Weather Imagery

With today’s land starved metropolitan cities expanding further into their rural surroundings, a strange consequence occurs that directly affects the local weather, in particular temperatures and rainfall. No, this doesn’t have anything to do with global warming. Instead, the principal involved has to do with how much of the sun’s energy is absorbed as compared to how much is reflected on a much smaller scale.

As a city increases in size, the amount of heat it absorbs from the sun increases. Vegetation and trees which cool the air through shade and evaporation are replaced with tar roof tops, dark colored roads, and asphalt parking lots which absorb more energy. The sun’s energy is then converted to heat which in turn heats the surrounding air. This is referred to

as the “urban heat island affect“. It has been well documented that cities like Houston, TX and Atlanta, GA are on average 7°F to 9°F warmer than their surrounding environments. In addition, tall buildings and other structures can alter wind patterns which can force the air to rise. Temperature differentials and rising air currents help to destabilize the atmosphere which can create clouds and help promote rainfall.

NASA and the University of Arkansas have used satellite mapping and ground-based weather station readings to figure out how widespread this phenomenon actually is. Turns out, Houston and Atlanta have quite a few days in which the “urban heat island affect” triggers thunderstorms. During the 1996 Olympic Games in Atlanta, scientists were allowed the opportunity scrutinize data collected by the National Weather Service’s ground-based weather collection equipment used to predict weather for the athletes. After scrutinizing the data, meteorologists Robert Bornstein and Qing Lu Lin of San Jose State University discovered 5 out of 9 days of precipitation were caused by the urban heat island effect.

Air that is warmer than the surrounding air tends to be more buoyant and as a result it wants to rise. Such is the case when a city’s temperature is 8°F warmer than the surrounding environment. As the warm air rises, cooler air fills in the void of the hot rising air. If the air contains sufficient amounts of moisture and has a high enough relative humidity level, clouds may form. As these clouds drift downwind, they can further develop and produce rain. NASA has proven that urban heat islands increase rainfall amounts on the down-wind side of the city:

Using the world’s first space-based rain radar aboard NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite, Shepherd and colleagues found that mean monthly rainfall rates within 30-60 kilometers (18 to 36 miles) downwind of the cities were, on average, about 28 percent greater than the upwind region. In some cities, the downwind area exhibited increases as high as 51 percent.

Can this phenomenon be reduced or eliminated? It’s unlikely it will be eliminated and there could be some argument in trying to reduce it. By creating more rain days, cities are showing signs of being more “green” due to an increase in vegetation. Considering cities will only get larger, it’s best to try and understand as much about the heat-island affect so that agricultural tracts of land can be better positioned and city engineers could design more efficient irrigation systems.