**Definition of Heat Index:** The heat index is an estimate of how hot the air “feels” to the human body and provides a relative indication of potential health risks. Among others, the two primary factors in the heat index equations are temperature and water vapor (i.e., moisture/humidity). Humidity affects the efficacy of perspiration to evaporatively cool the skin. When the air is very dry, sweat can evaporate efficiently, allowing the skin to cool via evaporative cooling. Conversely, very high humidity stifles the body’s natural cooling mechanism by limiting, even preventing, the evaporation of sweat from the skin. This can lead to heat exhaustion, heat stroke, heart attack, and other health complications.

Exposure to full sunshine increases the potential for heat exhaustion. A heat index above 120º is extremely rare, but it can occur. Listen to your body and drink plenty of fluids when the heat index is excessive. Use the charts below to estimate the heat index.

The heat index concept is meaningful and serves a purpose. The public’s critiques to the contrary stem from gross misconception and lack of understanding. Heat exhaustion arises from *both* air temperature and humidity (among other factors), parameters that some tend to overlook. Like the wind chill factor, the heat index was developed to raise public awareness to a weather hazard that affects some groups more than others, using real physical parameters that have known and detrimental impacts on the human body.

### What was the Hottest Heat Index Recorded on Earth?

✏️ *The highest theoretical heat index ever recorded was 176ºF on July 8, 2003, in Dharan, Saudi Arabia. This was calculated from the observed conditions and utilizing the widely accepted heat index equation with its inherent assumptions. This was the result of excessive humidity (dew point temperature of 95ºF) and an air temperature of 108ºF. Dew point temperatures that high are extremely rare. In fact Dharan’s 95ºF dew point is the highest ever recorded on the planet. Important Note: Bear in mind that this heat index calculation, like any other, is subjective and its merit can be debated and disputed ad nauseam; this is always an exhausting exercise in futility. In fact, all heat index calculations are purely theoretical since the index can’t be measured and is highly subjective. *

✏️ *In the United States, we rarely see dew points above 80ºF and those are usually found near the Gulf of Mexico. However, the highest dew point temperature ever recorded in the United States was 90ºF in Appleton, Wisconsin on Thursday, July 13, 1995. On that day, Appleton also recorded a high temperature of 101ºF resulting in a heat index of 148ºF, the highest heat index ever recorded in the United States.*

## Calculate the Heat Index

Heat Index Calculation | |||||||||

Enter in the air temperature (T_{air}) in degrees Fahrenheit and relative humidity (RH) in percent (without the % sign), then click on the Calculate HI to compute the heat index (HI) | Use the heat index tables | |||||||||

T_{air} | RH | HI= | |||||||

OR use dew point temperature | |||||||||

T_{air} | T_{dp} | HI= |

Heat Index Calculation | |||

Enter in the air temperature (T_{air}) in degrees Fahrenheit and relative humidity (RH) in percent (without the % sign), then click on the Calculate HI to compute the heat index (HI) | Use the heat index tables | |||

Temperature | ºF | ||

Relative Humidity | % | ||

Heat Index = | |||

OR use dew point temperature | |||

Temperature_{} | ºF | ||

Dew Point Temp_{} | ºF | ||

Heat Index = | |||

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## Printable Charts: Temperature to Heat Index

Heat index is a function of air temperature and humidity. We have several ways of expressing atmospheric moisture content, but the two most common are dew-point temperature and relative humidity. Aside from minor rounding errors in the numerical computations, both techniques will yield the same result. Ultimately, your choice of conversion table (or computational method in the calculator above) will depend on the type of data that you are using for the humidity parameter.

