Assertion (A) : Convectional rains occur during pre-monsoon summer in India. Reason (R) : Such rains occur due to adiabatic cooling.

Assertion (A) is true as India experiences significant convectional rainfall during the pre-monsoon summer (March-May). These are locally known as Kal Baisakhi in West Bengal and Bordoisila in Assam, occurring due to intense surface heating [t3, t5]. Reason (R) is also true and provides the correct scientific explanation. Convectional rain is triggered when intense insolation causes air to expand and rise in vertical currents [t5]. As this air parcel ascends into layers of lower atmospheric pressure, it undergoes adiabatic cooling [c3, t5]. This cooling leads to condensation of water vapor, releasing latent heat which further drives the convection process and results in the formation of cumulonimbus clouds and subsequent rainfall [c3, c4]. Thus, adiabatic cooling is the fundamental thermodynamic mechanism that converts rising warm air into precipitation during these pre-monsoon convective events [t5].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > A Parcel of Rising Air > p. 298
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Orographic Rainfall > p. 339