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Question about atmospherics and time of evening


Ianladd
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Hi all,

I have noticed that the clearest skies for me in West London are just after dark.. Even with the light pollution there just seems to be more clarity to the air. Later on when the dreaded flood lights of the tennis courts turn off and a couple of neighbours kill their security lights I go out expecting the seeing to be better and paradoxically it's worse!

Do any of you know why this could be? And if so would you mind giving me the lowdown on it? Would be interesting to know...

Regards,

Ianladd

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Hi Ian, I have noticed the same thing, it doesn't always happen, 2 nights ago the opposite occurred. :)

A pattern I have noted that might explain the effect: The wind usually drops at night and allows mist to build up, the clearest nights I have seen here have been windy. I think it must be mist as I notice the tunnel effect increases until only the stars overhead remain visible without bins, the rest fade into the mist. 2 nights ago I was out until 0300 but last night it was initially clearer and calmer and viewing just got worse, packed in after less than an hour.

Tony.

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I was also thinking about this recently, I have heard that planetary viewing is best done from 3am until morning twilight. This is probably due to the earth warming up in the day, when it gets dark the warmth radiates back into space. It can take many hours for the atmosphere to stabilise, about 3am the warmth has gone and in theory the atmosphere becomes less turbulent.

Unfortunately I have yet to try this as I am unable to get up at 3am :).

Interesting that you are getting the opposite effect, be interesting to see how the seeing is from your location during the summer months when there will be more warm air radiating up from the city. Keep us posted...

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Hi mountainman, I agree Saturn looked great at 0230 the other morning, it was clear, there was very little shimmer even at mag, a slight breeze and by that late no mist. It was about 2 below freezing and scope was covered in ice, I think all the heat from the town had dissipated so seeing was very good. I was looking over a neighbours house and still no shimmer. I think they have a very well insulated loft as the snow stays on it for ages, it all helps.

Heat from buildings is likely to be a bigger problem after a warm day (remember those? :)).

Tony.

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I guess there are a lot of factors here. Some to do with weather and others your location.

For example, if there has been rain, followed by sun but little wind, there is moisture in the air. When solar heating stops, this tends to form mist. Just how much there is, depends on where you are. A sheltered valley holds the mist.

During any sunny day, buildings and structures heat up. Convection currents continue until they cool to be similar to their surroundings.

If you are viewing in an urban area, the buildings will lose heat, causing turbulence. If the heating goes off at bedtime, less of a problem. A busy main road will do the same. Nicely seen in some of F1 races with long lens shots of the hot air behind the cars.

All of these effects tend to strengthen the idea of early am viewing being best.

But of course in our climate, we may well get a reasonable evening with rain after midnight. Tonight will start clear with cloud cover (for me) by midnight.

So the message has to be, if you can see, look. Go for it when you can. There might not be another good time for a month or longer. Then that might coincide with something else to stop you getting the scope out.

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I too have heard pre-dawn viewing is best for planets. The atmosphere is very still. But this has to co-inside with a clear sky all night to be worth waiting up for.

This is true in my experience. I think it is due to the fact that the earth has lost all the heat it is going to lose overnight and this is why the atmosphere is very calm.

But only good if it has been clear all night......as you say.

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So the message has to be, if you can see, look. Go for it when you can. There might not be another good time for a month or longer. Then that might coincide with something else to stop you getting the scope out.

Thanks for all your replies folks! I think that David's quote sums it up for me though.. Sod the floodlights! Get out and see what you can when you can :)

As an old Arabian proverb states, "There are only four things which never come back.. A spoken word, a sped arrow, time passed by and a missed opportunity.."

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