Dew Point In Simple Terms

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abermarc
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Dew Point In Simple Terms

Post by abermarc » Mon Nov 04, 2019 6:31 pm

I'm trying to understand dew points but from a certain point of view. I understand how dew points work......sort of......but here is my thinking and what I am trying to understand.

If air temperature is going to be say 5c and the dew point is say 8c and assuming no rain forecast will the road be dry or wet?

If air temperature is going to be 10c and dew point is 5c is the road going to be dry or wet?

I guess what I'm getting at is if I can figure out in advance if there is going to be a frost or the water vapour in the air is going to cause the road to be wet I can them decide the likelihood of ice forming and whether or not it would be safe to ride.

I commute early morning in UK usually before sun up.

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Re: Dew Point In Simple Terms

Post by catcitrus » Mon Nov 04, 2019 7:09 pm

Relative humidity is measured by the difference in wet and dry bulb temperatures--essentially in wet bulb temperatures evaporation takes place from around the bulb (KEPT WET BY DIPPING A WICK INTO MOISTURE)--and that rate of evaporation (and consequent cooling effect) depends on the ability of the air to hold more moisture (the drier the air the greater the difference between wet and dry bulb temps). If wet and dry bulb are the same then NO evaporation is taking place and that means that the air is 100% saturated at that local temperature and pressure- THE DEW POINT and without any wind--hence the typical louvered enclosures for such instruments (forgot the name for it). So as SATURATED air cools( below the dew point) then it cannot hold more moisture and it starts to condense out in the form of mist(and in winter gives rise to that marvellous hoar frost as moisture condenses out onto surfaces and then freezes)--the air may start at , say, 80% saturated (above the dew point) , but as it cools it can hold less and less moisture (% saturation increases), resulting in the dew point being reached and mist formation in cool vallies etc (cold air sinks--more dense). You need to look for clear nights and some reference of relative humidity at dusk to give you the best possibility of mist formation and ice on the roads. You can tolerate much colder temperatures without the risk of ice if the air is pretty "dry" to start with--not something that happens in the uk as we are surrounded by warmish water in winter.
I hope this helps--I'm no expert--corrections invited?

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Re: Dew Point In Simple Terms

Post by Tonibe63 » Mon Nov 04, 2019 7:30 pm

I think there are too many variables to accurately surmise what the road conditions will be like on a commute to work. The temperature can drop by 3 or 4 degrees on my 18 mile commute and there is a bridge on the motorway which can have snow flurries on one side and be clear on the other.
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Re: Dew Point In Simple Terms

Post by zimtim » Mon Nov 04, 2019 7:34 pm

There's an app for that
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Re: Dew Point In Simple Terms

Post by garyboy » Mon Nov 04, 2019 8:07 pm

I agree there are too many variables .. temperature of the earth, road surface, how long its been at that temperature .. wind force and temperature and direction .. air temperature, humidity position at the top of a mountain, bottom of a valley .. tree cover, leaves, moss, algae, .. nearby bridges .. precipitation .. weather forecasts .. time of year .. recent weather trends … Atlantic or northern influences .. time of day .. water crossing the road .. underpasses out of the sun .. sunshine … etc etc

I guess that even in lab conditions it would be difficult to predict the point at which ice would form on a surface .. even the roughness of a surface will affect it.

I just look at weather trends and temperatures, then half my chances as I live in wales. :D

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Re: Dew Point In Simple Terms

Post by Asgard » Mon Nov 04, 2019 8:21 pm

If air temperature is going to be say 5c and the dew point is say 8c and assuming no rain forecast will the road be dry or wet?
Sack the forecaster, dewpoint is never higher than air temperature, if the dewpoint is the same as air temperature then the air is 100% saturated with moisture and you are riding in a cloud and the road is definately wet

Other scenarios get increasingly complicated, but if surface temperature and dewpoint are within 3 degrees of each other, i most likely wont go flying
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Re: Dew Point In Simple Terms

Post by abermarc » Mon Nov 04, 2019 8:36 pm

garyboy wrote:
Mon Nov 04, 2019 8:07 pm
I just look at weather trends and temperatures, then half my chances as I live in wales. :D
I've got riding jeans and normal jeans hanging together in the bedroom. My usual trick is to look out window as soon as I get up then decide which jeans to grab. I just wondered if there was a more scientific approach I could take - I also live in Wales :)

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Re: Dew Point In Simple Terms

Post by dave h » Mon Nov 04, 2019 8:42 pm

maybe this is where the value of lean sensitive traction control may outweigh the lower cost of paying less for a bike

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Re: Dew Point In Simple Terms

Post by Asgard » Mon Nov 04, 2019 9:04 pm

abermarc wrote:
Mon Nov 04, 2019 8:36 pm
garyboy wrote:
Mon Nov 04, 2019 8:07 pm
I just look at weather trends and temperatures, then half my chances as I live in wales. :D
I've got riding jeans and normal jeans hanging together in the bedroom. My usual trick is to look out window as soon as I get up then decide which jeans to grab. I just wondered if there was a more scientific approach I could take - I also live in Wales :)
You got it right, no amount of arsing about with dewpoint depressions will give as accurate a result as looking out the bedroom window, I just look out at the Blorenge and if I cant see the top I dont bother flying and go out on the bike, If I cant see the bottom I shuffle around whinging like a girl for half an hour till the missus kicks me out
“I fly because it releases my mind from the tyranny of petty things. ”

Antoine de Saint-Exupéry

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Re: Dew Point In Simple Terms

Post by redbikejohn » Mon Nov 04, 2019 10:41 pm

I commute all year round too including early starts. A few rounds are not gritted so I fitted a ice warning LED that comes on at 3 reg c (orange) and then again (red) at zero. With heated jackets and muffs/heated grips it's sometimes hard to judge if it's getting to freezing point. I travel out of London into bucks so London can be just above zero but below outside town and ice/frost a possibility. Anyway - the led warning has worked great and saved me a couple of times. Trying to predict "safe" conditions will be next to impossible..... until summer lol.
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