Types of measures of variability
With adequate measurements of climatic conditions covering many years, it is possible to define what is considered normal and what is an extreme event for any part of the world. Data gathered over the 30-year period from 1961 to 1900 define the latest Normals used for climate reference. At any given time of the year, an extreme high temperature might be defined as one that occurs only once in every 30 years. A cold winter or hot summer can be specified in a similar way, or in terms of the number of days below or above defined exceptional values. This means that when there is a succession of extremes, or more extreme events over a period such as a season, it is possible to estimate whether these extremes are part of the normal expectation for the locality, or are so unlikely that they can only be explained in terms of some more radical shift in climate.
The basic properties of any data series, for example temperature, can be defined in terms of the mean over time and the amount of variance about the mean. Other meteorological variables exhibit more complicated statistical properties. For instance, rainfall is episodic. In many parts of the world, much of the annual rainfall falls in a short rainy season. In addition, most of that rain may be concentrated in a few heavy falls and small shifts in the large-scale weather patterns from year to year may significantly alter the amount and distribution of seasonal rainfall. More complex techniques usually are needed to interpret variations in rainfall.
Some facts on global climate extremes
Since extreme meteorological events may be good markers of climatic change or variability, it is important to keep good records of such extremes. A worldwide collection of such events has been assimilated by WMO in conjunction with the University of Arizona. In 2006, the WMO Commission for Climatology has developed of a world archive for verifying, certifying and storing world weather extremes. Existing record extremes are  available to the general public on They cover temperature, pressure, rainfall, hail, aridity, wind, tornados and cyclones, and are displayed on maps of the world, the hemispheres and the continents.

From Climate extremes to disasters
On 10 February 1935 35 cm of snow fell on Laghouat on the edge of the Algerian Sahara. While this was certainly an extreme event, it was no disaster. Disasters occur frequently as a result of extreme climatic events, however, and also as a result of the accumulation of extreme events that constitute climatic variability or change. The word disaster is used to describe such events when they cause human sickness, death or migration on a large scale, or when they cause severe economic damage.

Although human misery cannot be adequately represented by statistics, it is helpful to have some measure of the global scale of the impact of weather-related disasters. Many data have been collected by the International Federation of the Red Cross and Red Crescent Societies (IFRC). The important features of these figures are:
  • droughts killed more people than all other disasters combined;
  • droughts and floods affected about an equal number of people, and far more than high winds (including hurricanes, cyclones, typhoons, storms and tornadoes);
  • floods were, however, by far the greatest cause of homelessness; and
  • for the limited time covered (1973-97), there were large variations in the numbers of people affected by different forms of disaster in successive five-year periods. This makes it difficult to draw any definite conclusions about trends, apart from noting that the number of people affected by floods appears to be rising.
Examples of Climate Events and extremes
When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events have a disproportionate impact on human populations, and so represent a vital aspect of climate variability and change.
Tropical cyclones
Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. On average 80 tropical cyclones form every year. They are called differently depending on where they are formed: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region.

Mid-latitude winter storms
Heavy rain and snow are dangerous for vulnerable communities. They can exacerbate rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.

As an example, the East Asian winter is dominated by cold and relatively dry winds. The Siberian anticyclone is a persisting climatic feature that blows Arctic air over Siberia and northern China. Part of the flow sweeps out toward the North Pacific and part southward through China to the equatorial regions. The winter monsoon throughout this region is characterized by successive outbursts of cold air, called cold waves, that produce sharp falls in temperature of more than 10°C and are accompanied by snow and, in the south, rain. Snowstorms can be especially violent over northern China with sub-zero temperatures and gales lasting many days. The snowstorms can be damaging to communities and are particularly disruptive to transport, including coastal shipping. Periods of frost following the cold outbreak last several days and are a major hazard over the south of China as they have far-reaching effects on agriculture, especially on plants and crops. The frequency of cold waves varies greatly from year to year; as many as 10 per year or as few as one have been experienced. An active period is associated with higher pressure within the Siberian anticyclone and an intense low pressure system near the Aleutian Islands.
Droughts and duststorms
The primary cause of any drought is deficiency of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.

Duststorms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.
Floods can occur anywhere after heavy rains. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surges caused by tropical cyclones, tsunamis or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide.

The term ‘monsoon’, of Arabic origins, refers to a steady seasonal wind, and became widely associated with the Indian subcontinent and the onset of the main rainfall season. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world, including India, East Asia, Australia, and both North and South America ( map of the global monsoon system) .

Monsoon forecasts have improved since the early 1980s. This is the result of a growing understanding of the empirical relationships between indicators around the world and the subsequent monsoon. One reason for these advances has been the rising quality of data. Recent satellite observations have also revived interest in Himalayan snow cover as a predictor. They show that the relationship first identified by Blandford is a useful guide, but that the extent of the all-Eurasian winter snow cover was a better indicator, given the geographically uneven and variable nature of snow cover over the Himalayas, Tibet and Siberia.
Heatwaves amd cold waves/frost
Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.

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