Parallel to the equator and extending between 2° and 3° on each side is a broad belt, where the north and south trades neutralize each other, producing what is called the “Region or Belt of Calms.” Though wind is absent, thunderstorms and heavy rains are of daily occurrence.
When Humboldt ascended Teneriffe the trade wind was blowing at its base in the usual direction, but on arriving at the summit he found a strong wind blowing in the opposite direction. Observation has shown that this upper current prevails north and south of the equator, and that, after passing the limit of the trade winds, it descends to form the south-west winds of the north temperate zone and the north-west winds of the south temperate zone; the westing being due to the same cause as the easting in the regular trades, viz., the rotation of the earth on its axis. These winds are called the Return Trades, but are not equal in constancy to the regular trade winds.
Periodical Winds.—Land and Sea Breezes occur on the coasts, chiefly in tropical countries, but sometimes in Great Britain during the summer months when the land during the day becomes very hot, causing an ascending column of air, which is replaced by a comparatively colder stream flowing inwards from the sea. At sunset the conditions are reversed, the earth becomes rapidly cooled by radiation, the sea continuing comparatively warm, the air over it ascends, and its place is supplied by a cold breeze, which “blows off the shore,” as illustrated by the diagrams and the following experiment—In the centre of a large tub of water float a water plate containing hot water, imagine the former to be the ocean and the latter the heated land, rarefying the air over it. Light a candle and blow it out and hold it while still smoking over the cold water, when the smoke will be seen to move towards the plate. The reverse of this takes place if the tub be filled with hot water and the water plate with cold. When this phenomenon takes place on a large scale, as in the case of the north trade winds being drawn from their course by the heated shores of Southern Asia, the gigantic sea breeze thus produced is called the south-west monsoon. This occurs from April to October, when the sun is north of the equator. When the sun is south of the equator—that is, from October to April—the analogue of the land breeze is produced, and is called the north-east monsoon.
Variable Winds.—The character of this class of winds is determined by the physical configuration of the country in which they occur. Some tracts are marked by luxuriant vegetation, others are bare. Here mountains lift their awful fronts and “midway leave the storm,” there an arid plain extends itself to the seashore, or inland, towards a chain of lakes. Within the tropics these purely local conditions are insufficient to overcome the force of the prevalent atmospheric currents: such, however, is not the case beyond the tropical zone. There the variable winds prevail, for which space permits only the mention of their names:—The Simoom (from the Arabic samma, hot), peculiar to the hot sandy deserts of Africa and Western Asia. The Sirocco blows over the two Sicilies as a hot wind from the south. It extends sometimes to the shores of the Black and Caspian seas, spreading death among animals and plants. The Solano prevails at certain seasons in the south of Spain: its direction is south-east. The Harmattan is another wind of the same class, peculiar to Senegambia and Guinea. The Puna Winds blow for four months over a barren tableland called the Puna, in Peru. They are a portion of the south-east trade winds, which, having crossed the Pampas, are thereby deprived of moisture, and become the most parching wind in the world. The East Winds, peculiar to the spring in Britain, blowing as they do through Russia, over Europe, are a portion of the great polar current, distinctive of that season of the year. They are dry and parching, every one being familiar with the unpleasant bodily sensations attendant on this much-abused and yet most beneficent wind.
The Etesian Winds are drawn from the north across the Mediterranean by the great heat of the African desert. The Mistral is a strong north-west wind peculiar to the south-east of France. The Pampero is a north-west wind, blowing in summer from the Pampas of Buenos Ayres.
As long ago as the year 1600 Lord Bacon remarked that the preponderating tendency of the wind was decidedly to veer with the sun’s motion, thus passing from N. through N.E., E., S.E., to south, thence through S.W., W.N.W., to N.; also, that it often makes a complete circuit in that direction, or more than one in succession (occupying sometimes many days in so doing), but that it rarely backs, and very rarely or never makes a complete circuit in the contrary direction. The merit of having first demonstrated that this tendency is a direct consequence of the earth’s rotation is due to Professor Dove, of Berlin, who has also shown that the three systems of atmospheric currents just treated of, viz., the constant, periodical, and variable winds, are all amenable to the same influence.
As to the mode of observing the wind, Admiral Fitzroy recommends that a true east and west line should be marked about the time of the equinox, and the north, south, and other points of the compass being added, to take the bearings of the wind in relation to a dial so prepared, the indications of the lower stratum of clouds in conjunction with vanes and smoke being preferred to any other.
The direction of the wind should always be given according to true, and not to compass bearings. Two points to the westward nearly represents the amount of “Variation of the Compass” for the British Isles, which yields the following table for the conversion of directions observed by the compass in Great Britain and Ireland to approximate true bearings.
| Compass bearings. | N | NNE | NE | ENE | E | ESE | SE | SSE |
| True bearings. | NNW | N | NNE | NE | ENE | E | ESE | SE |
| Compass bearings. | S | SSW | SW | WSW | W | WNW | NW | NNW |
| True bearings. | SSE | S | SSW | SW | WSW | W | WNW | NW |
“One may call a very simple diagram, a circle divided by a diameter from north-east to south-west, the thermometer compass. While the wind is shifting from south-west, by west, north-west, and north to north-east, the thermometer is falling, but while shifting from north-east, by east, south-east and south, towards south and south-west, the thermometer is rising. Now the barometric column does just the reverse. From north-east the barometer falls as the wind shifts through the east to south-east, south, and south-west, and from the south-west, as the wind shifts round northward to north-east, the barometer rises—it rises to west, north-west, north, and north-east.