed countries, the major effect is usually economic loss, while in developing countries the main effect is loss of life. For example, Hurricane Katrina, which hit New Orleans in 2005, caused 1,836 deaths while Hurricane Mitch, which hit Central America in 1998, killed at least 25,000 people and made 2 million others homeless. In both cases the greatest damage was caused by the huge amount of rainfall. Honduras, Nicaragua, El Salvador, and Guatemala were battered by 180-mile (290 km) per hour winds, and more than 23 inches (60 cm) of rain every day. Honduras, a small country of only 6 million inhabitants, was the worst hit. The Hamuya River, normally a calm stretch of water about 200 feet (60m) wide rose by 30 feet (9 m) and became a raging torrent, ripping out trees as tall as a city block from the ground. Eighty-five per cent of the country ended up under water. Over 100 bridges, 80 per cent of the roads, and 75 per cent of its agriculture were destroyed, including most of the banana plantations.

In New Orleans the worst damage by Hurricane Katrina was caused by both the intense rainfall and the storm surge. Together they caused 53 different levees to break, submerging 80 per cent of the city.

The storm surge also devastated the coasts of Mississippi and Alabama. Hurricane Katrina was not the worst storm that has hit the USA; a storm that hit Miami in 1926 was 50 per cent larger but did little damage because Miami Beach was then still undeveloped. In the USA coastal population has doubled in the last 10 to 15 years making the country much more vulnerable to storm related losses. There is also a large financial difference if a hurricane hits a developed or developing country. For example, the immediate economic impact of Hurricane Katrina was over $80 billion, but its subsequent effect on the US economy was to boost it slightly, by 1 per cent, that year due to the billions of dollars spent by the Bush administration to aid reconstruction of the region. Compare this with Hurricane Mitch in 1998, which set back the economy of Central America by about a decade. Hurricanes also occur elsewhere in the world. An average of 31 tropical storms roam the western North Pacific every year, with typhoons smashing into Southeast Asia from June to December; most at risk are Indonesia, Hong Kong, China, and Japan, otherwise known as ‘Typhoon Alley’. Why does Typhoon Alley get so many typhoons? And why can they occur almost any time of year. The answers lie in the oceans. The key is the ‘warm pool’ of ocean water that sits in the western tropical Pacific.

All year long the Trade Winds and the ocean current push the surface water warmed by the tropical sun to the far western side of the North Pacific. Hurricane seasons come and go in other parts of the world but the water of the ‘warm pool’ is always warm enough to start a hurricane-though they are most common between June and December.


Tornadoes are nature’s most violent storms. Nothing that the atmosphere can dish out is more destructive: they can sweep up anything that moves; and they can lift buildings from their foundations, making a swirling cloud of violently flying debris. They are very dangerous, not only because of the sheer power of their wind, and the missiles of debris they carry, but because of their shear unpredictability. Tornado strength and destructive capability is measured on the Fujita Scale. A tornado is a violent rotating column of air, which at a distance appears as an ice cream cone-shaped cloud formation. Other storms similar to tornadoes in nature are whirlwinds, dust-devils (weaker cousins of tornadoes occurring in dry lands), and waterspouts (a tornado occurring over water). Tornadoes are most numerous and devastating in central, eastern, and northeastern USA, where an average of 5 per day are reported every May. They are also common in Australia (15 per year), Great Britain, Italy, Japan, Bangladesh, east India, and central Asia. While the greatest number of fatalities occurs in the United States, the deadliest tornadoes by far have occurred in the small area of Bangladesh and east India. In this 8,000 mile2 (21,000 km2) area, 24 of the 42 tornadoes known to have killed more than a 100 people have occurred. This is likely due to the high population density and poor economic status of the area as well as a lack of early warning systems.

We can see tornadoes as miniature hurricanes. Although tornadoes can form over tropical oceans they are more common over land. The formation of tornadoes is encouraged when there is warm, moist air near the ground and cold dry air above. This occurs frequently in late spring and early summer over the Great Plains of the USA. Intense heating of the ground by the sun makes warm, moist air rise. As it does so it cools and forms large cumulo-nimbus clouds. The strength of the updraft determines how much of the surrounding air is sucked into the bottom of what becomes a tornado. Two things help the tornado to rotate violently; the first is the Coriolis force and the second is the high level jet stream passing over the top of the storm, adding an extra twist to the tornado. Because of the conditions under which tornadoes are formed they can easily occur beneath thunderstorms and hurricanes.

In the USA nearly 90 per cent of tornadoes travel from the southwest to the northeast, although some follow quick changing zigzag paths. Weak tornadoes or decaying tornadoes have a thin ropelike appearance. The most violent tornadoes have a broad dark funnel shape that extends from the dark wall cloud of a large thunderstorm. There have even been reports of some tornadoes practically standing still, hovering over a single field, and of others that crawl along at 5 miles per hour. On the other hand, some have been clocked at over 70 miles per hour. However, on average, tornadoes travel at 35 miles per hour. It has been noted that most tornadoes occur between 3pm and 9pm, but they have been known to strike at any time of day or night. They usually only last about 15 minutes, staying only a matter of seconds in any single place-but then some tornadoes just do not fit any of these rules, for example on 18 March 1925 a single tornado travelled 219 miles in 3.5 hours through Missouri, Illinois, and Indiana killing 695 people.

