It may happen almost every year at around this time, but a developing heatwave in South Asia is raising concerns about the potential health hazard posed by extreme temperatures.

In the last few days temperatures across many parts of India and Pakistan have been hovering around the 40 degree Celsius mark.

This, in itself, is not unusual. Temperatures of 40C occur in many parts of the region in advance of the monsoon rains, which usually do not reach northern parts until July.

What is of concern this year is that some of the hottest weather of the pre-monsoon period will likely coincide with the start of Ramadan. This will begin on May 26 and continue until June 24. During this time many Muslims refrain from eating and drinking between dawn and dusk.

During the latter part of Ramadan, and well before the cooling effect of monsoon rain arrives, there will be almost 14 hours between sunrise and sunset.

Strict observers of Ramadan will be placing their bodies under considerable stress if they abstain from hydrating themselves for the whole 14 hours.

Before Ramadan in 2015, a severe heatwave claimed the lives of 2,500 people in India. Then just one month later temperatures as high as 49C over a three day period claimed 2,000 lives in southern Pakistan.

The hot weather during 2016 claimed the lives of 700 people according to the Indian Meteorology Department.

There are often other contributory factors in the death tolls of heatwaves. Pakistan's infrastructure is such that electricity supplies are often under severe strain during hot weather. The failure of these supplies means that people are left without fans, refrigerators and air-conditioning units.

Rapid urbanisation and anthropogenic climate change are other factors which are likely to make heatwaves a growing and more frequent threat.

The current spell of hot weather is expected to continue for the best part of a week. It is likely to turn cooler, by around 3 to 4C, from Friday, with the chance of showers.