Though in most parts of India winter had withdrawn by late February and summer had started early, heat started becoming unbearable only from April onwards. In Northern cities like Delhi air had been blowing hot since March but its land started scorching only by late April. By mid-April land surface temperature had almost doubled and even exceeded the air temperature, thus creating urban heat island (UHI) effect, a condition where temperatures in urban areas are warmer or hotter than the surrounding area.
In general, UHI refers to the increase in air temperature and also the surface temperature of the material. Therefore, climate change is triggered by UHI due to the atmospheric and surface changes in urban areas and UHI has important implications for human comfort, urban air pollution, energy management, and urban planning.
In alleviating the heat island problem, there are three-pronged strategies beyond microclimate below trees: cool roofs, cool pavements, and vegetation for evapotranspiration. Vegetation cools microclimates by shading heat-absorbing materials, increasing the reflectivity of surfaces, providing evapotranspirative cooling and altering wind patterns.
Usually, high surface temperatures correspond to areas with low vegetation and desolate terrain. Rise in average land surface temperature affects city services and triggers auto-ignition of fires at landfills which has happened in Delhi during the present summer itself. Concrete structures as well as emissions from automobiles and domestic appliances further aggravate the situation.
The thermal environment of urban areas can be optimized through methods, such as developing green areas and greening the roofs of buildings. Surface and air temperature can be reduced by increasing the reflectivity of roof and land covers and at the same time reducing the energy consumption of buildings. Several studies have confirmed the huge impact of vegetation and the protection of existing green spaces and wooded areas in reducing the urban heat island effect.
In fact, vegetation achieves cooling through various processes, more specifically: seasonal shading of infrastructure, evapotranspiration, and minimizing ground temperature differences. Vegetation also provides other worthwhile and complementary benefits in urban areas, including improving air quality through oxygen production, CO2 capture, filtration of suspended particulate matter, reducing energy demand for air conditioning, improving water quality through retention of rainwater in the ground and soil erosion control and providing health benefits for the population in protecting from ultraviolet radiation, reducing heat stress and providing spaces for outdoor exercise.