Neck ache
Question: Do giraffes standing on the plains of Africa ever get struck by lightning?
Answer: Yes, and not only giraffes. Game, stock, humans and trees all get struck with varying frequency and effect. Some of the planet鈥檚 highest lightning strike frequencies have been recorded in parts of southern Africa, and loss of stock or human life is not particularly unusual. Giraffes are good targets, but there are not many of them and their remains are often quickly consumed by scavengers, destroying evidence of the strike.
Even in the Bushveld, a low-lying area where thunderstorms are common, lightning is not a leading cause of giraffe death. There seems to have been no evolutionary selection for specific avoidance behaviour 鈥攃owering giraffes are not a sensitive predictor of thunderstorms. It is rumoured, however, that llamas and their relatives in the Andes lie down in storms, heads together, and that one bolt sometimes kills a whole herd. If true, sheltering in this way must offer some protection, despite the occasional disastrous strike.
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Jon Richfield
Dennesig, South Africa
Effortless ascent
Question: When you are in a lift that is travelling down or up at speed, and you see a fly buzzing away in front of your nose, how come the fly is not affected by the speed of the ascent or descent? How does it manage to remain hovering in the same place?
Answer: The lift is a closed box, the air inside moves with the lift and the fly is carried along with the air. If the floor and ceiling of the lift were perforated, air would flow through the lift when it moved. Skirts would rise, and the fly would hit the floor or the ceiling depending on direction of travel.
J. Simpson
Weston-super-Mare, Somerset
Flat bottoms
Question: Why are clouds usually rounded and fluffy on the top, and smoother and flatter underneath?
Answer: When air rises and cools to the point at which condensation takes place, clouds start to form. This will usually happen at a fairly constant level, so the base of the cloud will be flat. As the cloud stretches high up into the sky it will draw in drier, clearer surrounding air. The cloud will then start to evaporate and erode, giving the 鈥渇luffy鈥� look.
Nathan Powell
The Meteorological Office
Bracknell, Berkshire
Answer: Clouds and fog form when the ambient temperature of the air reaches the condensation point, the point at which water vapour (an invisible gas) becomes a liquid. When it reaches this point, the liquid collects on the dust particles in the air and becomes visible.
We call the results clouds, unless the altitude of the condensation point is low enough for a cloud to form at ground level, in which case we call it fog. Various factors influence the exact point at which condensation occurs, such as the air pressure at a given altitude and the moisture content of the air, but air temperature is generally the deciding factor.
In a given volume of air, the temperature is usually uniform horizontally, but it varies vertically because temperature decreases with altitude. This means that the condensation point is also horizontally uniform at a given altitude. Above this altitude, water vapour will condense out to form clouds, below it water will remain in its gaseous state. Therefore, the bottoms of most clouds will appear flat.
Not all clouds, however, have a rounded, fluffy appearance on top. Those that do鈥攃umulus clouds鈥攁re formed as warm, moist air rises and cools to the condensation point. The air stops rising when it cools to the temperature of the surrounding air. But this cooling occurs unevenly, because the air on the outside of the rising parcel cools and slows more quickly than that in the middle. This produces the billowy appearance the questioner describes.
In contrast, stratus clouds form when the ambient temperature of the air reaches the condensation point by normal heat loss processes. There is no vertical movement of the air, so the clouds form in place and have a flat top and bottom. Both types have 鈥渇lat鈥� bottoms, but the tops vary depending on how the cloud formed.
Jeff Colbert
Novato, California
The blue room
Question: Why is it that a room illuminated by a television appears blue to an outside viewer regardless of the colours that are being displayed on the screen?
Answer: Colour television pictures are produced by three different colours: red, green and blue. These combine to create a whitish light with a colour temperature close to that of daylight (5500 to 6500 kelvin).
Colour temperature is a way of describing the relative amounts of the different colours that make up the light from a particular source. It can be thought of roughly as the temperature an object would need to be heated to give off light with that mixture of wavelengths. At low temperatures, low-energy colours such as red and yellow predominate; at higher temperatures the proportions of green or blue increase.
For example, standard tungsten light bulbs have a colour temperature of 3200 K and are very yellow. Your eyes adjust to this light, making it appear whiter and more like daylight. At twilight or at night, anything with a higher colour temperature than tungsten lights, such as a television, appears distinctly blue.
The same effect is seen if you look out of a room lit by tungsten light. The twilight appears particularly blue, and yet if you were to go outside and look at the same scene, the eye would no longer compare it to the lit room and the colours would appear more neutral.
The converse is also true. Viewed from outside, the windows of houses lit by tungsten light sources look distinctly yellow. Fluorescent sources generally have a spectrum much closer to that of daylight and don鈥檛 produce the same effect.
Bill Richmond
Barnet, Hertfordshire
This week鈥檚 question
Back and forth: Tides are caused by the Moon and the Sun and contain a lot of energy, but where does it all come from?
P. Perkin
by e-mail