So bats have barometers built in ? That, in any event, is what it said just yesterday in Weather Eye. But they also, as we know, are equipped with an acoustic kind of radar, which allows them, despite their dismal eyesight, to flit in hundreds around their belfries without close encounters of an undesired kind. As the American poet Richard Purdy Wilbur once described a bat on its nocturnal wanderings:
He has no need to falter or explore;
Darkly it knows what obstacles are there,
And so may weave and flitter, dip and soar
In perfect courses through the blackest air.
The bat accomplishes this feat by transmitting ultrasonic squeaks inaudible to human ears. The animal then listens for reflected sound, and by noting the direction from which the reflections are the loudest, and the time-lag detected between each squeak and echo, it can judge the location of obstacles to be avoided.
Meteorologists have a very similar technique for tracking rainfall. Weather radar uses microwaves to detect the presence of distant water droplets. The antenna of the equipment is a rotating parabolic "dish", rather similar to, but larger than, the antenna used for receiving satellite TV.
At short, regular intervals it transmits pulses of electromagnetic radiation, and after each pulse the equipment "listens" for the small fraction of the energy that may be scattered back in its direction from distant drops of rain. The strength of the returning signal is a measure of the intensity of the reflecting rainfall, and the time taken by the pulses to reach their target and return is used to calculate how far away each area of rain may be.
This process of transmitting and receiving is repeated many hundreds of times every second, as the antenna sweeps round, time after time, in a full circle. The end-product, assembled with the aid of a computer, is a colour-coded map of the region, the various colours on which signify rainfall of differing intensities.
The rain itself, of course, is colourless; the tints supplied by the computer according to a prearranged choice, usually the lightest rain as a pale shade of blue, with green and yellow for successively heavier falls, and areas of red for very heavy rain of such an intensity that thunder and lightning might well occur as well.
A sequence of radar pictures taken at, say, quarter-hourly intervals can then be animated to show the movement of the rain areas, or to monitor the development and dissipation of clusters of showers as they pass the country.