Lucknow, May 21 : The India Meteorological Department (IMD) has issued a heatwave warning for 18 districts in Uttar Pradesh for the next 48 hours (Sunday and Monday), an IMD official said. The IMD said that the day temperature in these places will cross above 40 degrees Celsius. Thereafter, it will rain from Tuesday evening for five days in isolated places, both in east and west Uttar Pradesh. The districts that will be in the grip of the heatwave are Banda, Chitrakoot, Kaushambi, Prayagraj, Fatehpur, Sonbhadra, Mirzapur, Mathura, Agra, Firozabad, Etawah, Auraiya, Jalaun, Hamirpur, Mahoba, Jhansi, Lalitpur and the adjoining areas. Lucknow Met office in-charge, Mohd Danish, said that heat wave conditions had prevailed in 12 out of the 18 districts mentioned above on Saturday as well. The IMD has issued a yellow alert for these 18 districts. The IMD uses four-colour codes for weather warnings -- green (no action needed), yellow (watch and stay updated), orange (be prepared) and red (take action). A heatwave is declared when the maximum temperature is 40 degrees Celsius or higher, or remains 4.5 degrees above normal over two consecutive days. The day temperature in Uttar Pradesh districts went up by 3.5 degrees Celsius above normal on Saturday. Jhansi was the hottest in the state with the day temperature soaring to 45.1 degrees Celsius, Prayagraj 44.8, Agra and Varanasi (BHU) 43, Kanpur 42.3, Lakhimpur Kheri, Fursatganj and Orai 42 degrees Celsius. Lucknow recorded maximum and minimum temperatures of 40.1 and 24.4 degrees Celsius, respectively. Mainly clear sky is forecast for the state capital. The heatwave warning comes when most schools in the state are closed for summer vacations, including government primary and upper primary schools. The monsoon enters India around June 1. It arrives in Uttar Pradesh between June 18 and 20 through eastern districts like Ballia and Gorakhpur. Meanwhile, as the heat poses a major health risk between 12 noon and 4.30 p.m., when the day temperature remains the highest, doctors said going outdoors during these hours should be avoided, if possible. N.S. Verma, senior faculty, King George's Medical University (KGMU), said, "Try to keep outdoor work limited in these hours at least until the monsoon hits the region and humidity levels go up and the temperature falls a little." P.K. Gupta, former president of Indian Medical Association, Lucknow branch, said, "Ensure your water intake is frequent all through the day. While going out of the house, drink an extra glass of water, cover your head with cap/cotton cloth if using public transport or helmet if using a bike. If you go out in a car, do not use air conditioning till the very end of your journey." Abhishek Shukla, an expert in geriatric medicine and head of Aastha Hospice, advised, "Keep the car AC low to ensure there is no big difference in the temperature outside (the car) and inside. Switch off AC and open the window a minute before reaching the destination. If there are elderly people travelling in a car, avoid using the AC on maximum mode, as this could disturb the body temperature quickly." /IANS