Mariners, it seems, have often maintained that sometimes, with the onset of a bout of heavy rain, the seas grow calm. Meteorologists could find no evidence of such a phenomenon and could not suggest any reason why it might be so until recently. But now satellite images show that the sailors were correct.
Some environmental spacecraft nowadays carry radar equipment which bounces microwaves from the surface of the Earth and reassembles the reflected beams into an image. Since the microwaves can penetrate cloud, pictures can be obtained in conditions when visual images would be quite impossible. A side-effect of this type of observation is that rough seas reflect the radar beams in a different way from calm ones. In this way, scientists have discovered that from time to time there are areas of relative calm in the ocean.
They are each five or 10 miles across and dotted over an area of ocean experiencing heavy showers at the time. When these calm "footprints" are compared with the images of thundery showers from Earth-based weather radar, it has been found that the two exactly coincide; there is an area of relative calm directly beneath each thundercloud.
The explanation is believed to lie in the flow of air within a shower cloud. Heavy showers have their origins in atmospheric updrafts, but the rising fountains of air have their counterparts in equally strong downdraughts, downward surges of air which hit the surface of the sea and burst out horizontally around the point of impact, rather like the stream of water from a tap when it strikes the bottom of the kitchen sink.
These vicious downdrafts are well known to aviators as microbursts, a term coined in the 1970s by a professor, Theodore Fujita of Chicago University, who discovered the phenomenon when he investigated a number of aircraft accidents which had happened in the vicinity of thunderstorms.
A possible interpretation of the anomalous calms revealed by the satellites is that the blast of descending air dampens the existing waves in an area immediately beneath the shower clouds, albeit causing even larger waves elsewhere in the vicinity. Since the core of the downdraft usually coincides with the zone of maximum heavy rainfall, it is easy to see why the heavy rain might seem, as the mariners had said, to have a calming effect upon the sea. As Byron puts it:
Of all the horrid, hideous notes of woe, sadder than owl songs or the midnight blast is that portentous phrase '`I told you so."