The birds
I was recently standing outside as a thunderstorm approached when I noticed a large flock of seagulls behaving strangely: they wheeled round and round overhead like vultures in front of the thunderclouds. This made me wonder if birds have an ability to avoid storms, and whether they ever get struck by lightning?
鈥� I suspect that the birds鈥� wheeling was a response to the storm鈥檚 gust front 鈥� the cold blast of air released by a storm as its downdraught descends, hits the ground and rolls outwards. Birds sometimes ride gust fronts as they do thermals, and are sometimes displaced by gust fronts. Swifts have been observed trawling the leading edge of sea-breeze fronts, which are very similar, and feeding on insects caught in the front.
Birds are struck by lightning, although proving this can be difficult. A skein of geese flying through a thunderstorm was reportedly struck and killed by lightning over Manitoba in Canada in the 1930s. This may well have been the result of the large number of birds flying together 鈥� 52 birds were killed in the strike. Single birds are less likely to attract a strike.
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鈥淎 seagull sitting on a telegraph pole was struck by lightning. It disintegrated with a loud bang鈥�
However, unless they are migrating, birds generally do not fly in bad weather. In order to attract a lightning strike they would have to acquire and retain a net positive charge, as ground-based objects do during thunderstorms, but the nature of atmospheric electricity makes it hard to judge whether or not this is likely. Gust fronts can propagate tens of kilometres ahead of a thunderstorm, so it is more likely that any bird activity related to a storm would be well away from lightning strike areas.
Peter Burt
Biometeorologist
Natural Resources Institute
University of Greenwich at Medway
Chatham, Kent, UK
鈥� The description of wheeling seagulls makes me suspect the birds were foraging for flying ants. This is not uncommon, and is often associated with anticyclonic conditions, when the ants hatch, and thunderstorms often occur. Other birds avoid storms. Common swifts (Apus apus) certainly make sometimes lengthy detours around storms, because they are dependent on aerial insects for food.
The most celebrated example of birds being struck by lightning in the UK occurred on 3 January 1978, when a line of thundery squalls killed 140 geese along a 50-kilometre corridor in Norfolk. The bodies showed injuries consistent with violent decompression or blast trauma, and a fall from a great height. Whether lightning discharge, severe downdraughts, or even tornado activity killed the birds is not known, but these birds would clearly have been better off avoiding the storms.
Simon Woolley
Winchester, Hampshire, UK
鈥� Seagulls can be struck by lightning in spectacular fashion as many people witnessed this summer at a clubbing weekend organised at Pontin鈥檚 holiday camp at Camber Sands in Sussex. Numerous people were sitting inside chalets watching the thunderstorm when a seagull sitting on a telegraph pole was struck by lightning. It disintegrated with a loud bang. However, it was, of course, earthed鈥�
Ali Brotchie, Cheltenham, Gloucestershire, UK
Bluto strikes back
My Italian recipe book says that I should cut cooked spinach with a stainless steel knife to avoid discoloration. Which would become discoloured if I don鈥檛 鈥� the knife or the spinach? And what would be the chemistry at work?
鈥� The reason you should you always use a stainless steel knife to cut your spinach is intriguing and is a major stumbling block to the fortification of food with iron. Remember that a lack of iron is the world鈥檚 most prevalent nutritional deficiency.
Both iron blade and spinach will become discoloured because of the reaction between the polyphenols in the spinach, and the iron blade. If you want to see a dramatic illustration of this, make yourself a cup of tea and add a few crystals of a soluble iron salt such a ferrous sulphate (don鈥檛 drink it).
The black discoloration you see is caused by the reaction between polyphenols, called tannins, in the tea, and the iron. The resulting black compound is highly insoluble. The implications for iron absorption in the body are huge because iron in this form is virtually unavailable for absorption. So whatever the source of strength in Popeye鈥檚 spinach, it is not the iron.
Polyphenols are found in many vegetables and, together with phytates, are the reason why many people who subsist on cereal and vegetable diets have iron deficiency. Fortifying such diets with iron salts creates two problems. Firstly the iron is not absorbed, and secondly the coloured iron-polyphenols make the food look unattractive.
Patrick MacPhail
Department of Medicine
University of the Witwatersrand
Johannesburg, South Africa
Tottering jumbos
My young daughter has been discussing pressure, stiletto heels and elephants. This set me thinking. If I plotted the weight of all known walking animals against the total area of their feet, would I find that they all exert more or less the same pressure on the ground?
(Continued)
One of the previous answers to this question, from Graeme Cumming of the University of Florida in Gainesville, which described a graph of the hoof area of African ungulates plotted against body mass, was unfortunately mangled in editing. With our apologies, here is what it should have said 鈥� Ed
鈥� The graph revealed a straight line with a slope close to 1. This means that wherever you start on the graph, a given increase in body mass will produce the same increase in hoof area. In other words, for African ungulates, the pressure (mass per area) they exert on the ground remains the same no matter what size they are.
This week鈥檚 questions
Gnab gib
If antimatter had prevailed over matter after the big bang, would anything be different in our universe?
Sam Hopkins
By email, no address supplied
Spectral images
When condensation forms on a clean bathroom mirror, you can draw pictures in it. When the condensation evaporates, the pictures disappear. But when it forms again, they reappear. Why?
Glyn Williams
Derby, UK