Tropical savanna climate (Aw)


Tropical savanna climate or tropical wet and dry climate is a type of climate that corresponds to the Köppen climate classification categories Aw (for a dry winter) and As (for a dry summer, which is unusual). The driest month has less than 60 mm (2.4 in) of precipitation and also less than  of precipitation.

This latter fact is in direct contrast to a tropical monsoon climate, whose driest month sees less than 60 mm (2.4 in) of precipitation but has more than  of precipitation. In essence, a tropical savanna climate tends to either see less rainfall in the rainy season than a tropical monsoon climate or have more pronounced dry season(s). The dry season is longer too, often three months or longer. Total annual rainfall is typically 1000-1500 mm (40-60 inches).

In tropical savanna climates, the dry season can become severe, with some months having no rainfall, and often drought conditions prevail during the course of the year. Tropical savanna climates often feature tree-studded grasslands, rather than thick jungle. It is this widespread occurrence of tall, coarse grass (called savanna) which has led to Aw and As climates often being referred to as tropical savanna. However, there is some doubt whether tropical grasslands are climatically induced. Additionally, pure savannas, without trees, are the exception rather than the rule.

Köppen–Geiger climate classification map for Tropical - source: wikicommons, authors: Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F. Köppen–Geiger climate classification map for Tropical - source: wikicommons, authors:  Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F., 2018

Tropical savanna climates are most commonly found in Africa, Asia, and South America. The climate is also prevalent in sections of Central America, northern Australia, the Pacific Islands, in sections of North America and some islands in the Caribbean. Most places that have this climate are found at the outer margins of the tropical zone.

Sometimes As is used in place of Aw if the dry season occurs during the time of higher sun and longer days, such as at Honolulu International Airport. This may also be due to a rain shadow effect that cuts off summer precipitation in a tropical area. This is the case in East Africa (e.g. Moi International Airport), Sri Lanka (e.g. Trincomalee/China Bay Airport) and coastal regions of Northeastern Brazil (e.g. Pinto Martins – Fortaleza International Airport), for instance. The difference between 'summer' and 'winter' in such locations is usually so slight that a distinction between an As and Aw climate is a quibble. In most places that have tropical wet and dry climates, however, the dry season occurs during the time of lower sun and shorter days because of reduction of or lack of convection, which in turn is due to the meridional shifts of the Intertropical Convergence Zone (ITCZ) during the entire course of the year.

Flight safety and planning considerations

During the rainy season, heavy rains can cause disruption to infrastructure and airport emergency services. Thunderstorms can be accompanied by severe downdrafts and heavy rain causing flooding on airport surfaces and the risk of aquaplaning on runways.Wildfires, with their associated problems, are common in the dry season.

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