Dewpoint too low?!

[pooksly]

Earlier in the snow discussion thread I read that a dewpoint temperature would be too low for snow to fall as it would dry out by the time it gets there. Can a kind soul please explain the science behind this?

[Nick F]

I don't think the surface dew point ever gets too low enough in the UK for it not to snow if there is sufficient moisture at cloud level to produce snow, though a large gap between the surface dew point and air temperature (low humidities) may mean any snow flakes falling out of cloud are more likely to be small on their descent due to evaporation in the dry air near the surface. Whereas, if the dew point and the air temps are close to freezing, then big snow flakes are likely - due to the high moisture content in the air. Hope that helps abit.

[pooksly]

Thank you very much that makes things much clearer now.

[Coopsy]

We get that here very regularly. It will be snowing at the tops of the mountains, and nothing in the valley, when dew points can be as low as minus 20oC

[SP1986]

As a rule of the thumb to produce frost the dewpoint must equal the temperature, but this is not always the case.

In the case of frost if the dewpoint = -2°C and the temperature is +1°C, generally the temperature needs to drop to -2°C to form dew however as it is lower less moisture will form, and obviously freeze.

The highest obtain conditions for frost and snow are about T2m=0°C and DP=0°C, this means that enough moisture can be collected.

That's why the heaviest snows are always in temperatures directly on 0°C because the moisture is at it's highest level of content at the dew point (hence the name dewpoint)

If the temperature however is 0°C and the dewpoint is -9°C you might expect to see a frost, but the different between the temperature and dewpoint are too significant to create water vapour.

It also works with snow structure and nature

The perfect definition for perfect wet snow is T2m=°0C/DP=0°C but if the dewpoint is below 0°C then dry snow becomes a feature - and this is a sign of the loss of water in the atmosphere so snow is almost pure ice.

when the dewpoint hits -10°C (assuming the Air temperature is 0°C), the snow is almost pure ice (graupel)

[BrickFielder]

Dewpoints can be a good indicator of humidity, so that when people say the dewpoint is too low they really mean the air is too dry for snow. Humidity is usually a reflection of the difference between the actual temperature and the dewpoint temperature. In real terms if the dew point is low then we need a very low surface temperature as well to get snow. Because snow comes from clouds and clouds keep temperatures up and we are surrounded by sea which also tends to keep temperatures up ,very low surface temperatures and snow are unusual in the UK. Low dewpoints become a good approximation for it is too dry to snow.