What is a heat wave?
- A heat wave occurs when the maximum and the minimum temperatures are unusually hot over a three-day period at a location. This is considered in relation to the local climate and past weather at the location.
- It takes more than just a high daily maximum temperature to define a heat wave. It’s also about how much it cools down overnight. Hot days without hot nights allow some recovery from each day’s heat, but if the temperature stays high overnight, the maximum will be reached earlier the following day and will last longer. When unusually high night and daytime temperatures persist, heat stress becomes a critical factor in human health and whether infrastructure functions properly.
- In heat waves, hot nights make it harder to recover from the heat of the day and this puts more stress on the body.
Heat wave intensity
- For each part of the country, we compare the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday-Wednesday, Tuesday-Thursday) to what would be considered hot for that location, and also to observed temperatures over the last 30 days.
Heat waves are classified into three types, based on intensity.
- Low-intensity heat wavesare more frequent during summer. Most people can cope during these heat waves.
- Severe heat wavesare less frequent and are likely to be more challenging for vulnerable people such as the elderly, particularly those with medical conditions.
- Extreme heat wavesare rare. They are a problem for people who don’t take precautions to keep cool—even for people who are healthy. People who work or exercise outdoors are also at greater risk of being affected.
Why do heat waves cause deaths?
- High temperatures alone aren’t fatal in nature. It’s when high temperatures are combined with high humidity, known as the wet bulb temperature, heat waves become lethal. For instance, in April this year, 13 people died from an apparent heatstroke while attending a government award function in an open space in Navi Mumbai. Although the city wasn’t experiencing heat wave conditions — the maximum temperatures were in the range between 30 and 35 degree Celsius — experts said high humidity levels at the venue could have been one of the reasons behind the unusual death toll.
- The deaths in Ballia might have happened due to a similar reason. As per IMD, the relative humidity in the city on June 18 was 31 per cent (at 5:30 pm) and the maximum temperature reached 43.5 degrees Celsius. This means that the Heat Index (HI), or “real feel” temperature, touched 51 degree Celsius according to the calculations of the National Oceanic and Atmospheric Administration’s (NOAA) HI calculator. Such conditions could severely impact human bodies, sometimes leading to death.
- Experts suggest that high temperatures along with high humidity are dangerous for a reason.
- Humans lose heat generated within their bodies by producing sweat that evaporates on the skin. The cooling effect of this evaporation is essential in maintaining a stable body temperature.
- As humidity rises, sweat does not evaporate —just like clothes take a long time to dry in humid locations – and makes it difficult to regulate body temperature. And this could cause a heat stroke, which takes place only when the body temperature goes above 40 degrees Celsius.
- What happens is that excessive heat increases metabolic activity in the body, leading to a drop in blood pressure and oxygen levels with increased sweating — this is a condition called hypoxia.
- Metabolism goes haywire (in such conditions), creating a toxin overload which affects multiple organs… At that stage, things are extremely difficult to manage outside ICU care.
- The report also pointed out that prolonged exposure to even moderate heat, with poor nutrition and hydration levels in these circumstances, can lead to hypoxia. Those with pre-existing metabolic disorders like diabetes, the obese or the elderly, are more vulnerable.