A weather station in Delhi recorded a high of 52.9 degree Celsius on May 29 — unheard of in the capital. But another station in the same city recorded 45.2 degrees, pretty normal for the end of May. Leafier neighbourhoods often seem cooler than pockets of treeless concrete. Why does this happen?
The weather station at Mungeshpur in Delhi recorded a maximum temperature of 52.9 degree Celsius on Wednesday (May 29). On Tuesday, the same location had seen a high of 49.9 degree Celsius.
However, the maximum temperatures recorded at other places in Delhi were at least 6 or 7 degree Celsius lower than those seen in Mungeshpur. For instance, at Raj Ghat and Lodhi Road, maximum temperatures on Wednesday were 45.2 and 46.2 degree Celsius respectively.
There are several weather stations across Delhi, each of which records the temperature at a particular place.
Several observatories and automatic weather stations are located at different places within the city — and there is no single observatory or station that gives an average temperature of Delhi as a whole.
Temperatures are recorded at Palam, Lodhi Road, Ridge, Ayanagar, Jafarpur, Mungeshpur, Najafgarh, Narela, Pitampura, Pusa, Mayur Vihar, and Raj Ghat.The weather/ temperature app on your mobile phone shows the temperature at the nearest station, which may not necessarily be that of the official India Meteorological Department (IMD) station. (The same goes for the AQI/ air pollution data on your phone.)
So, if you drive across the city from, say, Pitampura to Raj Ghat, you will likely see different temperatures on your phone.
But why is the temperature different at different places within the same city?
Although temperatures experienced by a particular region are largely governed by weather, several anthropogenic factors also play a role, especially in a large urban centre such as Delhi.
These factors include the concentration of pavements, buildings, roads, and parking lots — in general, hard and dry surfaces provide less shade and moisture, thereby leading to higher temperatures.
The material used to build infrastructure also has an impact. For instance, places where most of pavements and buildings are made of concrete, witness warmer temperatures. That’s because concrete can hold nearly 2,000 times as much heat as an equivalent volume of air.
The geometry and spacing of buildings are a factor as well. If a location is densely populated by buildings, surfaces and structures there become “large thermal masses” as they fail to release heat readily. Very narrow streets and tall buildings obstruct natural wind flows that generally bring temperatures down.
The heavy use of air conditioners in shopping malls and residential areas result in localised higher temperatures — ACs release an enormous amount of heat outdoors.
These factors can collectively lead to the creation of ‘urban heat islands’ at a location. These ‘islands’ experience higher temperatures relative to outlying areas.
The likelihood of a place becoming an urban heat island is higher when it does not have trees, vegetation, and water bodies. Natural landscapes bring down temperatures because they provide shade, and the processes of transpiration from plants and evaporation from water bodies produce cooling.
This cooling effect is in evidence in the vicinity of large parks or urban forests in Delhi.
Written by Alind Chauhan
Source: Indian Express, 30/05/24
