Describe the characteristics of the equatorial climate and explain the factors that cause them

What this dot point is asking

SEAB wants you to describe the characteristics of the equatorial climate and to explain the factors that cause them. The central insight is that one factor, the place's low latitude near the equator, drives everything else: it keeps the sun high overhead all year, which keeps temperatures high and uniform and powers the daily convection that brings heavy rain.

The answer

Characteristics of the equatorial climate

Places within a few degrees of the equator, such as Singapore and much of the Amazon and Congo basins, share a distinctive climate:

• High temperatures all year: mean monthly temperatures stay around $26\ \text{C}$ to $28\ \text{C}$ every month.
• A very small annual temperature range: typically only $2\ \text{C}$ to $3\ \text{C}$, because there is no cold season.
• Heavy rainfall every month: usually over $150\ \text{mm}$ a month, with annual totals often exceeding $2000\ \text{mm}$, well distributed through the year.
• High humidity: the air is constantly moist, often around $80$ to $90\%$ relative humidity.
• Afternoon convectional rain: rain frequently falls in heavy afternoon thunderstorms.
• Small daily and seasonal variation: days and nights are of nearly equal length, and conditions change little through the year.

The factors that cause them

The characteristics flow from a few linked factors:

• Low latitude and the overhead sun. Near the equator the sun is overhead or nearly so all year. Its rays strike the ground at a high angle, concentrating energy on a small area, so heating is intense and constant, keeping temperatures high with no cold season.
• Strong convection. Intense surface heating warms the air, which rises rapidly. As it rises it cools, and the abundant water vapour condenses to form towering cumulonimbus clouds, giving heavy convectional rainfall, usually in the afternoon.
• High humidity. The hot oceans and dense rainforest release large amounts of water vapour into the air through evaporation and transpiration, keeping humidity high and feeding the daily rain.

Why the seasons are weak

Because the sun stays near overhead all year and day length barely changes, there is no strong summer or winter. The "seasons" are better described as slightly wetter and slightly drier spells rather than hot and cold ones.

Examples in context

Example 1. Singapore. Lying just north of the equator, Singapore shows the equatorial climate clearly: mean temperatures hover around $27\ \text{C}$ all year, the annual range is barely $2\ \text{C}$, and rainfall exceeds $2000\ \text{mm}$ a year with rain in every month, much of it in sudden afternoon thunderstorms. Its position near the equator, with the sun always high, is exactly why the heat and rain are so constant.

Example 2. The Amazon rainforest, Brazil. The Amazon basin, straddling the equator, has the same hot, wet, humid climate, with daily convectional storms feeding the world's largest rainforest. The forest itself recycles huge amounts of water vapour through transpiration, reinforcing the high humidity and rainfall. It shows how the equatorial climate and the rainforest ecosystem are linked, both driven by the overhead sun and intense convection.

Try this

Q1. State two characteristics of the equatorial climate shown on a climate graph. [2 marks]

• Cue. High and uniform temperatures all year (a small annual range of only a few degrees) and heavy rainfall in every month with no dry season (a high annual total).

Q2. Explain why the equatorial climate has only a very small annual temperature range. [2 marks]

• Cue. The sun is overhead or nearly overhead all year near the equator, so heating is intense and constant in every month with no cold season, keeping mean temperatures almost the same throughout the year.

Q3. Explain why convectional rain is common in the afternoon in equatorial areas. [3 marks]

• Cue. Strong heating through the morning warms the moist air, which rises by convection, cools and condenses into towering clouds; by the afternoon these clouds are large enough to release heavy rain, so the storms typically build up and break in the afternoon.