/ 100% Humidity, but no rain

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humptydumpty - on 19 Sep 2016
What does it mean if a weather forecast says that "chance of precipitation" is 0%, but "humidity" is 100% overnight? My understanding is that 100% humidity means that the air is holding as much water as it can without rain, so would this mean heavy dew and wet rock are likely early in the day?
harold walmsley - on 19 Sep 2016
In reply to humptydumpty:

No real knowledge on this but my guess is that rain falls from on high and so depends on humidity levels at height. Dew forms on the ground and so depends on the reported ground level humidity?
humptydumpty - on 19 Sep 2016
In reply to harold walmsley:

Sounds reasonable. I've just rechecked the forecast, and it gives the "Dew Point" (in degrees Celsius) as being suspiciously close to the temperature. So I suspect it will be very dewy, and perhaps foggy.
Valkyrie1968 - on 19 Sep 2016
In reply to humptydumpty:

It's a while since I underwent any kind of formal education, but I'm quite confident that 100% humidity means that you're underwater.
CurlyStevo - on 19 Sep 2016
In reply to Valkyrie1968:

erm right
ablackett - on 19 Sep 2016
In reply to Valkyrie1968:

When humidity is quoted it is always relative humidity, so 100% humidity is taken to mean 100% relative humidity means as humpty said, the air is holding as much water as it can.
stp - on 19 Sep 2016
In reply to humptydumpty:

I think when the dew point equals the temperature that is 100% humidity.

Apparently dew point is meant to be a better indicator of humidity that using percent because the latter is relative to the air temperature. As with most things the more you look into them the more complicated they become.
Valkyrie1968 - on 19 Sep 2016
In reply to ablackett:

Fair point - can we fairly extrapolate, then, that 100% humidity would essentially be equivalent to standing in a cloud?
harold walmsley - on 19 Sep 2016
In reply to Valkyrie1968:

But as pointed out above it is more complicated if you get into details. Condensation needs something to start on so it is possible to get 100% relative humidity or even more without forming a cloud if the air is clean. However solid surfaces will usually start condensaton and the ground is generally colder than air at night hence we get dew.
phizz4 - on 19 Sep 2016
In reply to humptydumpty:

Harold is right. Rain needs condensation nuclei to allow it to form (dust, soot, ash etc). Relative humidity refers to the amount of moisture that the air can hold at any given temperature. The higher the temperature the greater its capacity to hold water vapour. Hence, if you lower the temperature (by contact with a cold surface, such as your car body) the lower temperature air can't hold all of it's water vapour so it gets precipitated as dew.
Mostro - on 19 Sep 2016
In reply to phizz4:

I spent several years paragliding, trying to understand what made it rain (and why it didn't). I came to the conclusion that you need both (a) 100% humidity over a range of altitudes and (b) a catalyst to trigger the accretion of individual H2O molecules into raindrops. I think latent heat of vapourisation also affects the process. As raindrops start to form, the molecules give up their latent heat - this heats up teh air slightly, reducing the relative humidity a tiny bit and slowing the process down.

It's always seemed to me that the more water there is in the atmosphere, the more scope there is in the weather forecast being wrong (i.e. cloudy & humid, instead of raining).

You could probably do a PhD in why it rains.
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Valkyrie1968 - on 19 Sep 2016
In reply to humptydumpty:

Thanks all for taking the time to explain - interesting stuff. I'm a little old-fashioned, though, so won't be forgetting to pack my snorkel and trunks when heading out on a high-humidity day in the near future, science be damned!

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