CLIMATOLOGIES: ABOUT THE MAPS

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.

Maps Available:

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.

Why are western states omitted?

While Heat Index is commonly used east of the Rocky Mountains, lower humidity in higher elevations coupled with varied terrain make Heat Index less useful.  The interpolated nature of these maps also leads to inaccuracies with the varied terrain.

 

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.

Maps Available:

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 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.

Why are western states omitted?

While Wind Chill is commonly used east of the Rocky Mountains, varied terrain and local features decrease its usefulness.  The interpolated nature of these maps also leads to inaccuracies with the varied terrain.