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6 months. From October to April the northeast trade wind blows down toward the equator with clear weather; shortly after entering the southern hemisphere, however, it turns to the left (see wind chart, Fig. 9) and changes into the northwest monsoon up to the limits of latitude already mentioned, bringing with it sultry, damp weather and torrents of rain. During the months of April to October the conditions are reversed. The southeast trade wind, after having crossed the equator, turns to the right (see wind chart, Fig. 10) and becomes the southwest monsoon, bringing with it sultry and wet weather. In both cases the wind that brings the rain comes from the equator. The northwest monsoon is also known as the winter monsoon and the southwest monsoon as the summer monsoon. The latter is of a comparatively greater velocity than the former.
The periods of change of direction of these winds, which occur about April and October, respectively, are called the breaking of the monsoon. These changes are always marked by variable winds that alternate between dead calms and furious hurricanes.
27. Wind and Pilot Charts.-On the wind chart, Fig. 9, is represented, by means of arrows, the general direction of prevailing winds during the period of January and February, while Fig. 10 represents the predominating wind directions for July and August. The black dots attached to some of the arrows indicate winds of hurricane force. These charts, it should be understood, are intended to serve merely as a general guide to the students. For full and accurate information on this subject he should consult the monthly Pilot Charts issued by the United States Hydrographic Office. These charts, besides serving as weather maps, contain a great amount of valuable information not elsewhere to be found.
LAND AND SEA BREEZES 28. These winds are invariably met with along coast lines in the tropics and in the temperate zones. The sea breeze begins in the morning hours and brings in pure cool air from
the sea. In the late afternoon, and after sunset, the land breeze springs up and blows gently out to sea until morning. In the tropics this process is repeated in regular order along the shore of such countries as are not directly affected by the trade winds. For instance, on the northeast coast of Brazil, which is constantly swept by the southeast trade winds, the phenomenon of land breezes is not experienced.
29. The cause of the diurnal winds may be briefly explained as follows: During the day the land is heated more rapidly than the sea and during the night it is more rapidly cooled. In the morning, therefore, the air over and near the land being heated by the sun will rise and colder air from the sea will flow in to supply its place; this produces the sea breeze during the day. During the night and shortly after sunset the land becomes colder than the sea and a flow of the cooled air will begin to move seawards to take the place of the warm air over the sea, which then ascends. This movement of air produces the land breeze. To vessels engaged in the coasting trade these winds are of particular importance.
VARIABLE WINDS 30. The Simoom and Sirocco.-Among the variable winds, or winds that blow without any marked regularity as to time and place, those prevailing on the deserts of Africa and Arabia are, perhaps, the most remarkable, on account of their extreme dryness and intense heat. In Arabia and on the shores of the Red Sea this wind is known as the simoom, signifying hot, poisonous, or dangerous, while in Upper Egypt it is called khamsin. In Sicily, South Italy, and adjoining districts it is called the sirocco and is considered poisonous by the inhabitants. However this may be, it certainly exercises an unhealthy influence on the regions through which it passes, and is especially dangerous to those who do not know how to protect themselves.
31. The Puna.- Another wind remarkable for its dryness is the puna, a mountain wind of Peru. This wind
is a continuation of the trade winds, which, after having crossed the lofty range of the Peruvian Andes, are cooled and parched to an extent that has perhaps no parallel in any other country of the world.
32. The Pampero.-A sister wind of the puna is the pampero, which blows from the Andes across the pampas of the South American Continent toward the Atlantic Coast. This is also a very dry wind, frequently darkening the sky with clouds of dust and sand and drying up the vegetation of the pampas to a considerable extent. It often carries dust and insects hundreds of miles out to sea. To vessels plying on the Rio de la Platà and adjoining rivers this wind is quite dangerous, on account of its fierceness and effect on the rise and fall of the water.
33. The Bora.-On the south coast of Europe north winds are notorious for their violence. Of these winds the most noted is the bora, which means “furious tempest.” The bora is greatly dreaded in the upper part of the Gulf of Venice, where annually a number of vessels are sacrificed, and entire districts of the shore are nearly rendered uninhabitable by the destructive effects of this wind on the vegetation. No sign or warning of any kind is given of the approach of the bora, which usually takes place a couple of hours after sunset. The only thing indicating its near presence is a big drop of the atmospheric pressure about a quarter of an hour before the storm comes. Its duration, however, is brief.
34. The Nortes. —Another variable wind that has attained a general reputation, usually owing to the dangers to shipping that its prevalence entails, is the nortes (northers) of the Mexican Gulf. This wind is indicated by unusually fine precedent weather, a light bank of clouds in the north, followed, perhaps, by a faint northerly breeze coming in puffs and barometer always rising. Nortes occur from September to June.
35. Revolving storms are known under various local names. In the West Indies and in southern parts of the United States they are known as hurricanes, while in the Indian Ocean and the China Sea they are termed typhoons. They may, however, be classed under the general term of cyclones, owing to the more or less circular direction of the winds that constitute them. Among the distinctive features by which revolving storms may be distinguished from an ordinary gale, the following are prominent.
CHARACTERISTIC FEATURES OF A CYCLONE 36. Motion.-Cyclones have a rotary motion around a center, or focus, and a progressive motion varying from 4 to
15 miles per hour; the latter motion, however, depends to
great extent on local circumstances. Thus, the path of a cyclone advancing toward a coast of high land is often changed considerably.
Direction of Rotary Motion.—The peculiarity of their rotary motion is that in each hemisphere the rotation invariably takes place in different directions. Thus, in the northern hemisphere
the rotation is contrary FIG. 11
to the motion of the hands of a watch, that is, from right to left; in the southern hemisphere the rotation is with the hands of a watch, that is from left to right, as shown in Fig. 11.