October 5, 2010
There may be a shortage of blog posts over the next two weeks because I am busy compiling the all new 2011 abc27 Weather Almanac. I know many folks who rely on this each year for weather data, interesting weather stories, and lots of other fun facts. It also makes a great stocking stuffer and will once again be out just before Christmas this year. I am very excited about this year's edition, but it is requiring a lot of my time that I will have to steal from here in the coming days. Therefore, I wanted to post a story today that will appear in the Almanac this year and give you a little taste of what's inside.
This story deals with the issue of urban heat islands and why big cities are often warmer than rural areas nearby. I hope you enjoy the read and I hope you pick up your copy of the 2011 abc27 Weather Almanac in a couple months when it's released. Enjoy!
Urban Heat Islands
Changes often occur in the landscape around urban areas as they develop. Infrastructures such as buildings, roads, and traffic lights replace open land and vegetation. Permeable and moist surfaces become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures within the landscape.
On a hot and sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50-90 degrees Fahrenheit hotter than the air. Shaded or moist surfaces, often in more rural locations, remain close to air temperatures. Urban heat islands at the surface are usually present both day and night, but tend to be strongest during the day when the sun is shining.
Heat islands not only occur at the surface, but can also form within the atmosphere. However, in contrast with their surface counterparts, atmospheric heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4 degrees Fahrenheit warmer than its surroundings. On a clear, calm night, however, the temperatures difference can be as much as 22 degrees Fahrenheit.
The graph below shows how urban temperatures are typically lower at the urban-rural border than in dense downtown areas. It also shows how parks, open land, and bodies of water can create cooler areas within a city or town.