 Poster: A snowHead
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Can someone explain what Dew Point is?
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brian
brian
Guest
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It's the temperature at which the air will reach 100% relative humidity and therefore water will start to condense (as dew, clouds, whatever).
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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You need to Login to know who's really who.
You need to Login to know who's really who.
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it is the temperature at which relative humidity for a specific sample of air reaches 100%, which is when the moisture in the air condenses out (hence the dew point).
pass warm dry air over water and it will pick up water vapour. it will hold that ad infinitum provided that the air is not cooled. when it cools the water vapour condenses out, initialy as steam or moisture (dew) on any cold surface. take a glass of iced water into the bathroom next time you've run a bath and look at the outside of the glass. air touching the glass is cooled and condensation occurs.
edit: damn my one finger typing speed 
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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How do they work out what temperature this will occour at on a certain day?
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You'll need to Register first of course.
You'll need to Register first of course.
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| johnboy wrote: |
| How do they work out what temperature this will occour at on a certain day? |
It's relative to the temp and mositure content of the air.
http://www.imagepermanenceinstitute.org/shtml_sub/dl_dewpointcalc2.asp
The more water in the air the less you have to cool it before water starts to condense out.
Air at 20 Deg C 50% relative humidity will start to condense out water at ca 9 Deg C whereas at 70% humidity it condenses out at ca 14 Deg C.
http://www.decatur.de/javascript/dew/index.html
A good analogy is to think of air as a sponge (they can both contain water). Cooling the air is equivalent to squeezing the sponge. If the sponge is sopping wet then the water starts to fall out of the sponge with only a little squeeze (a small temp drop).
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If you need something to calc dewpoint without a computer then you could use a psychrometric chart
http://www.fao.org/docrep/S1250E/S1250EEW.GIF
Example
From X axis air temp of 20 Deg rise vertically until you hit the curved 50% humidity line. Go horizontally left from this line until you hit the curved edge of the chart to read off the correspnding saturation/wet bulb /dew point temperature of 9 Deg C.
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It might sound obvious but another important thing to know about the dewpoint temperature is that snow can't falland settle unless the dew point temperature is close to or below freezing point. In other words, snow can be as heavy as it likes but it can't settle unless the dewpoint allows the moisture in the air (and by the same token, on the ground) to remain in a frozen state.
There's nothing more frustrating than watching heavy snow and seeing it all hit the grass and turn into slush and water when the air temperature is just a bit too warm. That doesn't always happen though and sometimes when you see snow begin to settle in previously marginal temperatures, what you're watching is not snow sticking because it is freezing the grass but actually that the dewpoint is dropping because of evaporative cooling in the airmass above the grass and way up into the air where you see those big snow flakes slowly sailing down towards you.
This is why heavy snow and dry air make powder snow i.e. there is a double-whammy effect when some of the snow and moisture in the air actually evaporates, cooling the air still further and guaranteeing that none of the snow that reaches the ground has any wetness to it. 
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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So what is wetbulb temperature then?
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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Yoda, ta 
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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to determine the relative humidity of air at any time, measure dry bulb temperature (ordinary thermometer) and the wet bulb temperature (thermometer with wet cotton wrapped around the bulb). the wet bulb temperature will normally be lower because water is evaporating off the cotton and is cooling it.
once you have your readings find corresponding lines on a psychrometric chart (dry bulb across the bottom figures, follow green line vertically up and wet bulb follow top red curve to reading then pale blue line diagonally down from top left to bottom right.) where the two cross, follow around red curvy line, that's your reading.
for example 20c dry bulb, 15c wet bulb = 60%RH. then draw a line horizontaly from the 20/15 intersection point horizontally to the left until it hits the 100% RH (highest red curvy line) to discover the dew point - about 11deg c. in our example.
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