The Convolvulus arvensis (small bind-weed), Anagallis arvensis (scarlet pimpernel), and Calendula pluvialis shut up their flowers before approaching rain. (The pimpernel has been called the poor man’s weather-glass.)
Dr. Jenner’s well-known lines are very good, as they contain references to most of the natural prognostics. (See page [110], General Prognostics.)
It will be seen, however, later on, that the sayings about the rainbow, distant hills, and whirling dust, though good prognostics, do not belong to a depression.
As the depression centre approaches still nearer, rain sets in and continues till the barometer turns to rise. The passage of the trough is often associated with a squall or heavy shower, commonly known as “a clearing shower.” Immediately the air becomes cooler and loses the former muggy sensation, and soon small patches of blue sky appear.
When as much blue sky is seen as will make a Dutchman a jacket (or a sailor breeches) the weather may be expected to clear up.
A small cloudless place in the northeast horizon is regarded both by seamen and landsmen as a certain precursor of fine weather or a clearing up.
The steady rain breaks up into showers or cold squalls, which are followed by hard detached cumulus or strato-cumulus till the sky becomes blue again:
When after a shower, the cirro-strati open up at the zenith, leaving broken or ragged edges pointing upwards, and settle down gloomily and compactly on the horizon, wind will follow, and will last for some time.
This is a description of a clearing up before the windy portion of a depression has passed.
In the south of the depression near the outskirts, windy cirrus and “mare’s tails” are observed, which indicate wind rather than rain, as they are south of the rainy portion.