### Heat Index Chart Calculated from Dew Point Temperature

### Heat Index Using the Relative Humidity

### Can Heat Index Be Computed with a Relative Humidity Below 40%?

Yes, the humidity can be lower than 40%. There is a misconception that a relative humidity of 40% or higher is prerequisite for the heat index equation, but this is simply not true. Granted, heat index is subjective by its very nature, and we could randomly apply any upper/lower limits anywhere we want; however, the 40% rule doesn’t make sense, mathematically. Unfortunately, a few of the heat index calculators available online *unintentionally* perpetuate this notion by displaying an error message when a relative humidity below 40% is entered. This is mathematically illogical given that the relationship between the heat index and relative humidity is nonlinear. As a result, the corresponding equilibrium temperature is also nonlinear (see the chart above). Consider that an air temperature of 100º combined with a relative humidity of 40% results in a heat index of 109º. It’s illogical to assume that the calculation for this example is valid at RH=40%, but not at 39%. There is clearly plenty of room for a reduction of heat index by lowering the relative humidity until the equilibrium point is reached (i.e., temperature = heat index). In the case of air temperature = 100º, the equilibrium humidity is 25%. This is evident in our heat index chart, noting the superimposed dashed equilibrium line. Heat index is a strong function of humidity, and the equilibrium relationship (temperature = heat index) is *non-linear*. **Therefore, the equilibrium humidity is not a constant and the 40% rule is invalid**. The higher the air temperature, the lower the equilibrium humidity. At temperatures above 98º, the equilibrium RH is less than 30%. It drops into the teens at temperatures above 108º.

### What Factors are Included in the Heat Index Calculation?

As a result of numerous bio-meteorological studies, a long list of variables emerged as key contributors to heat discomfort, exhaustion, and heat stroke. The majority of those factors have been reduced to assumptions in order to simplify the equations. Biological factors pertaining to the human body vary from person to person, and an “ideal” heat index model would be highly customizable to each individual human being. Unfortunately, such computational methods would be impractical and the assumptive constants used in the standard heat index models have proven to be sufficient. Ultimately, the main factors are ambient air temperature and humidity. Without going into technical detail, the following are the bio-meteorological factors that comprise the heat index equation:

**1) The human body**

- Surface area and typical diameter of the human body
- Fraction of the human body typically covered by clothing (assumed 84%)
- Core body temperature, vapor pressure, and salinity
- Typical activity level (metabolic output)

**2) Clothing**

- Characteristics of typical clothing (affects heat transfer from the body)
- Assumption about the clothing’s efficacy for and resistance to heat transfer
- Assumption about the resistance of our clothing to moisture flux from the body

**3) Heat Transfer and Ventilation from the Skin**

- Effective wind speed is assumed to be 5 knots. This is the sum of an assumed average wind speed and a person’s movement (i.e., if you are walking at 5 mph into a wind blowing toward you at 10 mph, the vector sum would be 15 mph). Extremely hot days are often associated with relatively weak winds as would be the case in the middle of summer within a synoptic-scale ridge characterized by large-scale subsidence, and wind speeds generally less than 10 mph. Furthermore, we tend to move more slowly in the summertime (unless you’re one of the few who enjoy running in the sweltering heat). All things considered, a relatively weak effective wind speed (5 knots) is assumed in the heat index computations.
- An assumption about the effective radiation and convection (both are forms of heat transfer) from the skin. Previous studies have shown the significance of these factors.
- Skin efficacy to heat transfer (a function of skin temperature & activity level)
- Moisture flux from the skin (a function of skin/air vapor pressure gradient, i.e., relative humidity).

### The Primary Heat Index Equation

Accounting for all of the factors above, the majority of which have been reduced to assumptions, we arrive at the final heat index equation that is most widely used (i.e., by the National Weather Service and most private-sector entities, and used to create the tables above):

**Note:** As you can see, the only *variables* in the equation are ambient air temperature (T) and humidity (R). However, all of the biological, clothing, and wind contributions discussed above are accounted for via the constants. Therefore, yes, the wind is a factor, albeit a small one (I get this question often). Furthermore, the computational methods used for solving this equation (multiple regression analysis) result in a statistical error of ±1.3°F.

✏ **Tidbit:** Because the apparent temperature is a strong function of humidity, the heat index can actually be lower than the air temperature when the air is very dry. As I’ve explained, when it’s extremely humid, the body cannot cool itself via evaporative cooling from the evaporation of sweat. The drier the air is, moisture fluxes increase and perspiration can evaporate more efficiently. This results in a more efficient cooling of the skin via evaporative cooling. We often hear about the proverbial “dry heat”. Well, it’s really a thing. Very hot temperatures combined with an extremely dry airmass can result in a theoretical heat index that is lower than the air temperature. While we’re on the topic of moisture flux, contrary to popular belief, evaporation from any body of water increases with lower humidity. Moisture flux is directly proportional to the difference between the air temperature and the dew point temperature (known as the dew point depression). Water can even evaporate from a swimming pool more efficiently on a cool/dry day in the middle of autumn than on a hot, sweltering day in August.

**➡ Reference: ** Steadman, R.G., 1979: The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science. *J. Appl. Meteor.*, 18, 861-873.