On a calm, windless evening in 1970, the zoologist David Pye was watching bats hunting over a lakeside in Suffolk when he noticed the bats suddenly swerved to avoid a bank of fog. Using a bat detector, he could listen to the ultrasonic signals of the bats, which the detector converted into much lower frequencies that humans hear as rapid clicks. “The bats were seen to fly and hunt right up to the wall of fog but turned back and were not seen to enter it,” Dr Pye wrote. “A few paces within the fog bank the ultrasound detector could not register the sounds of bats foraging . . . and no bats were seen or heard within the fog. The whole situation was very striking and intriguing.”
Bats beam out ultrasound signals into the air and then catch the echoes to guide them to insects and navigate their way through the air. Experiments with artificial fog made with dry ice — solid CO2 — and water showed that ultrasound could not penetrate the fog. And a recent study in the journal Weather used a vibrator over water to create a dense mist, and this showed that the mist absorbed the ultrasonic signals.
So fog or mist present a big danger for bats finding their ultrasonic signals would vanish and leave them flying “blind”, and even if prey flew into a patch of fog it would be too dangerous for the bat to follow. The tiny water droplets in mist or fog are just the right size to absorb the ultrasonic signal, but larger water drops in drizzle or rain are not a problem, and bats can be seen hunting even in the finest drizzle, although they do it rather half-heartedly.
Bats are also equipped with their own barometers. One type of bat living deep inside caves only comes out at night when plenty of insects are flying. The bats were found to sense when conditions were right for hunting when the atmospheric pressure dropped, which is when light, rising air would bring out the insects. The bats detected the air pressure from a sensor inside their ear.
