Hot day, poor seeing?

[Camelopardalis]

Folks, it's a hot one in Sydney, feels like standing in front of an open over door :P

But SkippySky says the seeing is looking 8-9, transparency 80%, slow jet stream and gentle northerly wind. Could be a warm night out.

Any bets on how decent Jupiter will look tonight? Do we get many nights like this in summer or should I force myself out of my comfort zone tonight?

Had a minor wildlife experience last weekend, only a beetle, but I gained some new grey ones until I identified it...we've seen big nasty spiders around the garden before. What are my risks at night?

[The Mekon]

Big spiders are just harmless Orb Weavers. Their webs can be a nuisance though. Biggest problem I have with the hot weather is getting the instrument to cool down. My reflector mirror is at 30 degrees right now, and the temp drops quite rapidly here after sunset. I reckon I would have to stay up until 0400 to get everything even.

[glend]

It does take the earth awhile to cool down after a hot day, and your seeing can appear twichy as thermal rising will distort the atmophere. If your looking at Jupiter tonight, before the moon rises, exect to have some twich. I was looking at Jupiter last night and there was certainly some heat wobble. Waiting until later, say at least two hours, should improve seeing but then the moon will be flooding the sky with light - however Jupiter is bright enough to stand out well. Jupiter is going to be with us for some time, and next week when there is more darkness the views will improve.

[Camelopardalis]

Thanks guys, some would say I'm a bit of an arachnophobe :sadeyes: and not really used to the wildlife yet. But you're right, Jupiter will be there next week too :D

[Wavytone]

Dunk the heat does not mean much seeing-wise, in winter the air is just as turbulent and there are thermals too.

What matters most ts the microclimate in your immediate vicinity - hot walls or cliffs facing west, or dark roofs that have soaked up the heat and over the evening release thermals under your line of sight.

Also bare fields or grassy paddocks with dark soils spell trouble - farmed paddocks or urban areas, the dark soil heats up and also holds a lot of moisture.

The best areas are those like the Katoomba airstrip - trees providing shade to keep the sun off the ground during the day, no hot buildings, cliffs or walls to create thermals under your line of sight, and pale sandy soil doesn't absorb much heat and releases it quickly, and also the sandy sandstone plateau doesn't hold much water so dew isn't an issue.

Conversely if observing from home in an urban area... The atmospheric turbulence will be bad if it has been a full-sun day, any tine of year...

FWIW flying the paraglider provides a very first-hand feeling of turbulence in the air, the force on the glider tips and the rough air is immediately felt in the control lines much the same as a soaring bird would feel in their tip feathers.

The other issue in winter is that humans gave this awkward tendency to heat their home at night, which causes really bad turbulence. Not good if trying to observe in suburbia !

There's another phenomenon too - laminar airflow vs turbulent airflow. At Hargraves or Mt Blackheath the west-facing cliffs hold quite a lot of heat and it takes hours for them to release it - as thermals - during the evening. While that might seem bad, the prevailing breeze is a westerly and thanks to the width of the valley the airflow is laminar - smooth, in other words - so warm riding air isn't really troublesome. As experienced in the paraglide, in the late evening the warm air rises very smoothly out of the valley and is free of turbulence. A very nice flight.

At the airstrip however it's another matter - being so far back (kilometers) from the western ridge means the breeze over the airfield is inevitably turbulent. Hopeless for paragliders and only tolerable with a prop and very big motor. And not ideal for telescopes.

[Camelopardalis]

Thanks for all the info Wavytone, it's fascinating stuff! I must say last month at Hargraves was the best-ever view of Jupiter for me...maybe not a huge surprise given my previous microclimate :D but it was high in the sky up there the past few years.

I have a clear-ish view north so it probably would have been OK, so I'm kicking myself a bit now. Learning experience :thumbsup:

[Wavytone]

Mmmmm I think the trick is to be in laminar airflow. This is the reason why Hargraves works, also the ridge at Mt Banks, where you're on a ridge with a long valley floor stretching upwind; at night the wind is usually a southwesterly.

Even in the UK you could get the same effect if the wind is from the right direction blowing onto say Mam Tor, Stanage Edge in the Peak District, or the Lothians south of Edinburgh. But the scope needs to be right on the high point of the ridge, no more than 30 metres from the edge if there's a cliff.

[AstralTraveller]

Thanks Wavytone. You discussion about cliff-edges makes we wonder about my local circumstances. I'm on the coast, to the east of an east-facing escarpment. I've long thought that westerly winds produce turbulence as they come over the escarpment. Does that match with your experience? Also, I've tried observing from the communications towers on Maddens Plains, about 50m from the cliff edge and it never seemed to be a good site. Could turbulence generated at the cliff edge affect seeing upwind?