�Pre-monsoon rains sharp and intense, but monsoon brings longer, recurring spells�
GUWAHATI, July 6 - How do we differentiate monsoon showers from pre-monsoon rains? The direction of the wind, intensity of the rain and several other factors go into determining whether a rainy spell qualifies as monsoon or not.
This was the observation made by Dr Rahul Mahanta, a leading researcher on climate and the coordinator of the Interdisciplinary Climate Research Centre at Cotton University (ICRCCU).
Talking to The Assam Tribune, Dr Mahanta said that with respect to temperature, pre-monsoon season is synonymous with heat and humidity. It brings forth uncomfortable conditions throughout the day and night. Conversely, fairly stronger winds bring down temperatures to a comfortable level during monsoon in India.
The pre-monsoon season is marked by routine churning of the atmosphere as heating of the land causes formation of cyclonic circulations, favourable for setting up thunderstorms, lightning, heavy rain, even snow or hailstorm. Monsoon means seasonal changes in atmospheric wind circulation and the rainfall associated with the asymmetric heating of land and sea. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern. They are notable across much of South and Southeast Asia, including India, and Cambodia.
Prominent sea and land breeze: Due to differential heating and huge day-to-day variation of temperatures, the sea and land breezes remain prominent before the arrival of Southwest Monsoon in India. But, during monsoons, with the available humidity and overcast skies, the breezes are not that noticeable.
During the pre-monsoon season, clouds are more vertical in nature and mostly come up during late afternoon and early evening. They are triggered by high temperatures and towering cloud upsurge. On the other hand, the monsoon season is known for layers of clouds, which are mainly sheet-like continuous layers of clouds. Depths of these clouds are less but the layers are thick and moisture-laden.
Pre-monsoon rain is sharp and intense and gets over for the day, after just one spell. But, the Southwest Monsoon brings longer spells of rain which are also recurring in nature. Moreover, pre-monsoon rain is patchy in nature but Southwest Monsoon covers large areas and the weather remains similar in vast pockets. Chill in the air persists irrespective of the terrain.
Pre-monsoon rainfall is usually convective rainfall, that is, they are heat induced rainfall. Such rainfall usually follow a few hot days, and occur generally in the evening or anytime after noon. But monsoon rains could arrive first in the morning or continue to pour throughout the day or the rain can commence during any time of the day, though the preferred time is generally late evening.
Some experts are of the view that a heavy pre-monsoon season tends to disrupt the heating process of the land, which is crucial for the temperature-pressure gradient (between land and sea) to evolve and ensure a normal South-West monsoon, said Dr Mahanta.
During summer season, the sun apparently moves in a northward direction. Due to this movement, temperature rises at a particular place, which is called local heating, and it finally leads to thunderstorm. This is called pre-monsoon showers. This occurs before the monsoon season, that is summer.
As per the India Meteorology Department (IMD), March to May are the pre-monsoon months and hence, any rainfall in this period can be attributed as pre-monsoon showers. The classic features of these showers are the form of precipitation which is mainly convective, that is, thunderstorms. Various weather systems, say troughs, bring in moisture and thermal instability. Daytime heating triggers the convection process and hence we get the thunderstorms after the post-noon period, Dr Mahanta said.
