By analyzing satellite images and field measurements, researchers have found that urbanization causes high surface temperatures, creating urban heat islands1. Urban heat islands can modify the climate, increasing the occurrence of heatwaves in summer and hence increasing the risk of mortality in urban populations.

The land surface temperature (LST) is an important factor for climate change, and hence it is an important climate parameter in global climate models. Previous studies estimated LSTs in urban areas from satellite images of land use and land cover (LULC), but they did not consider the effects of the atmosphere or the varying LULC patterns in urban areas. This limitation may introduce a temperature error of 4–7°C in models.

To reduce this error, the researchers estimated daytime and nighttime LSTs in Delhi from images captured by ASTER and LANDSAT satellites. In addition, to confirm the accuracy of the results obtained from the satellite images, they measured daytime and nighttime LSTs using an infrared thermometer.

The central and eastern districts of Delhi had the highest LSTs because they mainly consist of residential areas and networks of paved roads. Some parts of northwest Delhi had lower LSTs due to the presence of wasteland and fallow land. On the other hand, water bodies had the highest LSTs during the night.

Residential areas, industrial zones and commercial hubs exhibited the highest LSTs. This implies that urban development does increase the LST through the replacement of natural vegetation with artificial surfaces such as stone, metal and concrete.

"The spatial layout of the LULC in Delhi has a great impact on the development of urban heat islands, which may affect the climate in the region. This highlights the need for urban planning to alleviate such effects," says Chander Kumar Singh, a co-author of the study.