Tropical climates share strong heating. Rainfall amount and seasonality separate the humid and arid climate zones.
The tropics receive concentrated sunlight and have high temperatures compared with higher latitudes. The geographical tropics lie between about 23.5 N and 23.5 S. The syllabus uses a wider climatic region whose margin fluctuates around 30-35 degrees north and south. Altitude still matters: a tropical mountain can be cold.
The humid tropics include tropical rainforest (Af), tropical monsoon (Am) and tropical savanna (Aw) climates. Af is wet throughout the year; each month has at least 60 mm of rain in the Koppen classification. Am has a short dry season and a large annual total. Aw has a longer, marked dry winter and a wet summer. For Aw climates, summer means summer in the station's own hemisphere. Northern and Southern Hemisphere summers occur at opposite times of year.
The arid tropics include hot sub-tropical steppe (BSh) and hot tropical desert (BWh). BSh is semi-arid; BWh has a greater moisture deficit. Aridity reflects low rainfall relative to evaporative demand. The Koppen dry-climate threshold depends on temperature and rainfall seasonality, so one fixed rainfall total cannot classify every desert.
Read a climograph by calculating annual total rainfall, the range between the highest and lowest monthly mean temperatures, and the mean of the 12 monthly mean temperatures. Then describe rainfall distribution: wet and dry months, their duration and any peaks. An annual total alone can hide a long dry season.
Step by step
Temperature
Check whether monthly means stay high; note altitude and annual range.
Rainfall amount
Add all 12 monthly rainfall totals.
Rainfall timing
Identify how long dry conditions last before assigning a climate type.
Worked example: Changi: warm and wet all year
For Changi's 1991-2020 normal, the 12 rounded rainfall totals sum to 2113.2 mm; MSS reports 2113.3 mm using its underlying annual calculation. The mean of the 12 monthly temperatures is about 27.8 degrees C. Monthly mean temperature ranges from 26.8 to 28.6 degrees C, a range of 1.8 degrees C. Even the driest month averages 105.1 mm. This supports Af, despite a wetter end to the year.
Watch out for this
Every tropical place has heavy rain in every month.
Tropical refers primarily to the warm climatic belt. Descending air, seasonal circulation and distance from moisture sources create dry tropical climates.
Check your understanding
Two warm stations receive 1800 mm annually. Station A has no dry month; Station B has five very dry months. What is the best comparison?
- Their climate types must be identical.
- Their annual totals match, but their seasonal water availability differs.
- Station B must have lower temperatures.