San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

Urban Tornadoes and the Heat Island Effect

With the general paranoia about climate change which has been promoted over the couple of decades, it is very easy to attribute an increased incidence of tornadoes in urban areas to increased levels of carbon dioxide in the atmosphere. However that is an error. The increased incidence of tornadoes in urban areas would be due to the heat island effect. This is something caused by human activity but not due to any greenhouse effect.

The heat island effect is the increase in temperature as a result of the clearing of vegetation in urban areas and its replacement with concrete and asphalt. This results in a substantial rise in temperature in cities. The warmer air at the surface is lighter than the air above it. It cannot rise as a unit because it must be replaced by the cooler air above the surface. When a break through between the warmer and cooler air develops it is like water going down a drain; i.e., a vortex develops. That vortex involves rotation speeds around the center that can be extreme. That is a tornado.

The tornado devastation in the South on April 27th, 2011 was terrible, but there were even worse ones in the past. In the later afternoon and evening of April 11th of 1965 37 tornadoes killed 271 people and injured more than three thousand in Iowa, Wisconsin, Illinois, Indiana, Michigan and Ohio. The property damage was about $300 million. Even worse was the toll of 689 killed by tornadoes on March 18, 1925 in Missouri, Illinois and Indiana.

The statistics on the incidence of severe tornadoes do not indicate any upward trend, but there is an upward trend in the total number of tornadoes. This is attributed to the greater monitoring of weather conditions so that there is a greater chance of detecting the less severe tornadoes. There can be a greater incidence of urban tornadoes because of the heat island effect, but also because cities are covering a greater area.


It is well known that the frequency of tornadoes is not highest during the hottest months of the year. Instead their occurence is related to the difference in density between air at the surface and the upper atmosphere. The difference in air density depends not only upon the temperature of the air but also upon its humidity (moisture content).

Against all intuition moist air is lighter than dry air. This is explained by noting that the average molecular weight of air is about 29. This is the weighted average of the molecular weight of nitrogen, oxygen and other gases. The molecular weight of water is about 18. Thus moist air has molecules of weight 18 replacing molecules which are on average of weight 29.

Here is the frequency of tornadoes by month.

The high incidence of tornadoes in the late spring in the southern states can be attributed to an inflow of warm moist air from the Gulf of Mexico moving north and cold dry air from the north moving south.

(To be continued.)

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