Dewpoint

he dew point (sometimes spelled dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.

When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease....

Humans tend to react with discomfort to a high dew point (> 60 °F), as higher dew points correlate with higher ambient temperatures. The body perspires and produces sweat to cool down, but the higher relative humidity that typically goes along with a high dew point prevents the evaporation of sweat and inhibits the cooling effect. As a result, the body may overheat, resulting in discomfort.

Lower dew points (< 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 to 68 °F). Most inhabitants of these areas will consider dew points above 21 °C (70 °F) to be oppressive.

Dew Point °C    Dew Point °F    Human Perception    Rel. Humidity at 90°F (32.2°C)
>24°C    >75°F    Extremely uncomfortable, 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    31% - 41%
10 - 12°C    50 - 54°F    Very comfortable    31% - 37%
<10°C    <49°F    A bit dry for some    30%

Here is a formula for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is useful only when the temperature is at least 80 °F and the relative humidity is at least 40%.

,

where

    HI = Heat index (in degrees Fahrenheit)
    T = ambient dry-bulb temperature (in degrees Fahrenheit)
    R = relative humidity (in percent)
    c₁ = -42.379
    c₂ = 2.04901523
    c₃ = 10.14333127
    c₄ = -0.22475541
    c₅ = -6.83783×10⁻³
    c₆ = -5.481717×10⁻²
    c₇ = 1.22874×10⁻³
    c₈ = 8.5282×10⁻⁴
    c₉ = -1.99×10⁻⁶

Fahrenheit    Celsius    Notes
80–90 °F    27–32 °C    Caution — fatigue is possible with prolonged exposure and activity
90–105 °F    32–41 °C    Extreme caution — sunstroke, heat cramps, and heat exhaustion are possible
105–130 °F    41–54 °C    Danger — sunstroke, heat cramps, and heat exhaustion are likely; heat stroke is possible
over 130 °F    over 54 °C    Extreme danger — heat stroke or sunstroke are likely with continued exposure

Note that exposure to full sunshine can increase heat index values by up to 15 °F (8 °C).