Heat index and hot weather: facts and figures

Hot weather impacts people’s health

Heat index is a combined measure of air temperature and relative humidity that indicates how hot it really feels. Many health agencies throughout the United States, with assistance from the National Weather Service, use heat index to communicate health risk due to hot weather.

In 2015, 652 Minnesotans went to the emergency department and two died from heat-related illness. Any person exposed long enough to excessively hot temperatures is at risk of developing a heat-related illness. Older adults, infants and children, people with chronic conditions, and outdoor workers are more vulnerable to the heat.

Understanding other risk factors that increase susceptibility to heat can help to prevent and reduce heat-related illness. MDH developed the MN Extreme Heat ToolKit, which includes an extreme heat training module and extreme heat tip sheets. The tips sheet, available in multiple languages, identifies steps you can take to prevent heat-related illnesses and how to help your families, friends, and neighbors to stay safe. 


Monthly heat index and heat-related illness ED visits

 
 
Source health data: Minnesota Hospital Association, 2000-2015.
Source temperature data: NASA, NLDAS, 2000-2016 Average Max Heat Index

In Minnesota, the monthly average heat index is lowest in September and highest in July. More Minnesotans are treated for heat-illness in the emergency department (ED) in July, when the heat index is the highest, compared to other summer months.


Average summer heat index in Minnesota by county, 2014-2016

 

Heat index is a measure of the ambient temperature and humidity that estimates the “real feel" of the temperature outside. The average summer heat index from 2014-2016 was determined by averaging the maximum daily heat index from May through September for the three years for each county. Three years of data were used to determine the average heat index because temperatures can vary considerably every year. Looking at multiple years helps to provide a better understanding of typical temperatures. 

The map shows that the average heat index is higher in the southwestern and southeastern parts of Minnesota, as well as the 7-county metropolitan area, which includes Hennepin and Ramsey Counties. This corresponds with average higher temperatures in southern Minnesota versus northern Minnesota.


Number of extreme heat days in Minnesota by county, 2016

 

For this analysis, extreme heat days were defined as days that had a heat index over the 95th percentile of the baseline (based on the daily heat index for the years 1979-2016).

Although all county in the state experienced a heat index above their county's 95% threshold for at least four days in 2016, generally there were more extreme heat days in the northeastern part of Minnesota. That is many northeastern counties had more days where their daily maximum exceeded their 95% relative threshold.

Every county in the state had at least four days of extreme heat in 2016, which shows that every Minnesotan is at risk for heat-related illness due to a high heat index.


Download county data and maps

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