Pre-Monsoon Thunderstorms
Context:
Recent powerful thunderstorms, locally referred to as Aandhi in meteorological literature, have caused severe devastation across Uttar Pradesh.
These localized storms have tragically resulted in over 200 deaths, with Prayagraj being the worst-affected district.
Characteristics and Triggers:
Aandhi refers to intense dust storms accompanied by thunder, lightning, rain, and occasionally hail.
They are a common annual phenomenon in northern, west-central, and eastern India (including parts of Madhya Pradesh) occurring between March and sometimes extending into July.
While typical pre-monsoon storms record wind speeds of 40 to 60 kmph, the recent destructive events recorded speeds of 100 kmph, even peaking at an extreme 130 kmph in Prayagraj.
Speeds exceeding 90 kmph are capable of uprooting large trees, toppling power poles and buildings, and turning debris into dangerous projectiles.
The formation of these severe storms requires a combination of intense surface heating (with temperatures recently crossing 45 degrees Celsius) and the incursion of moisture-laden winds, which primarily originate from the Bay of Bengal.
The CAPE Factor:
The convergence of extreme heat and moisture creates massive atmospheric instability.
This instability is measured by Convective Available Potential Energy (CAPE).
In the recent events, CAPE values exceeded 2000 Joules per kg, an extremely high figure indicating very strong upward atmospheric motion that rapidly forms severe thunderclouds.
Nor'westers:
These are violent, convective thunderstorms affecting states like West Bengal, Odisha, Assam, Bihar, and Jharkhand.
They are locally called Kalbaishakhi (calamity of the month of Baisakh) in Bengal and Bordoisila in Assam.
The term "Nor'wester" is used because these storm systems typically travel from the northwest towards the southeast.
Intense daytime heating over the Chota Nagpur Plateau creates a localized low-pressure zone.
This draws in warm, highly humid air from the Bay of Bengal, which then collides with cooler, dry upper-air winds from the northwest, triggering explosive Cumulonimbus cloud formation.
Usually striking in the late afternoon or evening, they bring gale-force winds (often exceeding 100 kmph), torrential rain, and hail.
While they provide crucial relief from heatwaves and support agriculture (like jute and early paddy), they frequently cause immense damage to infrastructure and significant lightning-related casualties.