High Humidity

[asteria]

Hi there - does anyone know whether high humidity (85% or greater) contributes to poor seeing. I noticed on the hydrometer yesterday that the evening humidity was around 90%+. There was certainly a lot of moisture in the air because everything got soaked with dew. I would have rated the seeing as moderate at best.

Are there any meteorologists amongst us who can advise!?

Best wishes.

Ed

[James]

High humidity on its own shouldn't affect seeing directly as seeing is more affected by the stability/motion of the air which is more a function of the wind, the jetstream and convection. High humidity means that the air has more moisture in it so in theory it would affect transparency much more. This would be most noticable near light sources as the sky would appear more light polluted than normal with light reflecting of lots of water vapour.

Just to confuse the issue though humidity is an indicator of how much moisture the air is holding. Warm summers air can hold a lot more moisture than cold winter air so high humidity in winter is not as bad as high humidity in summer.

Hope that helps.

James

[asteria]

Hi James - that's very helpful. I see the difference now - I think what made it difficult last night was transparency. The night sky looked liked it was veiled in a very thin mist that made it difficult to pick out key stars. This also explains why the skyglow can be much more of a problem on humid nights. I've noticed that the night sky seems be very 'light' (for want of a better word!) when the humidity is high.

Many thanks.

Ed

[asteria]

Hi James - case closed! The humidity is much less tonight, and the viewing far better. Transparent skies! With great contrast between starlight and black sky. Got the binos out and saw a few Messiers that were impossible yesterday. First glimpse of the Orion Neb. Fantastically bright even in binos.

Thanks.

Ed

[radioactive]

try The Pleiades in your binos when you get a chance its a very nice bino object I find

[beamish]

Bl**dy he** ! is it clear outside Martyn ?