Tuesday, 3 August 2010

Busting the inversion layer

There are several reasons why Mauna Kea is one of the very best astronomical observing sites on the planet. One of them is the inversion layer which is caused by cool air above the summit sinking and preventing the warmer air nearer the surface from rising higher than a few thousand feet. It means the summit is free of cloud and fog for much of the year - weather can be a real problem for observatories closer to sea level.

That's not to say we don't get hit by clouds. Depending on how the jet stream is behaving we can get hit by high level cirrus clouds for days on end that don't care about the inversion layer. They provide the best sunsets for tourists and photographers but for optical and infrared telescopes they make observing rather tricky.

Then there are evenings when there's an inversion layer but convection is strong enough to force the tops of the clouds through the layer as in these pictures. Some of us who've worked at the summit long enough can often predict how good a night we'll have in terms of image quality just by looking at the clouds below us and how far they've risen above the layer. There's even one or two of us who think they can tell how much water is in the atmosphere above us simply by looking at the night sky! Kooks! Except I'm one of them!

Anyway, here are more clouds to look at from the summit, I hope you enjoy the pictures. At the top is the NASA IRTF with a very reflective dome and inversion layer busting clouds in the background.

The CFHT and one or two convective cumulus clouds to the north. The tops are just high enough to catch the last light from the setting sun. The shadows on the slope below the observatory are caused by the Kecks and Subaru.

You can tell where the inversion layer is in the two pictures above. Just look for the flat-topped cloud layers (click on the pictures for a larger version if you want).

The clouds are really bubbling up between the Big Island and Maui to the North-West. The summit of Haleakala on Maui is visible in the distance.

Final walk-around and check before observing starts. It's well after sunset, the Kecks are opening and the clouds down below shouldn't be a problem tonight.

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