When the normal value of the temperature is subtracted from its actual value, we get what is called the ‘departure from normal’. When the actual temperature is higher than the normal, the departure will be positive, and when the actual temperature is lower than the normal, the departure will be negative. The actual temperatures prevailing at different stations in a sub-division may be somewhat different because of factors such as their height, distance from the coast, proximity to mountains, etc., but their departures from normal are likely to be similar. Hence meteorologists prefer to describe the maximum temperature patterns in terms of departures rather than the actual maximum temperatures.   

When the departure is up to +2 deg C, the prevailing day temperature is said to be ‘normal’.

When the departure is more than +2 deg C but up to +4 deg C, the prevailing day temperature is said to be ‘appreciably above normal’.   

When the departure is more than +4 deg C, the prevailing day temperature is said to be ‘markedly above normal’.

In situations where the departure is negative, similar criteria are applied and the day temperatures are accordingly described as ‘normal’, ‘appreciably below normal’ or ‘markedly below normal’.  Under these conditions there is a welcome respite from the heat of the summer. It is the above normal temperatures that cause discomfort and are a matter of concern.

In summer, there are occasions when the meteorological conditions allow hot dry northwesterly winds to blow from Rajasthan all the way into Orissa and Andhra Pradesh.  The maximum temperatures then remain consistently above normal over an extended area, a condition that is termed as a ‘heat wave’. A heat wave is said to prevail when the maximum temperature departures are between +5 and +7 deg, and it is described as a ‘severe heat wave’ when the departures are +7 deg C or more. However, over regions which are so hot that their normal maximum temperatures are themselves more than 40 deg C, the limiting values of the departures are between +4 and +6 deg C for a heat wave and exceeding +6 deg C for a severe heat wave.

A peculiar situation arises some times when it is the minimum temperature that is higher than normal, perhaps due to night time or morning clouding. This results in a very uncomfortable and unbearable situation during the entire day. When the minimum temperature is +5 deg C above normal, and there is no heat wave, it is described as a ‘hot day’.

Advertisements