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Soon, weather agency IMD forecasts to include ‘real feel’ factor: What it means
IMD will calculate HI or “real feel” using multiple models that will factor in the day’s relative humidity, wind speed, maximum temperature and cloud cover.
The India Meteorological Department (IMD) is planning to issue Heat Index (HI) readings for weather stations across the country, including Delhi, this summer, as well as colour-coded forecasts in addition to the day’s maximum and minimum temperatures, officials said Monday.
The added IMD will calculate HI or “real feel” using multiple models that will factor in the day’s relative humidity, wind speed, maximum temperature and cloud cover. A colour-based warning – yellow, orange or red– will also be issued, based on the expected HI, listing precautions people can take.
IMD will utilise the data it gathered from weather stations across the country during a heat hazard analysis last year to prepare this HI. The model will be shared with all regional meteorological centres across the country, with the aim to release HI data this monsoon, officials said.
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Kuldeep Srivastava, scientist at IMD and head of the Delhi regional meteorological centre, said that initially, only one HI forecast is likely to be issued for the Capital based on the expected weather at Safdarjung, which is representational of Delhi’s weather. However, in the long run, the goal is to issue HI forecasts at the district level. “While we can calculate the day’s HI for each station, we will only issue one single forecast for Delhi, which will be calculated based on Safdarjung’s weather. This will give us an estimate on what kind of weather is expected across the whole of Delhi,” he said, adding IMD is hopeful of utilising the model this monsoon when humidity is high.
HI is used widely, particularly in tropical countries, to understand the extent of discomfort being felt by an individual by factoring in both humidity and temperature. The higher the humidity, the higher the heat index. Experts say HI is often highest during the month of June and early parts of July, with humidity levels constantly remaining around 40-50%, coupled with temperatures of over 40 degrees Celsius (°C), with the body’s ability to not effectively sweat (which is how human bodies cool) making it more dangerous than spells of dry heat. HT shared Delhi’s wet-bulb temperature (a combined measure of heat and humidity of a region) all of last summer, finding the heat index to be 5-10 degrees higher than the actual maximum of the day in several cases. For instance, on June 27, 2022, when the maximum temperature in Delhi was 40.2°C, humidity of 45% brought the wet-bulb temperature up to 52°C. Exposure to a sustained wet-bulb temperature of 35°C or higher can kill even healthy people.
“Mapping the heat index is most useful when relative humidity is high and that makes standing or working outside uncomfortable. This is normally seen between June and August and the aim is to start sharing this data by then. Prior to that, we see dry heat and while the maximum temperature can be fairly high, the heat index is generally low as the body can still cool down by sweating,” Srivastava added.
Akhil Srivastava, scientist at IMD, who was part of last year’s heat hazard analysis, said they gathered data from weather stations across the country between March and June, with the aim of analysing it beyond only the impact of relative humidity. “While HI across the world is calculated only using maximum temperature and relative humidity, we also analysed the impact of wind speed, cloud cover and also the duration of heatwaves across the country. All these additional factors will be utilised to calculate the heat index, for which we are currently testing various models,” he said.
Experts say that during the dry heat months of March and April, the body can cool off more effectively, owing to the low moisture content in the air – that same ability for the body to sweat out reduces when humidity levels are 50% or higher. And it is the why, for people working outdoors, wet-bulb temperature is most important.
On Monday, a report released by the Centre for Policy Research (CPR) which looked at 37 heat action plans (HAPs) across 18 states, found while most states had included measures on dry heat, humid heat spells had not been factored in when it came to policy-making. Aditya Valiathan Pillai, associate fellow with the Initiative for Climate, Energy and Environment (ICEE) at CPR and part of the analysis, said IMD working on releasing HI is a bright start, as it will allow states to prepare their HAPs accordingly.
“It is important to factor in humid heat, even more so than dry heat, as there is loss of life even at lower temperatures when humidity is high. The launch of a heat index can allow states to set their own thresholds and these action plans can be triggered automatically when a certain heat index is reached,” he said.
Currently, IMD issues a colour-based warning for all significant weather activities, including rain, gusty winds, hail, thunderstorm, cold waves and heatwaves.
IMD issues a green alert when there is no risk, and a yellow alert to warn the public of a weather phenomenon in place, which is upgraded to an orange alert if preventive action is required. A red alert is generally issued if an extreme weather event is expected, asking the public to be vigilant and step out only if required.
