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Dewpoint

 
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Clive
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Dewpoint
« on: July 29, 2008, 02:33:20 PM »

There's a weather data collection thingie about five miles from my house.  For the past several days, the dewpoint has never dropped below 70*.  Very uncomfortable outside.  Disgusted



Dewpoint:
Quote
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 90F (32.2C)
>24C    >75F    Extremely uncomfortable, oppressive    62%
21 - 24C    70 - 74F    Very humid, quite uncomfortable    52% - 60%
18 - 21C    65 - 69F    Somewhat uncomfortable for most people at upper edge    44% - 52%
16 - 18C    60 - 64F    OK for most, but all perceive the humidity at upper edge    37% - 46%
13 - 16C    55 - 59F    Comfortable    31% - 41%
10 - 12C    50 - 54F    Very comfortable    31% - 37%
<10C    <49F    A bit dry for some    30%

I also tried to calculate the Heat Index, using the simple formula found on Wikipedia:
Quote
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 = -6.8378310−3
    c6 = -5.48171710−2
    c7 = 1.2287410−3
    c8 = 8.528210−4
    c9 = -1.9910−6

Fahrenheit    Celsius    Notes
8090 F    2732 C    Caution fatigue is possible with prolonged exposure and activity
90105 F    3241 C    Extreme caution sunstroke, heat cramps, and heat exhaustion are possible
105130 F    4154 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).




Yes, I'm a nerd.  And yes, I'm bored.
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spacey
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Re: Dewpoint
« Reply #1 on: July 29, 2008, 02:41:25 PM »

Around here it's a dry heat, but it's still 100 *goshdarn* degrees, so those who say "at least it's a dry heat" can lick my *goshdarn* balls.
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Clive
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Re: Dewpoint
« Reply #2 on: July 29, 2008, 03:04:33 PM »



I can derelicte my own balls, thank you very much.
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Uisce Beatha
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Re: Dewpoint
« Reply #3 on: July 29, 2008, 04:35:34 PM »

Doopoints



What a moran.
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gleek
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E chu ta!

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Re: Dewpoint
« Reply #4 on: July 29, 2008, 06:27:52 PM »

I left the house this morning in a Polo with no jacket and it was downright chilly. It's now around 65 and bearable.
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twoiron
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Re: Dewpoint
« Reply #5 on: July 29, 2008, 06:33:38 PM »

HTFU buttercups!!
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stroh
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Re: Dewpoint
« Reply #6 on: July 29, 2008, 08:22:06 PM »

Our dewpoints have been very high here lately too.  But I haven't seen dew on the grass in the mornings for quite some time, so I guess MMV.

Kinda hot though.  I was sweating.
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twoiron
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Re: Dewpoint
« Reply #7 on: July 29, 2008, 08:40:14 PM »

Our dewpoints have been very high here lately too.  But I haven't seen dew on the grass in the mornings for quite some time, so I guess MMV.

Kinda hot though.  I was sweating.

Is that due to the weather or some other activity??
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