What this lesson teaches
I can compare temperature and rainfall patterns across the tropical climate zones.
Syllabus 9173, Topic 2.1, 1. Tropical Climates. What are the similarities and differences in the temperature and rainfall patterns in the tropics?
Make a guess
Two hot stations each get 1,800 mm of rain a year. Must they have the same climate type?
- No. The timing of the rain can differ.
- Yes, because their totals are the same.
- No, because one must be a desert.
- Yes, because tropical places are wet in most months.
Show the answer
No. The timing of the rain can differ.
A yearly total can hide a long dry season. One station may be wet all year, the other may have months with almost no rain.
All tropical climates are hot, and what separates the humid and arid zones is how much rain falls and when.
The tropics get strong, concentrated sunlight, so temperatures are high. They absorb more energy than they lose. The geographical tropics lie between about 23.5 degrees N and 23.5 degrees S. The syllabus uses a wider climate belt whose edge shifts around 30 to 35 degrees north and south. Altitude still matters: a tropical mountain can be cold.
Climate zones group places with similar temperature and rainfall. The humid tropics have three. Tropical rainforest (Af) is wet all year: every month gets at least 60 mm of rain. Tropical monsoon (Am) has a short dry season and a large yearly total. Tropical savanna (Aw) has a long dry winter and a wet summer. Summer depends on the hemisphere.
The arid tropics have two zones. Hot steppe (BSh) is semi-arid. Hot desert (BWh) is even drier. Aridity means rainfall is low compared with evaporation, so no single rainfall figure defines a desert.
To read a climograph, add the 12 monthly rainfall totals, find the temperature range, and average the 12 monthly temperatures. Then describe when the wet and dry months are. A yearly total can hide a long dry season.
- Af, Am, Aw
- Rainforest, monsoon and savanna: the humid tropics.
- BSh, BWh
- Hot steppe and hot desert: the arid tropics.
- Af rule
- Every month has at least 60 mm of rain.
Step by step
Temperature
Are monthly means high? Note altitude and range.
Rainfall amount
Add all 12 monthly totals.
Rainfall timing
How long do dry months last?
Worked example: Changi: warm and wet all year
For Changi's 1991-2020 normal, the 12 rounded monthly totals add to 2113.2 mm; MSS reports 2113.3 mm from its exact data. The average of the 12 monthly temperatures is about 27.8 degrees C. Monthly means range from 26.8 to 28.6 degrees C, a range of 1.8 degrees C. Even the driest month gets 105.1 mm. So Changi is Af.
Try it step by step
Problem 1
Changi, Singapore, 1991-2020. Monthly rainfall (mm), January to December: 221.6, 105.1, 151.7, 164.3, 164.3, 135.3, 146.6, 146.9, 124.9, 168.3, 252.3, 331.9.
Show the working
- Add the yearly rainfall: Add all twelve months: 2113.2 mm.
- Find the driest month: The driest month is February, with 105.1 mm.
- Apply the Koppen rules: Every month has at least 60 mm, so Changi is tropical rainforest (Af). No other test is needed.
Problem 2
Mumbai (Colaba), India, 1991-2020. Monthly rainfall (mm), January to December: 0.6, 0.4, 0.7, 0.2, 15.9, 506.0, 768.5, 471.9, 355.6, 81.7, 8.5, 3.4.
Show the working
- Add the yearly rainfall: Add all twelve months: 2213.4 mm.
- Find the driest month: The driest month is April, with 0.2 mm.
- Apply the Koppen rules: The driest month is under 60 mm, so it is not Af. The monsoon (Am) test needs the driest month to reach 100 - (2213.4 / 25), about 11.5 mm. April's 0.2 mm is far below this, so Mumbai is tropical savanna (Aw).
Problem 3
A practice station (invented numbers). Monthly rainfall (mm), January to December: 40, 45, 70, 150, 350, 500, 520, 480, 400, 250, 120, 75.
Show the working
- Add the yearly rainfall: Add all twelve months: 3000 mm.
- Find the driest month: The driest month is January, with 40 mm.
- Apply the Koppen rules: 40 mm is under 60 mm, so it is not Af. The monsoon threshold is 100 - (3000 / 25) = -20 mm, and 40 mm is above it, so the station is tropical monsoon (Am).
Watch out for this
Every tropical place has heavy rain every month.
Tropical means hot. Sinking air, seasonal winds and distance from the sea create dry tropical climates too.
Check your understanding
Two hot stations each get 1800 mm a year. Station A has no dry month; Station B has five very dry months. What is the best comparison?
- Station B must be cooler, because it gets less rain.
- They must have the same climate type, as their totals match.
- Their totals match, but water through the year differs.
Show the answer
Their totals match, but water through the year differs.
Yes. Timing matters as much as the total.