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DISTRIBUTION OF PRESSURE OVER THE GLOBE.

Over the above area the distribution of atmospheric pressure presents certain constant features, namely

1. An equatorial belt of nearly uniform low pressure. 2. A tropical belt of high pressure rising at intervals into great irregular elevations or anticyclones.

3. A temperate and arctic region of generally low pressure, but in which occasionally areas of high pressure appear for a considerable period.

WEATHER IN THE DOLDRUMS.

The equatorial belt constantly covers the Sahara and the Amazon valley, and always narrows over the Atlantic at about 30° west longitude, where it often does not reach higher than 10° north latitude. The shape and depth of this area are tolerably constant.

This is the "doldrums" of the Atlantic navigators. Our charts only show the north side of this area; the south side is formed by an anticyclone, which always lies over the South Atlantic. The doldrums, therefore, form a sort of long hollow, or col, between two anticyclones, and though on the one side the north-east trade blows moderately, and the south-east trade on the other, still in the centre there must be calm. This is well shown in Fig. 68, where we see the symbol of calm between the two trades. These sultry doldrums are much dreaded by sailors, for in them "a ship may lie for weeks on the hot smooth water, under a cloudless sky, with pitch oozing from her decks; a region of un

bearable calm, broken occasionally by violent squalls, torrential rain, and fearful lightning and thunder." The general appearance of the sky is a steamy haze, sometimes growing into a uniform gloom, with or without heavy rain; at other times gathering into small illdefined patches of soft cumulus, or the forms of cloud given in Fig. 13, a, b, and c. After dark there is always a great development of sheet lightning till about two in the morning. As the position of this area only varies very slowly in its annual course a little more north or a little more south, there is nothing to change the weather, which therefore remains of a uniform character.

WEATHER IN THE TRADE-WINDS.

The tropical belt comprises a region of high pressure, rising at variable intervals into great anticyclones. These anticyclones are usually the longest in an east and west direction, and often rise into two or more heads. Their position is generally variable, with the exception of one, which is always found over the central Atlantic. This anticyclone forms a very important factor of the weather both of Western Europe and of the United States, and will be constantly referred to as "the Atlantic anticyclone." The extension south and west of this anticyclone is tolerably constant, while north and east it is variable, sometimes rising as far as 60° north, and stretching over Great Britain and Continental Europe.

The wind blows round this as in all anticyclones. The north-east and east winds on the southern side of the Atlantic anticyclone constitute the celebrated "trade

winds." An inspection of Figs. 68-71 will both show their true nature and correct some popular fallacies as to their position and constancy. It is obvious, from the nature of anticyclone winds, that north or north-east winds must stretch far north on the easterly edge, which accounts for the north-east trade being often picked up off the coast of Portugal. But on the westerly edge of the anticyclone the wind must be more south-east or south, and in practice is lighter and more variable. The centre must be, and is, calm, so that the wind-maps which appear in physical atlases with the north-east trade described as a belt of wind parallel to the equator are most delusive. The degree of constancy in the direction and force of the trades is best gathered from an inspection of the charts. We then see at once that the position of the edges of the anticyclone is perpetually changing, and that the gradients are very variable; so that, as a matter of course, both the direction and strength of the trades vary very considerably.

The weather in the trades is usually fine, and the sky more or less covered with a peculiar small detached cumulus, often called "trade cumulus." This is a small isolated cloud bending backwards from the flat base, as in Fig. 11, a, in our chapter on Clouds, which often degenerates into the small lens-shaped mass as in Fig. 13, e. Sometimes a thin, hard, broken strato-cumulus covers the sky with such regularity that, when seen in perspective near the horizon, we look at a series of bars, like the leaves of a Venetian blind; but if the gradients are all steep, squalls and showers from cumulus cloud are of frequent occurrence in the trade-wind regions.

Cyclones are rarely, if ever, formed to the south of this Atlantic anticyclone; sometimes, however, they have their origin on its south-west side, when they work round the anticyclone, first towards the north-west, and then towards north-east. These are the West India hurricanes.

The north side of the anticyclone is the birthplace of innumerable cyclones of every size and intensity, which invariably move towards some point of east. These are the cyclone-storms which affect Europe.

Cyclones are also occasionally formed on the southeast side near Madeira; these either work very slowly round the high pressure to the south-west, or else leave the anticyclone and go east over the Straits of Gibraltar. In winter-time, another anticyclone generally lies over Mexico, and the col between this and the Atlantic anticyclone forms the most prominent feature in the meteorology of the United States sea-board.

WEATHER IN TEMPERATE ZONE.

The temperate and arctic region extends from the tropical high pressure to the pole. Though ordinarily low, the pressure is perpetually fluctuating by reason of the incessant passage of cyclones; yet occasionally persistent areas of high pressure appear in certain portions of it.

As a necessary consequence of this, the weather in this zone must be changeable, with variable winds.

From this brief survey, we see at once the broad features of the climates of the world-the persistent

equatorial calms and rains, the regular trades of the tropics, and the variable wind and weather of the tempe

rate zone.

We will now proceed to examine the weather of the temperate zone in some detail.

TYPES OF PRESSURE IN TEMPERATE ZONE.

In spite of the great variability of the temperate zone, there are-with reference to Western Europe-at least four constant types of weather which coincide with four distinct types of pressure-distribution.

1. The southerly, in which an anticyclone lies to the east or south-east of Great Britain, while cyclones coming in from the Atlantic either beat up against it or pass towards north-east.

2. The westerly, in which the tropical belt of anticyclones is found to the south of Great Britain, and the cyclones which are formed in the central Atlantic pass towards east or north-east.

3. The northerly, in which the Atlantic anticyclone stretches far to the west and north-west of Great Britain,

roughly covering the Atlantic Ocean. In this case, cyclones spring up on the north or east side, and either work round the anticyclone to the south-east, or leave it and travel rapidly towards the east.

4. The easterly, in which an apparently non-tropical anticyclone (or one disconnected with the tropical highpressure belt) appears in the north-east of Europe, rarely extending beyond the coast-line, while the Atlantic anticyclone is occasionally totally absent from the Bay of

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