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Skywatching

Sunlit clouds at midnight

For us living north of latitude 45 degrees, which includes pretty well all of Canada, summer nights are short, and it does not get really dark.

This is because the sun does not get very far below the horizon. Here in southern British Columbia, at a latitude of around 50 degrees north, the midnight sun lurks a mere 17 degrees or so below the northern horizon. If there are no towns in that direction we can see the glow. If we go to latitude 67 degrees or so, the sun will be on the northern horizon at midnight. Further north, it won't set at all.

This means that if we look northward on a clear summer night around midnight, although we might be in darkness, something high enough in the sky will have the sun shining on it. This makes possible a rare but rather beautiful feature of summer nights, noctilucent clouds. To see these, get to a place without the glare of a town to the north, with a good view of the northern horizon. If they are present you will see a layer of opalescent, lacy cloud, shining with lots of delicate colours, looking rather like a layer of mother-of-pearl.

As is the case with many things in nature, this beauty is produced by something prosaic: low-angle sunlight shining on a layer of ice crystals. However, these clouds are different in that they are made up mainly of material from space.

The clouds we see in the sky almost every day are due to water rising from the ground or sea. The sun warms the sea or land surface, evaporating water. The warm air rises, carrying the water vapour with it. Eventually it reaches a height where it is too cold for the air to hold all that water, and it starts to condense out, forming a cloud of water droplets. If the column of rising air is vigorous enough, the water vapour can be carried to a height where it is cold enough to freeze the water, forming tiny ice crystals. These form the fibrous clouds we call mare's tails. Since those ice crystals are little prisms, they break up sunlight or moonlight into its component colours, producing sundogs, moondogs and pearly rings around the moon. However, all this happens at a relatively low altitude, below around 20 kilometres. The noctilucent clouds lie around 80 kilometres above the ground. They lie far too high to be formed by wet air rising from the ground. They are made mainly from meteor debris drifting downward.

Our planet is under constant bombardment. On average, around 100 tonnes of meteor debris arrives at Earth each day. Most of this stuff comes from objects the size of gravel, grit or grains of sand entering our atmosphere at many kilometres a second and burning up. This is really the tail end of the process of planet building. Our world and the other planets of the solar system are the result of many small bits of material accumulating into bigger and bigger lumps. The result is very fine dust that remains in the upper atmosphere for a very long time, diffusing only slowly downward. It is a fine mixture of rock dust, metal dust and tiny ice crystals. Once it gets down to heights where rain and snow forms, it gets flushed down to the ground quickly.

There are high-energy ground level phenomena that can send material to a height of 80 km or so. These include volcanic eruptions and nuclear explosions. On occasion, small noctilucent clouds have been seen after launching space vehicles, particularly those using hydrogen and oxygen as propellants, because the exhaust is water, which instantly freezes at high altitude into tiny ice crystals.

Despite their dusty origin noctilucent clouds are beautiful features of some summer nights. They do not occur every night. But if you are away from the manmade light pollution on a clear summer night, and you have a clear view to the north, it is worth keeping an eye on the northern sky. Telescopes and binoculars are not needed, but something to take a picture or two would be a good idea.

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• Jupiter and Saturn rise before midnight.

• The moon will reach last quarter on the 31st.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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