by George Miller, Consulting Meteorologist
As I wander through the grocery lines, I keep noticing the headlines in the scandal sheets: "Hottest Summer Yet" or "U.S. To Swelter In Record Heat." While I do not take a lot of stock in those magazines, I do know that summer is coming and with it comes heat and humidity. The combination of those two weather elements can be deadly.
Evaporation is a cooling process that takes heat away from our bodies and transfers it to the air around us in a latent form. It is simply the reason you feel cool when you jump out of a swimming pool or step out of the shower in the morning. Millions of water molecules are escaping from your skin into the air as water vapor.
Water vapor is a gas that you cannot smell, yet it is present in the atmosphere in large quantities. That quantity varies with the temperature. Warm air is capable of holding more water vapor than cold air. For example, air at 60 degrees Fahrenheit can hold over twice as much moisture as air at 41 degrees Fahrenheit. At 95 degrees that factor is seven times as much.
That is why the term is called relative humidity. The "relative" part of it refers to the temperature. The same amount of moisture in air at 41 degrees will have a different humidity than if it was in air at 95 degrees. The more moisture there is in the air, the greater the vapor pressure or more simply the pressure exerted by those water vapor molecules.
If the relative humidity is high and the temperature is high, the vapor pressure of the air inhibits the evaporation of water (sweat) from our bodies. We then begin to fan ourselves to get that moist air away from the skin and replace it with drier air.
Too much moisture and too much temperature taxes the body's ability to disburse heat. From these two measurements, an apparent temperature called the "heat index" has been developed. In essence, the "heat index" is how hot we "think" it is. If our body "thinks" it is too hot, it begins to slow down. Heat cramps, heat exhaustion, and heat stroke can result. We sometimes call them, "the silent killers."
Humidity at both the low range (that is, less than 10 percent) and at the high range (greater than 90 percent) is hard to measure. Yet, the Capricorn instrument does this very well. With Weather View software, heat index is calculated automatically and available for display.
It could be a long, hot summer. Knowing the relative humidity and the temperature could help you survive it.
Heat Index and Possible Heat Disorders
130° and up: Heatstroke/sunstroke highly likely with continued exposure
105° - 130°: Sunstroke, heat cramps or heat exhaustion likely and heatstroke possible with prolonged exposure and/or physical activity
90° - 105°: Sunstroke, heat cramps and heat exhaustion likely with prolonged exposure and/or physical activity
80° - 90°: Fatigue possible with prolonged exposure and/or physical activity