Cumulus Support

Hi guys,

I've noticed that Davis' new Davis Vantage Pro2 rain cone doesn't seem to record dew like the older cones including the Davis Vantage Vue's. Is this true? There's been several frosts recently and no rain has been recorded from the melted frost. The rain gauge was shipped correctly calibrated after I tested this.

Thanks,

William

The air has been drier than average during this recent frosty spell, so perhaps there hasn't been enough condensate to trigger a tip?

Think I've read somewhere it's about 4.2ml of water required per tip.
Not sure if it was a metric or imperial tip, but if that amount was present on the cone it could easily evaporate rather than drain if the humidity is not that high.

Yes, I think it's just not dewy enough here, but we've had several hard frosts so there should've been a tip I would've thought. Rain is recorded fine.

Grimers wrote:we've had several hard frosts so there should've been a tip I would've thought

But the air has been dry so as well as melting and evaporating, the frost would've been sublimating too. All conspiring against leaving enough condensate behind to make a tip occur. What I'm trying to get at is that the fact that new rain cone wouldn't have been specifically designed to prevent tips from dew/frost - as all precipitation, whether it is in the form of water that falls from clouds or water that condenses onto surfaces, is of interest and should be recorded.

The Pro 2 New Rain cone which I think is far better than the original one, it does record dew, I've had plenty of 0.2mm recorded primarily from dew. Mine had to be replaced as the reed switched died even with a new one installed.

I find from personal experience that the debris shield that comes with it does block some of the water getting in so that has been removed, being in a very open spot so leaves will not be an issue.

As stated above, it will not really record it as dew due to the fact you have had more frosts than normal, frosts form when the dew point is below 0C or 32F as moisture will go straight from a gas to a solid state. Where I live we rarely get frosts, maybe 1 per 2 years. You will also have to gain sufficient amount of condensed water vapor as well to from in the cone, a gauge that is set to record 0.1mm will often record 0.1mm as it was sufficient enough to tip the gauge while the Davis will remain on 0.0mm.

Depending on the conditions also, the amount of liquid in the gauge may evaporate before it moves down as well into the two spoons below. My vantage Vue does not record dew tho it is up at 2m compared to the Pro 2 at 30cm as my set up is a semi professional type.

One thing I've noticed with 2 brand new rain gauges; Both my VP2 & a cheap 4" digital rain gauge, is surface tension.

I noticed this particularly with the small one, which I could observe sitting on the ground. The new plastic surfaces had very little attraction to water.
The surface tension was causing the water to bead, rather than drain on the cone.
The 4" could collect enough water in a bead at the bottom to create 3 tips when the water bead became big enough to overcome the surface tension.

After watching this on the ground the first 2 days, I even sprayed it lightly with the hose. The same thing.
In terms of physics, the molecular attraction between the water molecules was greater than the molecular attraction between the water molecules & the newly manufactured plastic surface.

End result, the water tended to bead on the cones surface until it finally ran, then when it did, it produced a larger bead in the bottom of the cone, which didn't flow until it reached a given size.

Easy fix, just wiped the cone lightly with some detergent to act as a wetting agent to break the surface tension.
Small amounts ran to the tipper immediately.

Presume the same could apply for collected dew, which could not also resist draining, but evaporate as the morning warms.

This waxy, water beading characteristic is probably typical of all new moulded plastics due to a release agent or some other coating.
I'm pretty sure it's a surface characteristic that would vanish once the plastics spent a few days/weeks in the sun.

Slightly bizarre theory, but 100% observed it on the brand new 4" cone.

Phil