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monad

why is the sky clearer on colder nights?

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Hello,

this might be a stupid question, but i cannot figure it out.

last week there were a couple of days with clear skies, and i've noticed that when it was hotter, so the visibility was much worse compared to a colder night. is this somehow related to pollution?

or is it just because of the wind?

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Warm air tends to be more turbulent than cold air. This can be seen on a hot day when you look at the road in front of you. The heat haze is caused by the air being heated by the tarmac and rising. Cool air tends to settle down a lot more - a bit like dry ice in a warm room.

Also, warm air tends to be a lot more moist. Moisture causes refraction in the air and this causes haze. Cold air is normally dryer, and so allows light to pass through it more directly.

Wind won't affect the 'seeing' so much, although it will knock your scope over!

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Partly related to the converse: Why is it cold on clear nights. The heat has no blanket of cloud so escapes. Hence colder.

The 2 conditions are probably related to an extent.

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Indeed, better to say "why colder on clear nights" rather than the other way around. But it's all to do with shifting heat about. Lots of cloud = more trapped heat/reflection of heat from said cloud. Simplification, but close enough.

Actually BadgerChap, warm or cold air can both be moist or not, there is no rule on that. But warm air can hold more moisture than cold air so I see where you are coming from. Even so, it is the relative differences between different parcels of air that affect seeing the most.

Wind does affect seeing actually. Totally still nights all the way to the upper atmosphere gives you very stable seeing but that could equally mean very stable but awful seeing.

Or, if the wind is light but steady that can actually mix the boundary layer up a bit so you don't get sudden changes of temp and humidity as light passes through it. But then stronger and gusty winds can make things worse again by changing the refractive index quickly over time. You'll still get pockets of good seeing though.

If you do any solar astronomy on windy days, and your scope is looking upwind, you can usually "see" a gust of wind coming before it hits you on the ground. It looks a little like someone just kicked your tripod leg.

But assuming all conditions are good at ground level, you could have a strong jet stream up around 300mb which just trashes seeing altogether.

Now throw in transparency, on which water humidity has a much larger effect (plus trapped pollution, dust etc.) and you have another whole issue to deal with.

I sometimes wonder why we bother with this hobby :hello2:

Cheers

Ian

Edited by iwatkins

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I sometimes wonder why we bother with this hobby :)

It's something to do between the arguments on cloudy days :hello2:

James

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I'd say a warm atmosphere is also full of dust and insects, but that's just my assumption.

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As has already been said, the warmer the air the greater the agitation within it (and vice versa). I must admit to watching the moon through the scope on warm summer evenings and feeling a bit queasy because the shifting atmosphere can give a peculiar, almost undulating appearance to the moons surface.

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I'm sure it is to do with the clouds trapping the warm air. I always think that the clouds are acting like a duvet.:hello2:

Carole

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Exactly right Carole.

A "duvet"? I like that. Maybe we should measure clouds by their "Tog" rating rather than their thickness and water content :hello2:

But yes basically. At night, all the outgoing long wave radiation (we call it heat) is absorbed and reflected back again. So indeed acts as a duvet as that works in a similar fashion.

Cheers

Ian

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Wondered the same thing cloud wise & now I know... roll on Autumn...

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I've always thought that it was the cloudyness that altered the temperature. I thought that the clouds reflected heat back down to earth, therefore the less the clouds there are the colder it is.

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Thank you for all your thoughts, now it makes more sense to me, except the fact about the clouds, since what i originally meant was why when it is a clear night, the visibility is usually better because it is colder?

i also thought that it might be because its much darker when its colder.

what if you are in a dry place and its warm and clear? would the visibility be the same in that location as during a cold and clear night?

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Well, hot air rises so I would say is more dynamic than cold air. Think of the air above a Bunsen burner. The image above the flame is distorted. So I'd say that cooler is better.

Not that it will ever be that hot at night in Britain and I could be wrong anyway!

Edited by Ganymede12

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