IMD revises its rain alert for Delhi twice on sultry day

The India Meteorological Department’s (IMD) prediction of heavy rainfall for Delhi on Monday fell flat, with the Capital remaining bone dry and recording a sultry day due to high humidity levels. Delhi recorded a maximum temperature of 36 degrees Celsius (°C), but humidity of 59-82% kept the heat index or “real feel” temperature at 47°C.

On the day, the IMD revised its daily weather forecast for the Capital twice. The colour-coded alert — used to depict the severity of weather events — was downgraded to “yellow” at 9am, with light to moderate rain forecasted for the rest of the day. At 12.30pm, this was upgraded to “orange” again, with a forecast of moderate to heavy rain.

The IMD, at present, has an “orange” alert in place for Tuesday and Wednesday, with moderate to heavy rainfall likely on both days. The maximum temperature is likely to drop to 33°C on Tuesday, the IMD said, forecasting the minimum to be between 26°C and 27°C on the two days.

Despite the multiple updates on Monday, rain eluded Delhi, and the IMD said that forecasts for a small city like Delhi could be challenging. On the day, IMD announced plans to improve weather forecasting in the region by adding three more Doppler weather radars (DWRs) in the national capital region (NCR), in addition to three operational ones at Ayanagar, Palam and Lodhi Road.

Currently, the weather radars at Lodhi Road and Palam are under maintenance, IMD said.

Mrutyunjay Mohapatra, director general of meteorology, IMD, said: “Forecasts for any region, particularly during monsoon, can be challenging, but our accuracy over the years has improved by 10% to 20%. Since Delhi has a very small area, rain spells quite often miss Delhi, but end up occurring in the neighbouring Haryana or Uttar Pradesh.”

He cited the instance of Safdarjung receiving 91mm of rainfall between 5am and 6am on June 28, which was close to the cloudburst criteria of over 100mm in a single hour, as challenges in forecasting for a smaller area. “This extreme rainfall event was very confined, spatially and temporally, to some parts of the city, particularly Safdarjung. So, it is definitely a challenge we are trying to address,” he said.

A “yellow” alert is the first of three colour-coded alerts and simply alerts the public of an upcoming weather event. It is upgraded to “orange” when the public is to “be prepared” for an upcoming weather event. The last stage, a “red alert” is issued when the IMD wants people to be “most vigilant and take action”.

Last Saturday, IMD forecasted moderate to heavy rain for Delhi, issuing an “orange” alert for Sunday. However, Delhi recorded zero rainfall at the Safdarjung station, which is representative of Delhi weather, in the 24-hour period between 8.30am on Sunday and Monday. Its forecast for Monday was heavy rain and an “orange” alert, but there was no rainfall at any of Delhi’s five weather stations.

Mohapatra said IMD is trying to use “nowcasts”, which are short-range forecasts issued for weather events in the next two to three hours. He said that despite no rain in Delhi on Sunday, parts of Faridabad and Ghaziabad recorded light showers. “It is, therefore, more accurate to look at a much larger area of NCR for forecasts,” he said.

He said the addition of three Doppler radars will strengthen the forecasting system, and the under-maintenance ones at Lodhi Road and Palam will soon become functional.

On the day, Delhi recorded a maximum temperature of 36°C, which was a degree below normal and down from 37.1°C recorded a day earlier. Delhi’s minimum temperature was 28.4°C on Monday, a degree above normal, which was up from the 27°C recorded on Sunday.

The relative humidity oscillated between 59% and 82%, with Delhi recording a wet-bulb temperature — another indicator of how high humidity impacts comfort level outside — of 28.8°C at 2.30pm. However, this was down from the season’s high of 30.1°C, which was recorded on Sunday.

A wet-bulb temperature of 32°C or higher makes it difficult for even fit and acclimatised people to work outdoors for long and at a wet-bulb temperature of 35°C — the maximum threshold — humans can no longer regulate body temperatures, leading to heatstrokes and potential collapse.