Heat Index Statistics

Heat Index Tools and Statistics for West Central and Southwest Florida
The National Weather Service has multiple tools to assess the potential for heat stress due to extreme temperatures. Within the following tabs is information that can help understand the issuance of NWS heat watches, warnings, and advisories. Each of these tools integrate other weather parameters to provide a deeper level of information beyond what the actual air temperature can tell us. Across West Central and Southwest Florida we experience the conditions most favorable for heat indices reaching dangerously high levels when the winds are out of the west and thunderstorms are unable to form. If this pattern persists over several days or more, this can compound the risk for heat-related illnesses. Please visit https://www.weather.gov/safety/heat for more information. You can also visit the NWS Experimental Heat Risk page to visualize heat impacts in a new way. This index is supplementary to official NWS heat products and is meant to provide risk guidance for those decision makers and heat-sensitive populations who need to take actions at levels that may be below current NWS heat product levels.
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Heat Index
Tampa Stats
Fort Myers Stats
Sarasota-Bradenton Stats
Wet Bulb Globe Temperature
Links

What is the Heat Index?
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. In other words, the “feels-like” temperature. This has important considerations for the human body's comfort. When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body's temperature. When the atmospheric moisture content (i.e. relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases. The body actually feels cooler in arid conditions. There is a direct relationship between the air temperature and relative humidity and the heat index. This means that as the air temperature and relative humidity increase (decrease), the heat index increases (decreases).

It surprises many people to learn that the heat index values forecasted and measured are for shady locations. If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F. As shown in the table below, heat indices meeting or exceeding 103°F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.

What factors influence these values?
Proximity to the coast - The two sites with the longest running observational data are at Tampa International Airport and Fort Myers - Page Field. These sites are close enough to the coast to be routinely influenced by the west coast sea breeze. Particularly in summer when temperatures are the hottest, these sites experience a wind shift and a slight “cooling” (in a relative sense) as marine-influenced air moves overhead. Since the air over the water tends to be relatively “cooler”, the sea breeze helps to moderate temperatures (and subsequently the heat index) at these locations. However, this can be a double-edged sword; a westerly wind also carries air with the highest moisture content, or humidity. With high enough humidity, the cooling effects can be mitigated. Oftentimes if a west wind is persistent overnight, this can contribute to incredibly warm, muggy mornings. Thunderstorms - Climatologically, the Florida peninsula tends to see active summers with thunderstorms most days. These thunderstorms often form during the period when the air temperatures and subsequent heat indices would otherwise be at their hottest. However, these storms work like natural air conditioning and cool down the air temperature, thus lowering the heat index.
How is the Heat Index calculated?
The computation of the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23). The regression equation of Rothfusz is HI = -42.379 + 2.04901523T + 10.14333127RH - .22475541TRH - .00683783TT - .05481717RHRH + .00122874TTRH + .00085282TRHRH - .00000199TTRHRH T is temperature in degrees Fahrenheit RH is relative humidity in percent HI is the heat index expressed as an apparent temperature in degrees Fahrenheit If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI: ADJUSTMENT = [(13-RH)/4]SQRT{[17-ABS(T-95.)]/17} where ABS and SQRT are the absolute value and square root functions, respectively. On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI: ADJUSTMENT = [(RH-85)/10] [(87-T)/5] The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F. In those cases, a simpler formula is applied to calculate values consistent with Steadman's results: HI = 0.5 * {T + 61.0 + [(T-68.0)1.2] + (RH0.094)} In practice, the simple formula is computed first and the result averaged with the temperature. If this heat index value is 80 degrees F or higher, the full regression equation along with any adjustment as described above is applied. The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman. If this seems like a lot then use this Heat Index Calculator.

Tampa Heat Index Statistics:
Hourly observations at Tampa back to 1948 were analyzed and using the reported air temperature and dew point temperature values for the relative humidity were calculated and in turn, the Heat Index. The heat index is only calculated when the air temperature is above 80°F. Quality control was performed on the data, but errors could still exist.



Heat Index >=108°F	Heat Index >=105°F	Heat Index >=100°F
Link to >=108°F Table	Link to >=105°F Table	Link to >=100°F Table

Fort Myers Heat Index Statistics:
Hourly observations at Fort Myers back to 1977 were analyzed and using the reported air temperature and dew point temperature values for the relative humidity were calculated and in turn, the Heat Index. The heat index is only calculated when the air temperature is above 80°F. Quality control was performed on the data, but errors could still exist.



Heat Index >=108°F	Heat Index >=105°F	Heat Index >=100°F
Link to >=108°F Table	Link to >=105°F Table	Link to >=100°F Table

Sarasota-Bradenton Heat Index Statistics:
Hourly observations at Sarasota-Bradenton back to 1975 were analyzed and using the reported air temperature and dew point temperature values for the relative humidity were calculated and in turn, the Heat Index. The heat index is only calculated when the air temperature is above 80°F. Quality control was performed on the data, but errors could still exist.



Heat Index >=108°F	Heat Index >=105°F	Heat Index >=100°F
Link to >=108°F Table	Link to >=105°F Table	Link to >=100°F Table

What is the Wet Bulb Globe Temperature (WBGT)?

Heat stress is commonly measured by the heat index or the “feels like temperature” which is representative of the temperature and moisture in the atmosphere. An alternative method is the Wet Bulb Globe Temperature (WBGT), which uses temperature, humidity, and other weather parameters. WBGT has been shown to be an effective indicator of heat stress for active populations.

Why is it called Wet Bulb Globe Temperature graphic

The WBGT has been used for decades in many military agencies, by the Occupational Safety and Health Administration (OSHA), and organizations that facilitate marathons. The American College of Sports Medicine bases its guidelines for the intensity of sport practices on WBGT as well, and it is therefore utilized by athletic programs in many school districts.

WBGT versus Heat Index:

Comparison of Wet Bulb Globe Temperature and heat Index graphic

Who is WBGT Most Helpful for?

This parameter is best suited for active people such as outdoor workers, athletes, marching band, and others performing strenuous outdoor activities.

WBGT can be used to establish guidelines for activity modifications during exercise or outdoor work.

Guidelines vary geographically since temperatures fluctuate by region, and people’s response to exercising or working in heat may also vary by the geographic region.

The Occupational Safety and Health Administration (OSHA) recommends protective measures for outdoor work:

Acclimatize workers starting the first day working in the heat and after any extended absences
Provide shade for outdoor work sites
Schedule work earlier or later in the day
Use work/rest schedules
Limit strenuous work (eg., carrying heavy loads)
Use relief workers when needed

A good information reference on the WBGT can be found here.

Rain Showers and Gusty Winds Coming to the Mid-Atlantic; Remaining Unseasonably Warm in the Central U.S.

Heat Index Statistics