Tornado Alley

Tornado Alley is the nickname for the area in which most tornadoes occur in the USA, and it expands through spring and summer as the heat from the sun grows warmer and the flow of warm moist air from the Gulf of Mexico spreads further north. An area that includes central Texas, Oklahoma, and Kansas is at the hard core of Tornado Alley, but before the season is over it can have expanded to the north to Nebraska and Iowa. It shrinks and swells over time but there is only one Tornado Alley. Nowhere else in the world sees weather conditions in a combination so perfect to make tornadoes.

The key reasons for this special area are:

(1) Beginning in spring and continuing through summer, low-level winds from the south and southeast bring a plentiful supply of warm tropical moisture up from the Gulf of Mexico into the Great Plains;

(2) From down off the eastern slopes of the Rocky Mountains or from out of the deserts of northern Mexico come other flows of very dry air that travel about 3,000 feet above the ground; and

(3) At 10,000 feet high the prevailing westerly winds, sometimes accompanied by a powerful jet stream, race overhead, carrying cool air from the Pacific Ocean and providing a large temperature difference, which will drive the tornadoes and the twists to get started.

In 2011 there were 1,897 tornadoes reported in Tornado Alley in the USA beating the record of 1,817 tornadoes recorded in 2004. The year 2011 was also an exceptionally destructive and deadly year in terms of tornadoes, killing at least 577 people worldwide. Of those, an estimated 553 were in the United States, which compared to 564 US deaths in the prior 10 years combined. That year saw the second greatest number of deaths due to tornadoes in a single year in US history. However, this is still a long way off from the most deadly tornado on record, which occurred on 26 April 1989 in Bangladesh and killed over 1,300 people, injured 12,000 people, and destroyed everything but a few trees from Daultipur to Salturia.

Winter storms

For people living in the mid-latitudes weather seems to be a permanent topic of conversation. This is because it is always changing. In Britain there is a saying, ‘if you do not like the weather wait an hour and it will change’. This is because the climate of the mid-latitudes is dominated by the titanic clash between the cold polar air moving southward and the warm sub-tropical air moving northwards. This clash of air masses takes place at the Polar Front.

The Polar Front moves north and south with the seasons. In summer when the sub-tropical air is warmer it moves further towards the pole. During winter when conditions are much colder the polar air mass is dominant and the Polar Front moves towards the Equator. Where these two air masses meet rain is formed. This is because warm air can hold more water vapour and when it clashes with the cold air this vapour condenses into clouds, which in turn produce rain. But it is the upper atmosphere which really controls the shape and thus the weather of the Polar Front. The upper atmosphere is characterized by fast ‘jet streams’ that race around the planet. These powerful jet streams push the Polar Front around the Earth, but as it does so the Front wrinkles and becomes a mass of so-called planetary waves moving gradually round our planet. These waves have a great effect on our weather, causing us all to complain about the weather being so changeable and of course wet. One of these waves can pass over a town in about 24 hours. The weather will be experienced as starting out to be relatively cold but with clear skies. As the warm front passes overhead the conditions get warmer and it starts to rain-usually light rain or drizzle. As the centre of the warm air mass reaches the town the weather turns cloudy and muggy and the rain stops. Then the second front, the cold front, passes overhead; temperatures drop and there is a short period of very heavy rainfall.

Then it is back to cold, clear weather until the next wave reaches the town. As we have seen there are many storms that are associated with distinct areas of atmospheric circulation described in the section on Hadley Cells. Ice, wind, hail, and snow storms are associated with either the Polar Front or high mountain regions and are worse in the winter time. In the Northern Hemisphere these types of storms are common over North America, Europe, Asia, and Japan. For snow to reach the ground the temperature of the air between the base of the cloud and the ground must be below 4°C, otherwise the snowflakes melt as they travel through the air. For hailstones to form the top of the storm must be very cold. High up in the atmosphere water droplets can become super cooled to less than 0°C, which collide in the atmosphere to form ice balls or hailstones. If you cut open a hailstone you can see the layers of ice that have built-up like an onion. The stones can vary from between 2 mm and 20 cm. Their size depends on how strong the updraft of air is, as this determines how long they stay in the atmosphere before dropping out. The worst storm conditions are called blizzards. These combine strong winds, driving snow, ice, and hail, with air temperatures as low as −12°C and visibility less than 150 meters.

Caught in the cold

When your body loses the battle against the cold, it is often someone else who will notice it. This is why you should always be on the look-out for the symptoms of cold weather exposure in your companions. When the cold has started to affect you badly, you are not always the best judge of the seriousness of the problem. You still think that you are okay you just need another minute’s rest. These are the signs to look out for:

• You cannot stop shivering

• You are fumbling your hands

• Your speech is slow and slurred and may even be incoherent

• You stumble and lurch as you walk

• You are drowsy and exhausted and feel the need to lie down even though you are outside

• Maybe you have rested, but cannot then get up

A person acting like this needs to get into dry clothes and a warm bed. This is because the core temperature of that person has started to drop, which is extremely dangerous for the body; if it is not stopped it will result in death. They need a warm hot water bottle, heating pad, or warm towels on their body. They need warm drinks. They do NOT need an alcoholic or caffeinated drink, as these speed up the person’s heart rate, causing them to lose yet more heat; they also dehydrate the body, which hinders its recovery. Also do NOT massage or rub the person, as this again takes away heat from the body core where it is most required. The person should also always be seen by a doctor.

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