# Definition of HW and CW

There is no universal definition for HW or CW. Different definitions of HW and CW are used in different countries. However, a HW (CW) over a region represents an interval of hotter (colder) than normal weather over the region. The criteria used

4 Heat and Cold Waves Over India

Table 1 Criteria used in this study for defining heat and cold waves

 Criteria for declaring heat wave based on maximum temperature (Tmax) Heat wave over a station is declared only when the actual Tmax of the station is > 40 °C for plains and > 30 °C for Hilly regions. However, when the Tmax is > 40 °C for coastal stations and > 45 °C for other stations, conditions are declared as heat wave The following criteria are used for defining severity of the heat wave When normal Tmax < 40 °C and i. if (actual Tmax - normal Tmax) is 5-6 °C: heat wave ii. if (actual Tmax - normal Tmax) is > 7 °C: severe heat wave When normal Tmax > 40 °C and i. if (actual Tmax - normal Tmax) is 4-5 °C: heat wave ii. if (actual Tmax - normal Tmax) is > 6 °C: severe heat wave Criteria for declaring cold wave based on minimum temperature (Tmin) When the Tmin is < 10 °C for coastal stations, conditions are declared as cold wave The following criteria are used for defining severity of the cold wave over all the stations When normal Tmin > 10 °C and i. if (actual Tmin - normal Tmin) is -5 to -6 °C: cold wave ii. if (actual Tmin - normal Tmin) is > -7 °C: severe cold wave When normal Tmin < 10 °C and i. if (actual Tmin - normal Tmin) is -4 to -5 °C: cold wave ii. if (actual Tmin - normal Tmin) is > -6 °C: severe cold wave

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for defining the HW and CW are given in the Table 1, which signify increase (decrease) in the daily maximum (minimum) temperature at a station by a certain threshold compared to a climatological value. In this study, we have used the base period of 1971-2000 for computing the climatological values. HW (CW) of relatively higher intensity is classified as severe HW (severe CW) or SHW (SCW).