There is a species of frog, Rana wittei or mountain frog, common in the elevated marshes of the Ethiopian highlands and on Mount Kenya's northern slopes, which can act, allegedly, as a thermometer. This frog, it seems, will croak almost continuously as long as the mercury remains above 0C, but it falls suddenly silent when the temperature drops below the freezing point.
More commonly, however, in so far as the common frog resembles any instrument of meteorology, it acts like a barometer. Certain behaviour on its part, it is said, can be associated with the approach, or otherwise, of rain.
The idea is not totally implausible. Frogs, as we know, need abundant moisture to survive. An individual can absorb as much as half its own body weight of water in half an hour, and the whole of its weight in a few hours; it surrenders this water equally rapidly when placed in a warm, dry atmosphere. In their natural state, frogs retreat in dry weather to places where a residual dampness can be found, and often huddle together to keep each other moist. It is possible that with such a sensitivity to moisture, they may be able to detect minute changes in atmospheric humidity unnoticeable to human beings.
There are various ways in which their message comes across. Virgil, for example, has it as a sure sign of rain when veterem in limo ranae cecinere querelam - "the frogs have sung their old song in the mud". Likewise, in English folklore rain is on the way "when loudly croak the tenants of the lake". Alternatively, the animals may become darker in colour when rain is due: it was a sign of rain in olden times if . . .
The frog has changed his yellow vest
And in a russet coat is dressed.
For those who wished to use the frog's prognostic ability in a more scientific way, the country-folk of Switzerland and southern Germany designed a very simple device for harnessing the prognostic capability of frogs. They would place one of these unfortunate creatures in a jar half full of water, which also contained a branch of a tree that the frog could use as a kind of ladder to climb out of the water when it had the inclination. A frog thus equipped, anticipating bad conditions stays in the water and croaks; but as soon as it senses any improvement it edges its way slowly up its ladder, and if a period of fine, bright, sunny weather is on the way, our amphibian friend is invariably found on the topmost branches of its personal twig.