Extreme Weather Events

An extreme weather event is weather that is rare at a particular place and time of year. Climatologically, it lies near or beyond the upper or lower tail of the observed distribution of a weather variable, using a stated threshold and reference period. Its absolute magnitude is therefore place-specific and season-specific.

Statistical criterion of extremeness

Extremeness is determined against a probability distribution constructed from observations for a stated reference period. The IPCC indicates that an event would normally be at least as rare as the 10th or 90th percentile, although the appropriate threshold depends on the variable and purpose.

• Relative thresholds identify unusually hot, cold, wet or dry conditions for that location and season.
• Absolute thresholds, such as a specified temperature or rainfall amount, are also used in operational indices.
• A return period expresses rarity probabilistically; it is not a guarantee that events will occur at regular intervals.

Characteristics used for classification

An event may be extreme because of its magnitude, duration, frequency, timing or spatial extent. Climatologists therefore examine more than whether an observation breaks a record.

• Short-duration extremes include intense rainfall, severe wind and exceptionally high or low temperature.
• Persistent extreme weather can become an extreme climate event, such as seasonal drought or an exceptionally wet season.
• Compound extremes arise when multiple hazards occur together or in sequence, producing effects greater than those expected from each event separately.

Interpretation and limits

Climatological extremeness is distinct from disaster impact. An extreme event may cause little damage where exposure is low, while a less rare event may become disastrous where exposure and vulnerability are high.

• Because climate distributions change over time, the reference period and method must be stated.
• A single extreme event does not by itself establish climate-change causation; event attribution assesses whether human influence changed its probability or intensity.