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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.3.7.4. Potential for Cascading Effects <br />One complicating factor when discussing impacts of extreme heat, is extreme heat doesn't necessarily <br />immediately impact people when it sets in, instead it is when the periods of extreme heat last for days <br />and weeks that it takes its toll on people. Additionally, when overnight air temperatures do not cool below <br />70 degrees F, it does not give people's bodies a break from the heat. An additional complicating factor is <br />when extreme heat conditions are paired with another hazard. For example, if severe thunderstorms <br />affect an area and knock out power right before extreme heat sets in, you now have additional people <br />exposed to extreme heat without working air conditioners. Extended durations of extreme heat can also <br />exacerbate drought conditions and can also lead to excessive power consumption needs causing the <br />potential for brown- and black -outs, which would only make the exposure conditions worse. <br />Extended periods of extreme heat also contribute to wildfire hazard through a process wherein natural <br />materials, particularly sand and bare soil absorb solar radiation, holding the heat very near the surface, <br />resulting in extremely high surface temperatures. The hot surface heats the overlying air, which rises, <br />carrying the heat upward. The extremely hot surfaces generate strong updrafts, essentially creating local <br />winds that dry surrounding vegetation, increase fuel temperatures, and intensify and spread wildfires. <br />The dry vegetation, high fuel temperatures, and high winds increase the static electricity, increasing the <br />potential for spontaneous combustion, particularly during prolonged periods of drought. Extreme heat <br />temperatures can also force the closure of airports due to the lack of sufficient air density for take -offs <br />and landings. <br />4.3.7.5. Geographic Scope of Hazard Blc <br />When this hazard happens, it can be as small as a local hazard, or countywide with areas of highest <br />concern in the largest metropolitan areas because of the Urban Heat Island (UHI). Urban heat islands are <br />large metropolitan urban areas that are warmer in temperature than surrounding rural areas because of <br />pavement, blacktop, and buildings. The University of Minnesota conducted a study showing the Twin <br />Cities metro area temperature differences in 2011. <br />Graphic 4.3.7A illustrates measured temperature differences of up to 10 degrees just within Hennepin <br />County. <br />132 <br />