Annual Weather Review 2011



The word "monsoon" is derived from the Arabic word "mausim" which means season. A monsoon is a seasonal prevailing wind which lasts for several months. Monsoons blow from the land toward the sea in winter and from the sea toward land in the summer. The term was first used in India, Bangladesh, Pakistan, and neighbouring countries to refer to seasonal winds blowing from the Indian Ocean and Arabian Sea bringing heavy rainfall to the region.

Wind flow pattern during the Northeast MonsoonWind flow pattern during the Southwest Monsoon

Northeast Monsoon (Left) and Southwest Monsoon (Right)


Monsoons are similar to sea breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are of larger scale and seasonal.

Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty metres deep, through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a metre or so. Additionally, the specific heat of liquid water is significantly higher than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that land warms faster and reaches a higher temperature than the ocean. The hot air over the land tends to rise, creating an area of low pressure. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it.

Similarly, rainfall is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. No matter how the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation.

Characteristics of Northeast Monsoon

The Northeast Monsoon season typically starts with a wet phase (December to January) followed by a dry phase (February to early March). During the wet phase, the Northeast Monsoon season is characterized by short duration thundery showers in the afternoon and early evening, and about two to four episodes of monsoon surges. Monsoon surges refer to the steady strengthening of north-easterly winds blowing from the South China Sea. These monsoon surges usually bring periods of prolonged widespread moderate to heavy rain lasting two to five days, occasionally windy conditions and cooler temperatures. During the dry phase, generally drier and windy conditions can be expected.  

Based on long-term climate statistics, December is the wettest month of the year (287.9 mm), followed by November (256.4 mm) and January (243.3 mm) respectively. The mean daily minimum temperature is lowest for January (23.3 deg Celsius), followed by December (23.5 deg Celsius) and February (23.6 deg Celsius).