Depression or Cyclone (Low Pressure Area)

Unique identifier / Notation

MH0030

Synonyms

Low

A depression or cyclone is a region of the atmosphere in which the pressures are lower than those of the surrounding region at the same level (WMO, 1992).

Primary reference(s)

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

Additional scientific description

A depression or cyclone is represented on a synoptic chart by a system of isobars at a specified altitude level (or a system of contours at a specified pressure level) which enclose relatively low values of pressure (or altitude) (WMO, 1992).

Metrics and numeric limits

A depression or area of low pressure generally refers to a low pressure area with winds below storm strength of 89 km/h (WHO, 2015).

Key relevant UN convention / multilateral treaty

Not applicable.

Examples of drivers, outcomes and risk management

Depending on the size of a depression or cyclone, the impact can extend over a very wide area, with strong winds and heavy rain. However, the greatest damage to life and property is not from the wind, but from secondary events such as storm surges, flooding, landslides and tornadoes (WHO, no date).

Factors associated with vulnerability include settlements located in low-lying coastal areas (direct impact); poor building design, or construction; insufficient lead time for warning and evacuation; noncompliance with evacuation procedures where recommended; and inadequate shelter (WHO, no date).

The main impacts include direct impacts and indirect impacts.

Direct impacts: Injuries, trauma, and asphyxiation due to entrapment are observed and result from building collapse and windstrewn debris. Electrocution or drowning happen while securing property such as television antennas or boats (WHO, no date).

Indirect Impacts: Risk for water-borne disease and vector-transmitted disease can be exacerbated: human exposure to disease vectors can be increased due to changes in the physical environment. The impact on health infrastructures and all lifeline systems can be significant and may result in food shortages and interruption of basic public health services (water, etc) (WHO, no date). Short- and long-term mental health effects have been observed (WHO, no date).