Lines in the sky
Question: Sometimes the sky has long, straight and evenly spaced rows of cumulus clouds streaked across it. Why are the clouds lined up in this way, and what factors determine the gaps between the different rows? How frequently do the correct conditions occur?
Answer: Your correspondent doesn鈥檛 say whether the lines of cloud he has observed are across or along the direction of the wind. This is important because each orientation indicates a different phenomenon.
Cumulus clouds form when thermals carry moisture upwards. As a thermal rises, its pressure and temperature drop. Eventually it reaches the dew point, when the moisture condenses. This forms a cumulus cloud, and because the thermal will have been drifting with the wind during its ascent, the cumulus will be downwind of the point on the ground where the thermal started.
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Certain areas on the ground are better than others at triggering this convective process. In particular, locations where the temperature is a little higher than the surroundings will tend to start convection. Examples are slopes facing the Sun, wind shadows, dark-coloured areas and built-up areas where warm air can accumulate between buildings.
As a thermal separates from its ground-based source, cooler air gets sucked in at ground level to replace the rising air. This cools the surroundings and the convection is temporarily cut off. As a result, thermals may 鈥減ulse鈥�. However, if the air is particularly unstable or the trigger location is particularly effective, the convection may produce an unbroken stream of rising air. Under these conditions, the cumulus forms downwind of the source, continuously fed by the column of rising air, and drifts further in the same direction, creating the lines cloud. These are sometimes referred to as cloud streets and they provide glider pilots, such as myself, with an easy way to proceed cross-country, especially if the desired direction is approximately parallel with the cloud streets.
The spacing of cloud streets depends initially on the location of the ground sources that produce them. But once they have started to form, they tend to organise themselves at regular spacings. The reason for this is that once a cloud street has formed, the ground in its shadow cools, and this suppresses the tendency to trigger thermals along the line shaded by the cloud street. This suggests that the angle between the Sun鈥檚 rays and the wind direction, the height of the cloud street, and the Sun鈥檚 elevation above the horizon all play a part in determining the spacing. My observations during glider flights support this.
A completely different phenomenon causes clouds to line up across the wind as opposed to along it. This is the formation of standing waves in the atmosphere, which are often marked by parallel bars of cloud known as lenticulars. This usually occurs in the lee of mountains, when, under the right conditions, trains of lenticulars can be seen sometimes for hundreds of miles downwind. Once again, these are a delight to glider pilots because, under the right conditions, waves can rise to many times the height of the mountain that creates them, and gliders can use them to gain altitude. The British altitude record for a glider under these conditions is 14 000 metres.
Like cumulus, lenticulars are formed as air rises鈥攖ypically, just as it reaches the crest of the wave. A few minutes鈥� observation shows that although there is bound to be a strong wind at the height they form, the lenticulars themselves are stationary. The cloud forms continuously along the wave鈥檚 leading edge, and the airflow passes through the cloud. As the airflow turns downwards behind the crest, the increasing temperature causes the moisture to be reabsorbed and to disappear in the downside of the wave. The airflow in wave conditions is uncannily smooth and, as it passes through the lenticular, it gives the cloud top a characteristic silky appearance. The story is very different below the crest, however, where 鈥渞otor鈥� conditions are found. At its worst this produces extreme turbulence, presenting a hazard to aircraft.
A completely different cause of waves is atmospheric shear, when one air mass moves across another. Typically, this happens when a mass of warm air lying on top of a cooler layer is moving in a different direction. Under these conditions, waves may form at the interface of the two air masses. Unlike the lee wave described above, this type of wave is unstable, and 鈥渂reaks鈥� like a wave in the sea. A shear wave can also form lenticulars 鈥攖hough like the wave itself, these tend to be short-lived.
David Starer
Watford, Hertfordshire
Answer: In the case of cloud streets that are parallel to wind direction, when the Sun gets lower in the sky the spacing between the clouds increases until there is insufficient energy to heat the ground. Thus, in Britain, you will see cloud streets starting to form at about 11 am and finishing at 5 pm.
Glider pilots love this because you can travel for miles at speed without the need to turn. When the street is well defined, gliders can climb at over 300 metres per minute in the rising air.
In the case of standing waves, the amplitude of the waves can be 30 times the height of the mountain below, allowing glider pilots to climb in the rising section to 9000 metres or more. This has to be the ultimate surfing experience.
Adrian Loening (glider pilot)
Lamington, Clydesdale
Smelly surfaces
Question: What causes the smell when rain wets roads and paths after a dry spell?
Answer: The smell is given off by Streptomyces bacteria, a genus belonging to the Actinomycetales order of Gram-positive eubacteria, also called actinomycetes.
The bacteria grow in damp, warm earth before fine weather dries out the soil, which then blows around as dust. Rain hitting the ground kicks up an aerosol of water and soil and you breathe in fine particles of soil containing the bacteria.
If you invite somebody to smell a plate of these bacteria grown in the lab, they always comment on how it smells just like the soil after rain.
Cary O鈥橠onnell
Manchester
Answer: The smell comes from rain falling on dry soil near the path. It is caused by actinomycetes, filamentous bacteria that are very common in soil and are the source of most of our antibiotics. During a dry spell, actinomycetes produce spores that are released on contact with moisture.
Incidentally, actinomycetes were also responsible for the distinctive smell of Glasgow鈥檚 old underground system.
Duncan Simpson
Milton, Cambridgeshire
This week鈥檚 question
Burnt offerings: I have noticed that brown or wholewheat bread chars more quickly than white bread when you toast it. Why?
Andrew Tonison
Bedford