Extra-tropical Cyclone

Primary reference(s)

WMO, 1992. International Meteorological Vocabulary, WMO-No. 182. 2nd Edition. World Meteorological Organization (WMO). Accessed 13 October 2020.

Additional scientific description

An extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude or baroclinic storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. In contrast, tropical cyclones typically have little to no temperature differences across the storm at the Earth’s surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics (NASA, 2020).

Notes for clarification: Differences between an extra-tropical cyclone and a tropical cyclone:

• An extra-tropical cyclone is a low-pressure system that primarily gets its energy from the temperature difference in the horizontal direction across the cyclone (known as temperature gradient in meteorology). Structurally, tropical cyclones have their strongest winds near the Earth’s surface, while extra-tropical cyclones have their strongest winds near the tropopause – about 8 miles (12 km) up. These differences are due to the tropical cyclone being ‘warm-core’ in the troposphere (below the tropopause) and the extra-tropical cyclone being ‘warm-core’ in the stratosphere (above the tropopause) and ‘cold-core’ in the troposphere. ‘Warm-core’ refers to being relatively warmer than the environment at the same pressure surface (‘pressure surfaces’ are simply another way to measure height or altitude).
• A tropical cyclone will often transform into an extra-tropical cyclone as it recurves poleward and to the east. Occasionally, an extra-tropical cyclone will lose its frontal features, develop convection near the centre of the storm and transform into a full-fledged tropical cyclone. Such a process is most common in the North Atlantic and Northwest Pacific basins. The transformation of a tropical cyclone into an extra-tropical cyclone (and vice versa) is currently one of the most challenging forecasting problems (NOAA, 2004; Hong Kong Observatory, 2019).

Extra-tropical cyclones are large rotating weather systems that occur in the extra-tropics (generally more than 30° latitude away from the equator). They consist of an approximately circular region of low surface pressure, of a radius of 100–2000 km, accompanied by cold and warm fronts. They typically develop in regions of strong horizontal temperature gradients, which are commonly denoted on a weather chart as a cold or quasi-stationary front. In turn, such fronts often connect to a pre-existing decaying extra-tropical cyclone, which itself is situated some way downstream (typically to the north-east) (CCPO, no date).

At the same time, high up in the atmosphere (around 10 km altitude) a jet stream is typically found relatively close by. In fact, the intensity of an extra-tropical cyclone is closely related to the strength of this jet stream. The strongest extra-tropical cyclones occur in the winter months when the jet stream is at its strongest. Periods when the jet stream is unusually strong can lead to two or more strong cyclones occurring within days of each other. The total lifecycle of an extra-tropical cyclone from birth (genesis) through to development and on to decay (lysis) can occasionally be more than 10 days, although somewhere in the range of 2 to 5 days is more typical (Frame et al., 2017).

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

The major hazards associated with extra-tropical cyclones are high winds and precipitation (rain and snow). Precipitation occurs primarily along fronts and, on average, is not particularly intense relative to that delivered by tropical cyclones and convective storms (Frame et al., 2017).