Factors affecting urban warming

In a recent paper in Nature by Xhiu Lee and colleagues at Yale University, the role of local background climate in creating a difference in temperature between urban and surrounding rural area (Urban Heat Island or UHI effect) was studied.

For 65 selected cities across North America the annual mean midnight and mid-day difference between urban and rural temperature (delta T) were studied.

While midnight difference between urban and rural temperature was positively linked to population but not precipitation, solar radiation and air temperature, the mid-day difference was strongly correlated with precipitation and weakly linked to population size.

The relation of the urban heat island with population at night is indirect — i.e., through the number of buildings and their height. The long wave or infrared radiations get reflected into each other and back into the streets and are not lost to the free atmosphere. This happens at night because during the night, there is no heating from the sun and the buildings and the surfaces emit long wave or infrared from the heat they have built up during the day.

“The buildings are unable to send heat radiation upwards as the rooftop area can be much smaller than that of the sides of the buildings,” notes Dr. Raghuram Murtugudde, a climate scientist based in the U.S., in an email to this Correspondent.

Dr. Murtugudde explains that there are two kinds of UHI —atmospheric UHI and surface UHI. Atmospheric UHI peaks during the night because of the lack of evaporative cooling as in rural areas and due to the longwave radiation being bounced into each other and down to the street by the buildings as opposed to being lost to the free atmosphere. Surface Urban Heat Island Effect is there during day and night because of the heat capacity of the materials used for pavements, buildings, roofs, and so on.

“This is why the midnight temperature difference is related to population and not background climate but the midday difference is strongly correlated with background climate but not population,” clarifies Dr. Murtugudde.

In wetter climates, rural areas have more vegetation and thus higher surface roughness than urban areas and thus more turbulence and heat loss. In drier background climate, rural areas have less vegetation and are thus smoother than urban areas and create less turbulence and thus lose less heat so urban areas can in fact be cooler. This is the cause for the cities in eastern U.S., where the climate is wet and have densely vegetated rural surroundings to experience a greater delta T than cities of the American Southwest where the vegetation consists of bushes and scrub and the landscape is mostly desert with low levels of precipitation. In fact, Las Vegas is cooler than the surrounding desert which has a smooth topography and the value of delta T is negative.

What can be done to mitigate the heat island effect which takes a toll on the health of urban residents? One suggestion by the authors is to make rooftops reflective by painting them in bright colours to reflect back the radiated solar heat to the atmosphere (albedo effect).