Welcome New Members! We want to hear from you. Register, stop lurking and start posting!

dew

All general weather related discussion & questions, including model discussions, longer-term outlooks & anything non-breaking weather.
Post Reply
h
hotnbothered
New User
Reactions:
Posts: 6
Joined: Wed Feb 08, 2017 2:56 pm

dew

Post by hotnbothered »

No day-time dew:
In all the places I've lived, dew forms only at night. Sometimes it may have been raining on and off for days, with the BoM AWS (close by our house) recording a persistent rel. humidity of 100% over extended periods. By definition, the local air mass is then at the dew point. Yet in my experience, dew never forms during the day, even though there are multiple ways in which the temperature might drop a degree or two. Where I live (Melbourne suburban) dew only appears on a clear cold night. I don't understand the physics here. In theory, if the RH is 100%, a slight temperature drop ought to produce dew, whether it's day or night.

Dew shadows:
The other puzzling observation is that light dews are exquisitely sensitive to the presence of very lightweight barriers. Even an small amount of thin plastic film, or a small square of low % shadecloth located well above the ground (or even disposed vertically!), can cast a "dew shadow". A car under my open carport, completely open on 4 sides, typically shows dew only on the end of the car nearer to the edge of the roof, even though the roof is a full 2m above the car. I am aware that in principle these layers might change the radiative and convective transfer patterns - but sometimes you see situations where it's hard to understand how a distant flimsy barrier has any influence. The balance of forces must be very delicate. I wonder if anyone has tried to numerically model these effects.

I'd be interested in comments on either of these phenomena.
J
JasmineStorm
Supercell
Reactions:
Posts: 1870
Joined: Thu Sep 22, 2016 9:40 pm
Location: Kyneton 527 ASL

Re: dew

Post by JasmineStorm »

Interesting little topic.... this is my understanding on the forming of dew.

Daytime heating ceases on sunset and cooling commences. Radiation from the earth lifts into the air when the sky is clear. Surface temp cools to around the dew point. Dew appears.... I think (opinion only) crucial to your question is that daytime heating has to cease first, before the dew process can commence. I can't imagine that would be possible during normal daylight hours with a clear sky.
Post Reply