How are you ? During early Summer, weather in the eastern U.S. seemed much more like Autumn, but now heat characteristic of our longest season is being felt. This week I discuss an appropriate topic for this time of year - Heat Index (HI). This is also called apparent temperature. It is an estimate of the temperature which causes equivalent discomfort as existing conditions if dew point temperature is 14 °C (57.2 °F). I.e., if dew point temperature is > 57.2 °F, heat index is > ambient temperature and vice-versa, as a chart illustrates.
Charts & diagrams can be found at many WWW sites - formulas are more difficult to find though. After searching enough, I found different ones at 2 sites (and probably didn't find some 
). One formula is from USA Today's WWW site (their chart is very excellent, though I've seen a few incorrect expressions in related humidity equations). Too many significant figures are included, considering that a person is doing well measuring temperature within a degree F and relative humidity within a few %. It can be more conveniently written :
HI = -42.38 + 2.049 T + 10.14 R - .2248 T R - .006838 T2 - .05482 R2 + .001229 T2 R + .0008528 T R2 - .00000199 T2 R2
HI represents heat index (°F), T temperature (°F), and RH relative humidity (%). For example, suppose T = 93 °F and RH = 60 %. You can verify that HI = 107.4 during such conditions. I do not use ° for expressing heat index because it is not a (real) temperature - only estimation of a supposed equivalent one. Because various formulas are used, heat index charts may not exactly correspond, but are all similar. A few sites include meteorological calculators, using which you can determine heat index. I suppose a definitive reference for this is contained in an NWS Fort Worth, TX office report :
Rothfusz LP. The heat index "equation" (or, more than you ever wanted to know about heat index). Fort Worth, Texas: National Oceanic and Atmospheric Administration, National Weather Service, Office of Meteorology, 1990; publication no. SR 90-23.which I don't see available anywhere on the WWW (please inform me if you know of its location).
@ least 6 factors significantly affect human comfort : temperature, humidity, wind speed, insolation, skin type, and clothing. Effects of heat & humidity are explained at many WWW sites, so I don't state specifics regarding how sweat causes cooling when evaporation occurs, etc. Nor much about health considerations, though I think a good rule-of-thumb is being aware of what your body is indicating. I.e., if you are thirsty, drink water; and if you aren't, don't force it on yourself (contrary with what some people say). Mentioning a few things regarding recognizing developing conditions and interpretation of pertinent weather information may be helpful though.
For planning activities well in the future, knowledge of the general weather regime is helpful. Medium Range Forecast model images are helpful for determining such. For reasons too involved for a brief explanation, a strong upper air ridge typically indicates hot weather during summertime. If surface winds are from a favorable direction for high humidity (e.g., a trajectory from the Gulf of Mexico), conditions are favorable for great heat stress. Consider if you wish, the following example : A recent 500 mb chart from Purdue's WXP (description of upper air charts - now at Unisys Weather) indicates a broad ridge over the SE U.S. A surface chart (description of surface chart) indicates southerly winds are prevalent there. Thus, you may expect large heat indices. Largest values (> 100) for this situation were in eastern North Carolina to eastern Maryland, where temperatures were near 100 °F. Indices over most of Florida were < 100, though dewpoints of mid-upper 70's were larger, indicating that temperature is the main consideration, and humidity a modification of comfort. Considering the cyclonic circulation around Lows and anticyclonic circulation around Highs previously discussed, you can determine conditions above and forecast them.
Routine weather products provide temperature information, but the NWS provides much information - including long-term forecasts and specific heat advisories for situations when heat stress can become threatening.
Text is copyright of Joseph Bartlo, though may be used with proper crediting.