Kevin Myatt is The Roanoke Times' weather columnist.

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I have never been a huge fan of human comfort formulas such as the heat index and the wind-chill factor.

For one thing, they don't mean anything at all in relation to what is physically happening in the atmosphere. A wind-chill factor of 32 doesn't freeze water if the temperature is 45. When it's 96, the molecules in the air are acting as if it's 96, even if the heat index is 105.

Secondly, figuring out what temperature it really "feels like" is very subjective for the individual. I can be breaking out in a full sweat while my wife is reaching for a sweater. So a temperature of 82 may at once "feel like" 97 for me and, say, 47 for her.

But we shouldn't entirely dismiss the importance of either the heat index or the wind-chill factor, because excessive moisture in summer heat certainly magnifies the potential for heat stress just as strong winds in winter cold increase the likelihood of hypothermia.

The principle behind each of these human-comfort indexes is that some variable -- atmospheric water vapor or wind -- interferes with the human body's ability to cope with temperature.

With wind chill, increasing amounts of wind more quickly blow away the cocoon of body heat the human body produces. We'll leave any deeper consideration of wind chill for a more appropriate season.

In the case of heat index, water vapor in the air inhibits the body's ability to cool itself through sweating.

As sweat evaporates, heat is used up in the evaporation process, reducing the heat on the body. Sweat evaporates more readily the drier the air is because the air has more room to hold the extra water vapor.

So, the less moisture the air around you contains, the more easily your body cools through sweating. This is the grain of truth in a "dry heat" being easier to handle than sticky heat.

The more moisture that is in the air, the harder it is for sweat to evaporate. So you just get soaked with sweat and experience less and less of the cooling effects of evaporation.

The heat index is a scale designed to calculate the effects of moisture in the air on the body's stress level as evaporation of sweat is inhibited. The heat index calculates a temperature that it would generally feel like under a given temperature and dew point.

There are scales that use temperature and relative humidity, but I prefer to use dew point because it is a true measure of moisture in the air. Also, humidity is already calculated from temperature and dew point, so using it in the heat index scale is actually figuring in temperature twice.

Though heat symptoms can occur at lower readings, a heat index of 105 is considered the point at which a serious threat of heat stroke or heat exhaustion begins for most people in good health. A temperature of 95 with a dew point of 72 produces a heat index of 105, as does a 97 temperature and 69 dew point or a 99 temperature and 66 dew point.

If a heat index of 105 or more is expected for three hours or longer, the National Weather Service issues a heat advisory. We will be very near that range today, though as of Tuesday afternoon, no heat advisory had been issued for the Roanoke Valley. Several counties to the east are under a heat advisory.

A heat index of 110 or more for three hours or longer warrants an excessive heat warning. Those are rare here.

For me, I guess I just don't have to see any index that makes it seem hotter than it is. If it's 90 or above, it's too hot, period. Avoiding three straight months of that is part of why I moved from Arkansas to Western Virginia.

On the Net: Heat index charts tinyurl.com/2u4xld