[16] Thunder clouds sometimes rise and spread against the wind (lower-current). It is probable that there is a meeting, if not a contest of air currents, electrically different, whenever lightning is seen. Their concurrence, when the new one advances from polar regions, does not depress the barometer, except in oscillations of the mercury, which are very remarkable at some such times.
[17] Aneroids, metallic barometers, and oil sympiesometers, seem to be much more affected than mercurial barometers by electrical changes.
[18] Southerly, in North latitude; the reverse in the Southern hemisphere.
[19] A "high dawn" is when the first indications of daylight are seen above a bank of clouds. A "low dawn" is when the day breaks on or near the horizon. The first streaks of light being very low.
[20] Indications of weather, afforded by colours, seem to deserve more critical study than has been often given to the subject. Why a rosy hue at sunset, or a grey neutral tint at that time, should presage the reverse or their indications at sunrise;—why bright yellow should foretell wind at either time, and pale yellow, wet;—why clouds seem soft, like water colour; or hard edged, like oil paint, or Indian ink on an oily plate;—and why such appearances are infallible signs—are yet to be shown satisfactorily to practical men.
[21] In the trade winds of the tropics there is usually a counter current of air, with light clouds,—which does not indicate any approaching change. In middle latitudes such upper currents are not so evident, except before a change of weather.
[22] Much refraction is a sign of Easterly wind. Remarkable clearness is a bad sign.
[23] The "young moon with the old moon in her arms" (Burns, Herschel, and others) is a sign of bad weather in the temperate zones or middle latitudes, because (probably) the air is then exceedingly clear and transparent.
[24] Even in ordinary changes of weather it is interesting, as well as useful, to mark the formation or disappearance of clouds, caused by colder and warmer currents of air mixing: or intermingling.
[25] Depending on pressure and temperature.