Written and researched by Kestrel intern Joe Moore, Senior Meteorology student at Millersville University.

Heat index and wind chill are two of the most popular “apparent temperature” indexes which try to convey a more useful temperature to the general public. Most people understand the basic premise of these indexes (wind makes cold temperatures feel even colder, humidity makes it feel hotter), but did you know that because of how heat index was developed, the heat index can be lower than the actual air temperature?

Heat index was developed by R. G. Steadman in a 1979 paper published in the Journal of Applied Meteorology titled, “The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science.” As a researcher in the Textiles and Clothing Department at Colorado State University, his paper takes into account a number of factors to form what we know today as the Heat Index. These factors that Steadman accounted for include include:

• “Basic Dimensions” – A typical human, either sex, height of 1.7m (5’ 5.8”), 67kg (147.7lbs), which gives a person’s surface area of 1.78 square meters (19.1 square feet)
• “Effective radiation area” – The amount of radiation exchange between radiant heat and surroundings. Steadman uses the average ratio of 0.79 for clothed parts of the body, and 0.85 for bare (unclothed) parts under mild conditions.
• “Clothing cover” – The amount of clothing and the thickness of the clothing, which is assumed to be “long trousers and a short-sleeved shirt of blouse” to cover 84% of the body’s surface. While there can be differences in the exact apparel worn along with hair length, Steadman points out that these effects impacts are similar to individual variations in skin insulation.
• “Core temperature” – The comfortable body temperature (which the body is trying to achieve in equilibrium) is 37 C (98.6 F).
• “Activity” – The amount of actively a person in engaged in. Steadman uses a person walking at a rate of 1.4m/s (3.1 mph), which generates 800 W/m^2 of energy from the human body.
• “Effective wind speed” – This is the wind speed on the body. Meteorologists measure wind speed (Eg. At your local airport) at a height of 10 meters (33 feet). Using the average wind speed at 10 meters, and factoring in the diminished winds that actually reach the surface, Steadman uses an effective wind speed of 1.32 m/s (3 mph).
• “Ventilation rate” – The amount of heat exhaled by a person. For summer conditions, this is estimated to be about 8% of heat from the body lost through the lungs.
• “Skin resistance to moisture transfer” – What most people would call the effect of humidity on the skin.
• “Clothing resistance to heat transfer” – The amount of heat not escaping through garments.
• Along with a few other complex factors

It is important to note that there is no “official” formula to calculate heat index exactly. This is because heat index is a value from a table. Your Kestrel unit looks up the table value using a complex polynomial equation that gives a very accurate value from the table. This is why, if you use the temperature and dew point from the Kestrel and plug it in to some online calculators, you may see a difference in the heat index value. Rest assured, your Kestrel unit is using the most accurate method possible. (Most online calculators of Heat Index, such as this one on the national weather service website, use a modified formula which carries an error of +/- 1.3F. The Kestrel uses a much more sophisticated formula that is nearly identical to the index table.)

Looking at the official chart (or at your Kestrel indoors) you notice what seems to be an anomaly-the heat index may be lower than the current temperature. I noticed it this morning on the Kestrel meter I use at my desk. (See image on the right.)

The reason for this is that the index assumes a small amount of wind, and also assumes the person is walking. Note, the amount of wind and the walking speed (defined as “Activity” in the list above) is a constant value throughout the table. So in many low-humidity situations (under 30% humidity) with an air temperature of less than 98 F, the heat index will always be lower than the actual air temperature, as explained above. In the US, this can be common in desert areas that are very dry such as places in Arizona, New Mexico and Nevada.

For more information about Heat Index, check out this great resource from the National Weather Service: http://www.srh.noaa.gov/jetstream//global/hi.htm

To learn more about humidity and other environmental conditions that your Kestrel meter measures, check out Knowledge Center.

Tags: Kestrel Weather Meters, Kestrel Weather Meters, Meteorology