Mountain headgear
I鈥檝e seen mountain-top clouds similar to the one in this astounding image of a sombrero-like cloud atop Mount Fuji. What causes them? Do they only occur above volcanoes or do they occur above any mountain of suitable height?
鈥� This is a striking example of the cloud species known as altocumulus lenticularis (known in English as lenticular altocumulus). Such cloud forms when a stable, humid layer is forced to rise above the level at which condensation usually occurs, normally as part of wave motion. The uplift occurs above or downwind of an obstacle, and is certainly not restricted to volcanoes.
Depending on the exact atmospheric conditions, long trains of waves and clouds may be produced, and wave clouds have been observed far from any obvious source of motion. Such clouds tend to remain stationary as long as conditions, including wind strength and direction, remain constant. It is not uncommon for a series of humid layers to be affected, giving rise to a vertical stack of lenticular clouds separated by clear air, known to meteorologists as pile d鈥檃ssiettes (鈥減ile of plates鈥�).
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Just as the air is forced to ascend and cool, producing condensation, so as the air descends at the rear of the wave it warms and any cloud dissipates. Close examination (with binoculars) will often reveal how the cloud is forming on the upwind side and dispersing at the trailing edge.
This cloud species is related to a similar cloud known as pileus, where an ephemeral cap of cloud forms as air is forced upwards above an actively rising cloud tower. In this case, the convective cloud cell often breaks through the pileus, producing a collar of cloud that is normally entrained into the rising column.
Storm Dunlop
Chichester, West Sussex
鈥� This cloud type is a very well known phenomenon to glider pilots worldwide. The upper winds are blowing away from the camera towards Mount Fuji, probably at speeds in excess of 100 kilometres per hour at the mountain鈥檚 peak. As the wind strikes the slopes of the mountain, it is forced to rise and becomes colder and less dense, and the moisture in the airstream condenses out, creating what is called a cap cloud over the peak.
Immediately downwind of the peak 鈥� the other side of the peak in the photo 鈥� the airstream, through a combination of temperature and stability characteristics, spills down the mountainside and then rebounds upwards again before descending even further downwind. With the right atmospheric conditions, this series of stationary vertical oscillations, or standing waves, can continue for 100 kilometres or more downwind.
The top of each standing wave is often marked by what is called a lenticular cloud a few kilometres behind the mountain crest. The lenticular cloud appears stationary despite the very high wind speeds through it.
Massive standing-wave systems and their accompanying lenticular clouds are found above mountain systems all over the world, including Europe, the Andes, California and New Zealand, where the altitude record for a glider was set at 15,000 metres.
A miniature version of a standing wave can often be seen when a shallow flow of fast-moving water in a stream or gutter rides over a submerged object. This often creates two or three standing waves in the water downstream from the submerged object.
Max Hedt
Horsham, Victoria, Australia
鈥� What a beautiful lennie! As any glider pilot will tell you, this is a lenticular cloud formed in a standing wave. When flying a glider in such a wave, the ride is incredibly smooth, and if the wind speed is higher than the glider鈥檚 stall speed then you can head directly into wind and seemingly be suspended in space with just the aircraft instruments indicating that the glider is actually flying. Magic!
Mike Debney
Melton South, Victoria, Australia
This week鈥檚 question
Up in smoke
How many people are cremated each year and how much energy is consumed in the process? Will these numbers increase on current projections? And are there no better and environmentally friendly methods of disposal?
Jeremy Dawson
Laurencekirk, Grampian, UK