It is a common fact that when we go to a rural area, we find that the temperature is much lower there than what it is in an urban area. An important reason for such an occurrence is stated by the theory of Urban Heat Island, which for most part, has gone unnoticed by most of us.
According to Wikepedia, a heat island refers to any area, populated or not, which is consistently hotter than the surrounding area. With this background, they further define an urban heat island (UHI) is a metropolitan city which is significantly warmer than its surrounding rural areas. This phenomenon was first described by Luke Howard FRS in the 1810s. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. UHI is evident during both summer and winter. The main cause of the urban heat island, as proved by the study, is modification of the land surface by urban development which uses materials which effectively retain heat. As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature.
To elaborate on the cause, one major factor is the low vegetation cover in Urban Areas and the lack of property of reflecting back the heat , of building materials such as concrete, used in the construction of buildings. Such materials have a tendency to trap and retain the heat from the Sun. This results in the temperature soaring up during the day. At night, when the surroundings are cool, the buildings then release the heat in the atmosphere. This pulls up the average temperature of an Urban Area, which seems to be hotter at night too.
Moreover, the study shows that the tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the “urban canyon effect”. Furthermore, the buildings block the way of the wind which bars the option of cooling by convection. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere.
Thus UHI has a major impact on temperature and also uses it as a medium to exhibit itself in this atmosphere.
Other major impacts include changes in the wind pattern, development of clouds, fog, humidity, rates of precipitation. Due to upward movement of lighter winds (when heat is released back into the atmosphere at night – the air gets heated, becomes lighter and hence rises ), there’s also an increased risk of thunderstorms in such areas.