Relationship To Human Comfort
When the air temperature is high, the body's thermoregulation uses evaporation of perspiration to cool down, with the cooling effect directly related to how fast the perspiration evaporates. The rate at which perspiration can evaporate depends on how much moisture is in the air and how much moisture the air can hold. If the air is already saturated with moisture, perspiration will not evaporate. The body's cooling system will produce perspiration in an effort to keep the body at its normal temperature even when the rate it is producing sweat exceeds the evaporation rate. So even without generating additional body heat by exercising, one can become coated with sweat on humid days. It is the unevaporated sweat that tends to make one feel uncomfortable in humid weather.
As the air surrounding one's body is warmed by body heat, it will rise and be replaced with other air. If air is moved away from one's body with a natural breeze or a fan, sweat will evaporate faster, making perspiration more effective at cooling the body. The more unevaporated perspiration, the greater the discomfort.
A wet bulb thermometer also uses evaporative cooling, so it provides a good analog for use in evaluating comfort level.
Discomfort also exists when the dew point is low (below around −30 °C (−22 °F)). The drier air can cause skin to crack and become irritated more easily. It will also dry out the respiratory paths. OSHA recommends indoor air be maintained at 68 to 76°F (20 to 24.5°C) with a 20-60% relative humidity (a dew point of 24 to 60°F).
Lower dew points, less than 10 °C (50 °F), correlate with lower ambient temperatures and the body requires less cooling. A lower dew point can go along with a high temperature only at extremely low relative humidity (see graph below), allowing for relative effective cooling.
Those accustomed to continental climates often begin to feel uncomfortable when the dew point reaches between 15 and 20 °C (59 and 68 °F). Most inhabitants of these areas will consider dew points above 21 °C (70 °F) oppressive.
| Dew point °C | Dew point °F | Human perception | Rel. humidity at 32 °C (90 °F) |
|---|---|---|---|
| > Higher than 26 °C | > Higher than 80 °F | Severely high. Even deadly for asthma related illnesses | 65% and higher |
| 24–26 °C | 75–80 °F | Extremely uncomfortable, fairly oppressive | 62% |
| 21–24 °C | 70–74 °F | Very humid, quite uncomfortable | 52–60% |
| 18–21 °C | 65–69 °F | Somewhat uncomfortable for most people at upper edge | 44–52% |
| 16–18 °C | 60–64 °F | OK for most, but all perceive the humidity at upper edge | 37–46% |
| 13–16 °C | 55–59 °F | Comfortable | 38–41% |
| 10–12 °C | 50–54 °F | Very comfortable | 31–37% |
| < 10 °C | < 49 °F | A bit dry for some | 30% |
Read more about this topic: Dew Point
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