Extreme events are generally easy to recognize but difficult to define. This is due to several reasons. First, there is no unique definition for what is meant by the word "extreme": several definitions are in common use. Second, the concept of "extremeness" is relative and so strongly depends on context. Third, the words "severe," ''rare,'' ''extreme,'' and "high-impact" are often used interchangeably.
1.2.1 Severe, rare, extreme, or high-impact?
In an attempt to alleviate some of the confusion, here are some definitions of these terms.
* Severe events are events that create large losses in measures such as number of lives, financial capital, or environmental quality (e.g., loss of species). The severity can be measured by the expected long-term loss, which is known as the risk. Risk depends on the product of the probability of the event (the hazard), the exposure to the hazards (e.g., how many people are exposed), and the vulnerability (i.e., how much damage ensues when someone is hit by the event). In other words, severity is a function of not only the meteorological hazard but also the human state of affairs. For example, the severity of US landfall hurricanes has increased considerably in recent years, mainly owing to increased numbers of people settling in the US Gulf states (increased exposure).
* Rare events are events that have a low probability of occurrence. Because of the rarity of these events, human societies (and other ecosystems) are often not well adapted to them and so suffer large amounts of damage when they do occur. Hence, despite their rarity, the large vulnerability associated with such events can often lead to large mean losses (and hence they are a type of severe event).
* Extreme events are events that have extreme values of certain important meteorological variables. Damage is often caused by extreme values of certain meteorological variables, such as large amounts of precipitation (e.g., floods), high wind speeds (e.g., cyclones), high temperatures (e.g., heat waves), etc. Extreme is generally defined as either taking maximum values or exceedance above pre-existing high thresholds. Such events are generally rare; for example, extreme wind speeds exceeding the 100-year return value, which have a probability of only 0.01 of occurring in any particular year.
* High-impact events are severe events that can be either short-lived weather systems (e.g., severe storms) or longer-duration events such as blocking episodes that can lead to prolonged heat waves and droughts. The World Meteorological Organization (WMO) program THORPEX uses the phrase ''high-impact weather'' rather than ''severe weather'' to help people avoid confusing the term severe with only short-lived events such as individual storms (D. Burridge, personal communication).
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