Tropical Climates

7. 1 - Tropical climates - content

Global distribution and climatic characteristics of humid tropical and seasonally humid tropical environments: the roles of the intertropical convergence zone (ITCZ), subtropical anticyclones, and monsoons.

The key features of temperature and rainfall and their annual and diurnal variations in the humid tropical and seasonally humid tropical environments.

Climate Classification

The Köppen Climate Classification System is the most widely used system for classifying the world's climates. Its categories are based on the annual and monthly averages of temperature and precipitation. The Köppen system recognises five major climatic types; each type is designated by a capital letter.

Tropical moist climates, in which all months have average temperatures above 18° Celsius are denoted by the letter A

Tropical moist climates extend northward and southward from the equator to about 15 to 25° of latitude. In these climates all months have average temperatures greater than 18° Celsius. Annual precipitation is greater than 1500 mm. Three minor Köppen climate types exist in the A group, and their designation is based on seasonal distribution of rainfall. Af or tropical wet is a tropical climate where precipitation occurs all year long.

Monthly temperature variations in this climate are less than 3° Celsius. Because of intense surface heating and high humidity, cumulus and cumulonimbus clouds form early in the afternoons almost every day. Daily highs are about 32° Celsius, while nighttime temperatures average 22° Celsius. Am is a tropical monsoon climate. Annual rainfall is equal to or greater than Af, but most of the precipitation falls in the 7 to 9 hottest months. During the dry season very little rainfall occurs. The tropical wet and dry or savanna (Aw) has an extended dry season during winter. Precipitation during the wet season is usually less than 1000 millimeters, and only during the summer season.

Af - Iquitos, Peru 4° S , Elevation: 104 m

Am - Mangalore, India 13° N , Elevation: 22 m

Aw - Darwin, Australia 12.5° S , Elevation: 27 m

The most important control in tropical climates is the position of the Inter Tropical Convergent Zone or ITCZ. The ITCZ is an area of low pressure and marks the point of trade wind convergence. It is also directly related to the sub-solar point.

The ITCZ's location varies throughout the year and while it remains near the equator, the ITCZ over land drifts farther north or south than the ITCZ over oceans. This is due to greater variation of land temperatures. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean. In temperate latitudes relative migration of the sun between the Tropic of Cancer and Tropic of Capricorn is responsible for creating the seasons but in tropical latitudes it is responsible for the migration of low pressure and the resulting shifts in seasonal tropical rains.

Humid Tropical Environments

Humid tropical (Equatorial) environments are characterised by hot average temperature all year round and high monthly precipitation, typically no less than 60 mm a month with annual precipitation tending to be over 2000mm. The diurnal temperature range is greater than the annual temperature range. The reason for this regular climate is due to a feedback between low pressure convectional processes that result from the high altitude of the sun (ITCZ) and the high levels of soil moisture and interception of rainfall from the dense vegetation cover leading to transpiration. This feedback leads to a repetitive climate pattern of hot humid air, dry but misty mornings and late afternoon downpours and convectional storms.

Seasonally Humid Tropical Environments

Climates with less than 60mm in any single month and lower than 1000mm annual rainfall may be considered seasonal. They are characterised by dry and wet seasons that relate to their situation in regard to the equator and the migration of the ITCZ. Effectively as the ITCZ migrates away from the equator a zone of low pressure and converging air tracks it. At this zone of rising air seasonal rain occurs due to convectional forces. The map below shows the location of Seasonally Humid Tropical Environments, which can be found north and south of the Equator within the tropical belt.

What about Monsoons?

Pressure and the weather

Mean monthly and annual rainfall at sixteen selected stations in Peninsular Malaysia. The background map displays the orography of the area.

Seasonally Humid Tropical Environments - An example

Nigeria is in a Seasonally Humid Tropical Environment. The pattern of rainfall is quite different throughout the year in Lagos compared to Kano. Lagos has a more humid climate experiencing rainfall in all months of the year, with a maximum monthly rainfall of 16.3 inches in June. In addition Lagos appears to expericnce a rise, fall and rise in rainfall patterns. This pattern of rainfall occurs as a a result of the ITCZ passing by two times in the year. Once as it migrates north towards the Tropic of Cancer and secondly as it returns sourthwards in the direction of the Tropic of Capricorn. In contrast Kano experiences a much more clearly defined dry season and (singular) wet season. Between November and March, no precipiation falls in Kano. The rains arrive in May and remain until September/October, with its maximum rainfall remaining significantly lower than Lagos at just 11.3 inches. See the climate graphs below.

The diagram to the left illustrates the impact of the shifting ITCZ throughout the year in Nigeria. As you can see, the ITCZ shifts north to the Tropic of Cancer between March and June. It delivers rainfall to Lagos earlier than in Kano because the ITCZ reaches Lagos first. By June the ITCZ stands over the Tropic of Cancer and rains have reached a maximum in Lagos. The heat of the sun is now producing convectional rain for Kano as well but it won't be until July/August that Kano experiences its maximum rainfall. By August/ September the ITCZ has returned to Lagos and delivers increased preciptation for the second time. In December, the ITCZ has migrated south to the Tropic of Capricorn and so both Lagos and Kano experience their dry season.

Exam Question & Answer

Describe and explain the differences between the humid and monsoon climates in the tropics. [10]

Humid tropical climates have no distinct dry season; all months have at least 60mm of rain, whereas the monsoon climate exhibits a marked dry season. In both cases temperatures are high although more consistently so in the humid climates where average daily temps can be as high as 27°C The monsoon climates display greater seasonal variation in both temperature and ppt. Reasons are that the humid climates are found within the equatorial low pressure driven by intense heating and hence convectional rainfall. Monsoon climates are most commonly found in S. Asia where they are subject to seasonal changes in wind direction In Asia, during the summer, the high land temperatures produce onshore winds bringing moisture-laden winds. High pressure in winter brings NE winds that are dry.

Page updated

Report abuse