Reading IMD heat warnings: green, yellow, orange and red

When summer comes, IMD heat warnings appear on the news and on phones. Knowing what each colour means helps you act in time.

The colour codes. IMD issues impact-based heat warnings jointly with NDMA. Green means a normal day and no action. Yellow (be updated) means heat-wave conditions in isolated pockets for two days; heat is tolerable for most people but a moderate concern for vulnerable people such as infants, the elderly and those with chronic illness. Orange (be prepared) means a severe heat wave for two days, or a heat wave lasting four days or more; heat illness becomes more likely for people in the sun for long or doing heavy work. Red (take action) means a severe heat wave for more than two days, or more than six heat or severe heat-wave days in total; there is a very high likelihood of heat illness and heatstroke at all ages.

What counts as a heat wave. On the plains, a station's maximum must reach at least 40°C (30°C in hilly areas). It is a heat wave when that is 4.5 to 6.4°C above normal, and a severe heat wave when it is more than 6.4°C above normal. A maximum of 45°C or more is a heat wave, and 47°C or more a severe heat wave, regardless of normal. Coastal stations need 37°C or more with a departure of 4.5°C. The criteria must hold at two stations in a subdivision for two days in a row.

Where to look. IMD's website, mausam.imd.gov.in, and its daily bulletins. Heat waves occur mainly from March to June, peaking in May.

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