A red alert was issued for Delhi as it remained in the grip of an intense heatwave, with temperatures ranging from 40.9 to 45.0 degrees Celsius across various weather stations on Wednesday.

The heat index, a measure of how hot it feels when humidity is factored in, touched an alarming 51.9 degrees Celsius.

At 5.30 pm, Ayanagar was the hottest at 45 degrees Celsius, followed by Palam at 44.5 degrees Celsius, Ridge at 43.6 degrees Celsius, Pitampura at 43.5 degrees Celsius, Lodi Road at 43.4 degrees Celsius, Mayur Vihar at 40.9 degrees Celsius and Safdarjung --the city`s base station -- reported 43.3 degrees Celsius.

On the heat index crossing 50, the India Meteorological Department (IMD) said it depends on temperature and relative humidity and varies throughout the day based on changes in these values. Additionally, the heat index is not validated for Indian conditions, and no official records of it are maintained.

Heatwave conditions have been observed in Delhi and at isolated places, the IMD said, adding that while three stations reported heatwave conditions yesterday, only one station - Ayanagar - recorded such conditions today.

Temperatures are likely to remain in a similar range until tomorrow, with a gradual fall expected from June 13.

The IMD bulletin said these temperatures were above normal by three to four degrees Celsius, triggering continued heatwave conditions likely to last until June 12.

A red alert - the highest-level warning - advises residents to "take action," stay hydrated, avoid direct sun exposure, and limit outdoor movement, especially during peak afternoon hours.

"The severe heat is expected to persist till Thursday. A western disturbance is likely to affect northwest India from the night of June 13, which may bring light rain and thunderstorms in Delhi," said Dr Naresh Kumar, senior IMD scientist.

Humidity levels in the capital fluctuated between 31 per cent and 73 per cent, with dry northwesterly winds prevailing throughout the day, aggravating heat discomfort.

The forecast for Thursday predicts maximum temperatures of 43-45 degrees Celsius, along with gusty winds (40-60 kmph) and a chance of very light to light rain or thunderstorm activity by evening or night.

The intensity of the heatwave is expected to reduce from June 13 onward, with an orange alert replacing the red alert.

By June 14-17, Delhi could see a dip in temperatures to 37-42 degrees Celsius, accompanied by light to moderate rainfall and gusty winds, potentially easing the ongoing heat stress.

Meanwhile, Delhi`s 24-hour average Air Quality Index (AQI) was recorded in the "poor" category at 245, according to the Central Pollution Control Board (CPCB).

An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".