On instability from water vapour

This paper summarises the general criteria for the stability of a particle and of a layer in the atmosphere under conditions when condensation may bring the latent heat of water vapour into play. It discusses briefly the relationship between “Feucht‐Labilitat” (Refsdal), latent instability (Normand) and convective instability (Rossby), and gives the reasons for the introduction in India 01 the classification based on latent instability. The conditions for latent instability are defined with the aid of the tephigram, and an example is given to show how by generating latent instability within a closed system, e.g., by raising the wet‐bulb temperature in the lower layers and leaving all else unchanged, the energy available for transformation into kinetic energy may rise from zero to the equivalent of a wind of 50 mi./hr. throug‐hout the system, without taking account of the energy that may become available from the evaporation of rain‐drops, in descending air. If, however, cvaporation from rain‐drops takes place in the descending air in a thermodynamically efficient manner, the available energy in the closed system of the chosen example is nearly trebled, i.e., the equivalent average velocity rises to more than 90 mi./hr.