About Heat Index Climatology Maps

Heat Index affects millions of people across the United States during the summer months.  By combining air temperature and atmospheric humidity, Heat Index is often a better indicator of heat stress.  These maps of the annual number of days, hours, and days with at least three hours at a threshold are available for states east of the Rocky Mountains.

Image Maps Available:

  • Five thresholds are available: 90°F, 95°F, 100°F, 105°F and 110°F. 
  • Days are based on if at least one hour in a particular day met the threshold.  Daily averages are then computed as a sum of daily averages.
  • Days with 3 or more hours are based on if at least three hours in a particular day met the threshold.  These hours may or may not be consecutive.  Averages are once again computed as a sum of daily averages.  Three or more hours is commonly used by National Weather Service offices when issuing advisories and warnings for Heat Index.
  • Average hours are based on the actual number of hours where the threshold was met. 

GIS Interactive Map:

Values from specific stations can be accessed through the GIS interactive map.  All thresholds from 80°F through 115°F are available for hours, days and days with at least three hours.  Monthly and annual averages are available.  Note: Annual averages may not match the sum of monthly averages due to rounding.

Data Used:

Hourly data from stations across the eastern two-thirds of the U.S. were used to create these maps.  Observations were pulled from the Cli-DAP data access portal.  Most of these stations are part of the ASOS/AWOS Network.  Only top of hour observations are used in this climatology for consistency.

Data Quality:

Coarse quality control to eliminate erroneous values above or below physical or official record values for temperature and dew point temperature.  For the station to be considered for the climatology, at least 30 years of data must be available, where at least 90% of the data in those 30 years is available. While many stations have data prior to 1973, the period of record start date was chosen based on the increased number of stations that have quality data from 1973-present.  For a day to be counted for the Average Number of Days with a Heat Index, there must have at least 22 of 24 hours available for a given day. 

How Heat Index is calculated:

Heat Index values were calculated using the official National Weather Service Heat Index equation devised by Lans P. Rothfusz in 1990.

Heat Index = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH

Where T is temperature in degrees Fahrenheit, and RH is relative humidity in percent.  Temperature values must be above 80°F for Heat Index to be calculated.  No adjustments are made to this equation for extreme relative humidity or temperature values.

 

About Wind Chill Climatology Maps

Wind Chill affects millions of people across the United States during the winter months.  By combining air temperature and wind speed, Wind Chill is often a better indicator of susceptibility to frostbite.  These maps of the annual number of days, hours, and days with at least three hours at a threshold are available for states east of the Rocky Mountains.

Image Maps Available:

  • Fifteen thresholds are available at 5°F intervals from 30°F to -40°F. 
  • Days are based on if at least one hour in a particular day met the threshold.  Daily averages are then computed as a sum of daily averages.
  • Days with 3 or more hours are based on if at least three hours in a particular day met the threshold.  These hours may or may not be consecutive.  Averages are once again computed as a sum of daily averages.  Three or more hours is commonly used by National Weather Service offices when issuing advisories and warnings for Wind Chill.
  • Average hours are based on the actual number of hours where the threshold was met. 

GIS Interactive Map:

Values from specific stations can be accessed through the GIS interactive map.  All thresholds from 30°F through -40°F are available for hours, days and days with at least three hours.  Monthly and annual averages are available.  Note: Annual averages may not match the sum of monthly averages due to rounding.

Data Used:

Hourly data from stations across theContinental U.S. were used to create these maps.  Observations were pulled from the Cli-DAP data access portal.  Most of these stations are part of the ASOS/AWOS Network.  Only top of hour observations are used in this climatology for consistency.

Data Quality:

Coarse quality control to eliminate erroneous values above or below physical or official record values for temperature and dew point temperature.  For the station to be considered for the climatology, at least 30 years of data must be available, where at least 90% of the data in those 30 years is available. While many stations have data prior to 1973, the period of record start date was chosen based on the increased number of stations that have quality data from 1973-present.  For a day to be counted for the Average Number of Days with a Wind Chill, there must have at least 22 of 24 hours available for a given day. 

How Wind Chill is calculated:

Wind Chill values were calculated using the official National Weather Service Wind Chill equation.

Wind Chill=35.74+0.6215T-35.75(V0.16­)+0.4275T(V0.16)

Where T is temperature in degrees Fahrenheit, and V is wind speed in mph.  Temperature values must be below 50°F while wind speeds must be 3 mph or higher for Wind Chill to be calculated.

What is the Snow Year?

Because the winter season overlaps two different years, the Snow Year, defined as July 1-June 30, is used for wintertime climatologies such as snowfall and wind chill.