A common feature of the weather during the pre-monsoon season, i.e., March, April and May, over the Indian region is the outburst of severe local convective storms which often turn into thunderstorm and lightning. The intense convection activity due to strong heating of land mass over Chhotanagpur plateau and movements towards southeast and mixing with warm air mass triggers the Nor Westers to develop which is locally named as Kal-Baisakhi (Bengal region) and Bardoli Cheerha (Assam). Apart from this, Mango Showers or Blossom Showers in Kerala and coastal Karnataka regions are some famous local storms of pre-monsoon season in India. Thunderstorms are characterized by towering cloud masses (mostly Cumulonimbus) with sudden electrical discharges manifested by a lightning and thunder enjoined with vigorous circulations and thus has the potential to spawn severe weather.

Formation of thunderstorms evolves in three stages

1.Cumulus Stage

• The Sun heats the Earth’s surface during the day.
• Warm plume of rising air starts.
• Condensational heating causes air to remain warmer than the surrounding environment.
• If the air is moist, then the warm air condenses into a cumulus cloud.

2. Mature Stage

• When the cumulus cloud becomes very large, the water in it becomes large and heavy.
• Raindrops start to fall through the cloud and meanwhile, cool dry air starts to enter the cloud.
• Because cool air is heavier than warm air, it starts to descend in the cloud (known as a downdraft). The downdraft pulls the heavy water downward, making rain.
• This cloud has become a cumulonimbus cloud because it has an updraft, a downdraft, and rain.
• Thunder and lightning start to occur, as well as heavy rain.

3.Dissipating Stage

• After some time, the thunderstorm begins to dissipate.
• This occurs when the downdrafts in the cloud begin to dominate the updraft.
• Since warm moist air can no longer rise, cloud droplets can no longer form.
• The storm dies out with light rain as the cloud disappears from bottom to top.

Impact of pre-monsoonal rainfall

• Southern India
  • Mango Shower: A common phenomenon in Kerala and coastal areas of Karnataka. They help in the early ripening of mangoes.
  • Blossom Shower: With this shower, coffee flowers blossom in Kerala and nearby areas.
• Eastern India: Nor Westers are dreaded evening thunderstorms in Bengal and Assam. However, these showers are useful for tea, jute and rice cultivation.
• Northern India: The pre-monsoon rainfall is vital for soil moisture required to raise paddy nurseries and subsequently to sow the crops.
• Western India: In Sahyadri range, Pre-monsoon showers are important for the growth of insects, which help in maintaining the fauna in the region.

Pre-Monsoon storms mark the beginning of agricultural activities as it helps in ploughing and getting the field ready for the season. However, these storms are also associated with hailstorm and high speed winds of short duration, which are harmful for crops like mango and other standing crops.