ANTICYCLONE PROGNOSTICS.
Having spoken of cyclones, we shall now direct our attention to anticyclones. In the daily weather charts we sometimes see but two or three isobars, and these are a considerable distance apart, and extend over a large area. The pressure is highest in the centre, and gradually diminishes outwards. The air is calm and cold in the central area, but on the outskirts the wind blows in the direction of the hands of a watch. These are the special features of an anticyclone. The weather in an anticyclone is almost the opposite of that in a depression; that in the latter being wet and unsettled, while that in the former is usually settled and fine, with more or less haze in the air. Another great difference is that while depressions are generally rapid in their movements, anticyclones are nearly stationary; and it is for this reason that they are associated with “settled” fine weather. In the area of high pressure the characteristic features are largely modified by what is termed “radiation” weather, as determined by diurnal and seasonal variations; and as the pressure is nearly stationary, these diurnal and seasonal variations are the chief features of anticyclonic weather.
We shall now give the prognostics due to the variations in some detail, Fig. 5. The sky being generally clear and the air calm, the temperature is high in the day and low at night. In summer brilliant sun shine prevails during the day, and at night there is heavy dew, and in low-lying places mist.
Heavy dews in hot weather indicate a continuance of fair weather, and no dew after a hot day foretells rain.
If mist rises in low ground and soon vanish, expect fair weather.
Thin, white, fleecy, broken mists, slowly ascending the sides of a mountain whose top is uncovered, predicts a fair day.
When the mist creeps up the hill,
Fisher out and try your skill.
When in the morning the dew is heavy and remains long on the grass, when the fog in the valleys is slowly dissipated and lingers on the hill-side, when the clouds seem to be taking a higher place, and when a few loose cirro-strati float gently along, serene weather may confidently be expected for the greater part of that day.
Fig. 5.—Anticyclone Prognostics.
These all refer to night radiation, mist being dispersed by the sun’s rays. Fine, light, genial weather raises the spirits and exerts an enlivening influence not only on human beings but also on animals, birds, insects, &c. Hence the saying:
If larks fly high and sing long, expect fine weather.
When sea birds fly out early and far to seaward, moderate winds and fair weather may be expected.
If rooks go far abroad it will be fine.
Cranes soaring aloft and quietly in the air foreshow fair weather.
If kites fly high, fine weather is at hand.
Wild geese, wild geese, ganging out to sea,
Good weather it will be.
When owls whoop much at night, expect fair weather.
Bats, or flying mice, coming out of their holes quickly after sunset and sporting in the open air premonstrates fair and calm weather.
Chickweed expands its leaves boldly and fully when fine weather is to follow.
In winter frost is generally prevalent in the central area of an anticyclone, accompanied frequently by fog, which is most dense in the neighborhood of large towns. This is all due to the radiation of calm weather.
White mist in winter indicates frost.
In the evenings of autumn and spring vapor arising from a river is regarded as a sure indication of coming frost.
This comes from the air being colder than the water.
When fires burn faster than usual, and with a blue flame, frosty weather may be expected.
This is caused by reduced temperature of the outer air making a better draught in the chimney.
In winter when the sound of the breakers on shore is unusually distinct, frost is indicated.
These last two prognostics are explained by the fact that the atmosphere is very dense and still in an anticyclone.
In those places where fog does not form the sky will often be clear at night, whence—
Clear moon,
Frost soon.
In winter when the moon’s horns are sharp and well defined, frost is expected.
The wind in an anticyclone system blows in the direction of motion of the hands of a watch, but slightly outwards, and as the anticyclone is nearly always stationary, the wind blows from the same quarter for several days together.
When the wind turns from northeast to east and continues two days without rain, and does not turn south the third day, nor rain the third day, it is likely to continue northeast for eight or nine days, all fair, and then come to the south again.
If the wind is northeast three days without rain,
Eight days will pass before south wind again.
The wind is usually very light in force.
It is said to be a sign of continued good weather when the wind so changes during the day as to follow the sun.
If wind follows sun’s course, expect fair weather.
This “veering with the sun,” as it is called, is the ordinary diurnal variation of the wind, which in this country is very obvious with the shallow gradients of an anticyclone. At sea-side places in summer very often “the wind is in by day and out by night,” which is the equivalent of the land and sea breezes of the tropics. Like two preceding prognostics, it is only in anticyclones that local currents of air, probably due to unequal heating of sea and land, can override the general circulation of the atmosphere in this country.
Sometimes in winter, on the southern side of the anticyclone, bitter east winds, with a black-looking sky, will prevail for several days together, when it may be truly said:
When the wind is in the east
’Tis neither good for man nor beast.
Sometimes also the sky in this region will be covered with a uniform stratus cloud, which is not of any great thickness, and when breaks occur, the sun is seen to be shining brightly above.
On the northeast side of the anticyclone in summer, light, cumulus clouds frequently form in the morning, gradually increase till after the maximum temperature has passed, and then decrease and disappear towards evening.
If woollen fleeces spread the heavenly way,
Be sure no rain disturbs the summer day.
When the cumulus clouds are smaller at sunset than they were at noon, expect fair weather.
Clouds small and round like a dapple gray with a north wind, fair weather for two or three days.
The cirrus cloud is usually seen on the outskirts of an anticyclone, if in the front it gradually disappears, but if in the rear it is a sign that there will be a change in the weather, hence:
If cirrus clouds dissolve and appear to vanish, it is an indication of fine weather.
If cirrus clouds form in fine weather with a falling barometer, it is almost sure to rain.
When, after a clear frost, long streaks of cirrus are seen with their ends bending towards each other as they recede from the zenith, and when they point to the northeast, a thaw and a southwest wind may be expected.
Both these latter prognostics refer to a depression coming in and “breaking up the weather” and the anticyclone.