NWS Ft. Worth

Watching Hurricane Erin; Heavy Rainfall Threat for Upper Midwest; Heatrisk for Mississippi Valley

Hurricane Erin continues to track westward with its outer bands bringing heavy rainfall and gusty winds for Puerto Rico and US Virgin Islands. Life-threatening surf and rip currents are occurring and will spread across the Atlantic coasts this week. Meanwhile, heavy rainfall and possible flooding are in the forecast for upper Midwest through Monday. Heatrisk continues for the Mississippi Valley. Read More >

It's the Heat AND Humidity

It has been said that "It's not the heat, it's the humidity". Well, actually it's both. Our bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as the last extremity is reached, by panting. As the body heats up, the heart begins to pump more blood, blood vessels dilate to accommodate the increased flow, and the tiny capillaries in the upper layers of skin are put into operation.

The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere by one or a combination of three ways...

radiation,
convection, and
evaporation.

At lower temperatures, radiation and convection are efficient methods of removing heat. However, once the air temperature reaches 95°F, heat loss by radiation and convection ceases. It is at this point that heat loss by sweating becomes all-important. But sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation (sweat changing to water vapor). The downside of this method of cooling is that high relative humidity retards evaporation.

Relative humidity is a measure of the amount of water vapor contained in the air, divided by the maximum amount the air can hold, expressed as a percent. A relative humidity of 50% means the air contains ½ of the water vapor it can actually hold. The maximum amount of water vapor the air can hold is dependent upon the temperature (the "relative" in relative humidity). The higher the temperature, the more water (actually water vapor) the air can hold. For example, air with a temperature of 32°F can hold about 0.16 ounces of water. Air with a temperature of 80°F can hold about an ounce of water.

So, what does this all mean? Sweat is evaporated (changes from a liquid to a gas, i.e. water vapor) when heat is added. The heat is supplied by your body. The results are summed up in the table below...

Relative Humidity	Capacity for air to hold water	Amount of Evaporation	HEAT removed from the body
LOW	LARGER	HIGHER	MORE
HIGH	SMALLER	LOWER	LESS

If the condition where the lack of heat removed from the body is prolonged, the results can be very serious and even fatal.

DID YOU KNOW...

In a normal year, approximately 175 Americans die from extreme heat. Young children, elderly people, and those who are sick or overweight are more likely to become victims.
Between 1936 and 1975, nearly 20,000 people succumbed to the effects of heat and solar radiation.
Because men sweat more than women, men are more susceptible to heat illness because they become more quickly dehydrated.

Knowing how the body deals with excessive heat is critical in understanding the dangers to you and proper steps to take to prevent becoming a victim of excessive heat.

NEXT: The Heat Index