Sheetali pranayama is rather done using mouth instead of nose for inhalation to produce the cooling effect. Sheetali literally means cooling and hence, the purpose of the pranayama is to reduce the body temperature (using the principle of cooling effect of evaporation on tongue ) and calms down the mind and reduces the stress.
Even Hatha Yoga Pradipika says that person becomes attractive by practicing this pranayama.
In this pranayama the tongue is rolled and rolled tongue is used to inhale through and then retain and exhale as per the given ratio maybe in 1:1:1, 1:2:1, etc.
Cold, low b.p.
rainy / winter season