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In page Urban heat island:

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Research has been done in a few areas suggesting that metropolitan areas are less susceptible to weak tornadoes due to the turbulent mixing caused by the warmth of the urban heat island.[5] Using satellite images, researchers discovered that city climates have a noticeable influence on plant growing seasons up to 10 kilometers (6.2 miles) away from a city's edges. Growing seasons in 70 cities in eastern North America were about 15 days longer in urban areas compared to rural areas outside of a city's influence.[6]