Its damn nice to know the dew point if you've hung some clothes out to dry......

when the temperature reaches the dew point, water in the air will 'condense' and collect on things that are outside. if your bike is outside overnight, for example, if the temperature drops below the dew point then water will begin to form on it.

/instaed/instead/

Well, unless you really are concerned about water condensing on your bicycle, that leaves little to be learned about what good the dew point is.

Yes, it certainly is an indication of how close the air is to holding all the water vapor it can.

If the temperature gets lowered a smidgen below the dew point (or, if the PRESSURE goes up, which will RAISE the dew point), then the air can hold less water and the difference will condense out. That''''s as simply as I can state the fundamentals.

Another practical use of this informnation is that it can give you an idea of your potential comfort level outside or whether or not it pays to open your windows. At a given temperature ... let''''s just stick with summer time ... the closer the dew point is to a given (what''''s known as "dry bulb") temperature, the less readily moisture (sweat) will evaporate from your skin.

The evaporation of sweat from you skin will cool your body by roughly 1000 British Thermal Units per pound of sweat evaporated.

Duh... yeah, real useful information. Well, to make it more useful, a pound of dry air can only move 1/4000th as much heat per degree (Fahrenheit) of change as the evaporation of a pound of water. Hope that puts it in a little more intuitive perspective.

Of course, the irony here is that if the air was TRULY dry, the sweat would evaporate from your skin quite quickly and generate PLENTY of relief from the heat.

This is why 110°F in West Texas can be quite comfortable (in the shade, we won''''t broach radiant heating yet) with a dew point of 65°F, but 95°F in Houston is murder with a 90°F dew point. There''''s much less driving force for sweat evaporation when the air is more full of water ... the rate of sweat evaporation is what makes the difference in comfort level.

Alas, this is all second nature to me (chem engineer) but it gets me nowhere with my wife (liberal arts)when it''''s 70°F outside and I scream that the dewpoint is 65°F and NO WAY do I want the air conditioner shut off and the windows thrown open to refill the house with EXPENSIVELY removed moisture ! The air out of my vents is around 50°F in this situation. Since the AC is condensing moisture still, I know the vent dew point is 50°F.

To get the comfort level back after the "airing out" is complete, I know it''''s going to cost me mucho kilowattage to pump 15°F worth of water back out of the air by condensing it through my AC coil.
> >

Of course, if it''s 110°F in the day time but the dewpoint will stay low overnight, like 50°F for instance, then you dang sure ought to throw open the windows an hour or two after sundown because by morning (especially aided by a fan or fans) you can have a house full of pretty near 50°F air and you get up when the sun does, close the windows and pull the shades/blinds !





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Hey, at least it wasn't 500 lines of incomprehensible ravings on the 4th dimension.

so it comes all down to: leave that dew pnt out of the sys's

#12

4th dimension

Dewpoint is derived as follows:

exp(x) is the exponent function, the constant e raised to the power (x).
e_sw is the saturation vapor pressure over a plane surface of water.
e_vp is the vapor pressure

e_sw = 6.1078 * exp(5.0065 * log(273.15/kelvin_temperature) *
exp(24.846 * (1 - (273.15/kelvin_temperature)))

e_vp = (humidity/100) * e_sw

dewpoint = (237.3 * log(e_vp/6.1078)) / (17.27 - (log(e_vp/6.1078))


Hope that clears things up...

#15
Not really, since you have two sets of parentheses hanging open. It may be logic, but it certainly is "fuzzy" at this point !

I see, and "real life" examples of the significance of dew point given in #7 are chopped liver, right ?

There''s no practical reason I can think of to calculate relative humidity from the dew point, mainly because it''s a pretty expensive entailment to measure dew point directly. You can "calculate" anything from anything else as long as there''s a monotonic mathematical correlation (i.e., "curve-fit") of their relationship. BUT, where you gonna get the dew point ? (besides the Weather Channel, who''ll give you the relative humidity along with it).

Measurement of relative humidity (to better than Wal-Mart wall hanger accuracy) entails first measuring the wet bulb temperature, the dry bulb temperature, and, if you want it within a gnat''s ass, the barometric pressure.

From there you can plug these data into empirical equations similar (but with balanced parentheses) to those given by Fuzzy Logic or you can look up the information on a Psychrometric chart, take your pick.

Dew point is much more meaningful to me, especially along with the dry bulb temperature (like Xymantix said), than relative humidity.

If I know the relative humidity but not the temperature outside, I know very little.



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