Variable Weather & Changing Climate KQ1 - Rainfall & Monsoon Winds

Rainfall or Precipitation refers to water in any form that falls from the atmosphere to the surface of the earth.

Precipitation types include

• Hail (balls of ice)
• Snow (white or translucent ice crystals)
• Sleet (mixture of rain and snow)

The main form of precipitation in the tropics (in which Singapore is located in), is rain. It is measured in millimeters (mm) using a rain gauge.

Areas with rainfall above 1,500mm are classified as having high rainfall.

Areas with rainfall below 250mm are classified as having low rainfall.

Rainfall is the result of air instability, which is the tendency of a parcel of air to rise (unstable state). This air parcel is heated, expand and rise as a result of becoming less dense than the surrounding air. 

Two Main Types of Rain

1. Convectional
2. Relief

Convectional Rain (often associated with lightning and thunder and generally lasts only for a short time but brings intense rainfall over an area, Most common in Singapore)

Relief Rain (also known as orographic rain occurs when warm, moist air is forced to rise over landforms such as a mountain range. The side of the mountain where moist, warm air from the sea is blown by the prevailing wind is called the windward side, while the other is called the leeward side which is usually dry as most of the moisture has fallen on the windward side).

Wind travels from an area of high to low pressure. They also also affected by the earth's rotation, also known as the Coriolis Effect. This effect causes the bending or deflection of winds, to the right in the northern hemisphere and left in the southern hemisphere. It is strongest near the north and south poles, while weaker in the tropics (negligent at the Equator). 

Monsoon Winds are regional wind patterns that reverse direction seasonally. This brings about seasonal changes in precipitation in different areas with the occurrence of monsoons, which may lead to wet or dry seasons.

Click on the picture to see the Monsoon Animation and better understand the phenomenon

Monsoon occur in a number of locations around the world, with the major monsoons affecting Asia and Australia. Singapore commonly experiences high rainfall in the month of December because of the north-east monsoon.

North-East Monsoon (10 to 2)

The North-East monsoon takes place between the months of October (10) to February (2) due to a difference in pressure between the northern and southern hemispheres.

• During this period of time, it is summer in the southern hemisphere (forming an area of low pressure in Australia) and winter in the northern hemisphere  (forming an area of high pressure in the Indian sub-continent and Central Asia).
• Dry cold air moves from the Indian sub-continent and Central Asia to Australia as the north-west monsoon winds. 
• These winds change direction to north-east as it crosses the Equator, where the Coriolis effect deflects the winds to the left. 
• These winds warm up, picks up moisture as it travels across the Indian Ocean and brings heavy rain to Australia.

South-West Monsoon (6 to 9)

The South-West monsoon takes place between June (6) to September (9) due to a difference in pressure between the northern and southern hemisphere.

• During this period of time, it is summer in the northern hemisphere (forming an area of low pressure in the Indian sub-continent and Central Asia) and winter in the southern hemisphere (forming an area of high pressure in Australia).
• Dry cold air moves from Australia to the Indian sub-continent as the south-east monsoon winds. 
• These winds change direction to south-west as it crosses the Equator, where the Coriolis effect deflects the winds to the right. 
• These winds warm up, picks up moisture as it travels across the Indian Ocean and brings heavy rain to the Indian sub-continent.