Summer heat, is it really hotter in the city?
7-13-10
The summer heat has been turned on and cranked up the last week. With the heat come some meteorological phenomena that occur in cities and urban areas. If you haven’t noticed on the news, the East Coast has been subject to even more extreme heat than southeast Ohio has. Why is this? Well there are two main reasons, one synoptic reason and another microscale reason. First, they are located directly underneath the large ridge that has developed in the jetstream. This ridge allows very hot, humid, tropical air to swell northward much farther than it would in a zonal pattern. On the microscale level, this occurs because the large populated areas are subject to something that is called the urban heat island effect. This is most easily described as the concrete, buildings, etc absorbing the sun’s rays and holding the heat in during the overnight and also making it hotter during the day than surrounding rural areas. This can also be caused by the large amount of motor vehicles in the city and also running mechanical devices such as air conditioners, etc. This is called microscale because it occurs on a much smaller scale than country, state, or even county levels.
On the East Coast, the urban heat island effect is noticed the most because there are much more populated areas than in much of the rest of the country. The massive amount of people living in large cities, bunched together, the urban heat island effect will be felt by more people than a smaller city. This in no way affects the entire East Coast, but only the people living within the megatropolis areas.
With the ridge in the upper atmosphere that was in place last week, there was very minimal synoptic movement of air because of the lack of air mass differences. This stagnant air allowed the urban heat island to become a much greater factor in local temperatures. On July 7th the temperature at Newark International Airport in Newark, NJ (about 30 miles west of Stanton Island) recorded a high of 101 degrees Fahrenheit, which is only 2 degrees warmer than that of Morristown Municipal Airport in Morristown, NJ (about 20 miles west of Newark). This isn’t much of a difference for the day time, however, overnight on the 6th/morning of the 7th the temperature at Morristown was 8 degrees cooler than that at Newark with a reading of 80! That is a drastic difference! Morristown is in a much less populated area and much more rural (though still urbanized somewhat) compared to Newark.
This also occurs closer to home as well. The Port Columbus International Airport recorded a high of 93 degrees Fahrenheit on the 7th and the Newark Airport in Newark, OH reached 92. Once again, not much difference in highs, but the Newark low reached 64 degrees while the Columbus airport only made it to 69, 5 degrees warmer. Newark is less populated and more rural than near downtown Columbus, where the airport is located. The different surfaces of the city that absorb the suns' energy, coupled with the generated heat by the associated sources, Columbus remains warmer during the overnight than its surrounding regions.
This can be also seen in the forecast models. They pick up the warmer temperatures over the major metropolitan areas while the surrounding regions cool off faster overnight. Below is the forecast model for Tuesday morning at 5am visibly showing the cities circled.
At some points, like Cincinnati, the temperature is predicted to be up to 4 degrees Celsius (about 8 degrees Fahrenheit) warmer than the surrounding rural regions. Here is an image from last September during a warm spell also depicting the heat island effect. Circled are the cities once again.
This time the heat islands are noticeable in Detroit and Pittsburgh also.
National Weather Service graphical forecasts also pick up on the warmer cities as can seen by the predicted temperatures for Cleveland, Pittsburgh, Columbus, Cincinnati, and Indianapolis (the very warm Lake Erie also helps keep Cleveland and Erie warmer as well).
At night, with such large differences in temperatures over small areas, winds can develop similar to lake-sea breezes. These winds aren’t usually as strong as the ones that occur on the coast, but they can be substantial enough. These are caused the same way as warmer air over the city ascends into the atmosphere creating a micro-low pressure system. This in turn develops a micro-high over the surrounding rural areas of descending air.
Typically, winds blow from high to low pressure, so often these breezes will blow into the city. With the ascending air over the city, it is also possible for clouds to develop. If enough moisture exists, rain showers can also develop. Though this is being discussed at night, it is also possible during the daytime as well. The whole system will get altered, becoming much more complicated, and create downstream showers and clouds with a synoptically/mesoscale driven wind. This means that they will not necessarily sit over the city but form to the east of the city if there were a prevalent west wind.
Christopher Redmond
cr898005@ohio.edu
