August, 1913. 



KNOWLEDGE. 



309 



METEOROLOGY. 



By William Marriott, F.R.Met.Soc. 



"THE DOCTOR."— At the June meeting of the Royal 

 Meteorological Society a paper by Mr. H. W. Braby was read 

 on the Harmattan Wind of the Guinea Coast, which was 

 based upon the results of five years' observations made at 

 Zungeru, in Northern Nigeria. The harmattan blows during 

 the winter months along the coast of Upper Guinea, from 

 French Guinea to the Cameroons. It is exceedingly dry and 

 brings with it fine sand which enters the crevices of doors and 

 windows, covering everything with a film of dust. The sun 

 is partially obscured, and distant objects become invisible. 

 It blows intermittently from November to March, and is, 

 generally speaking, health-giving, although its extreme dryness 

 is trying to new residents. It is locally known as " the 

 doctor." 



The harmattan almost invariably blows from the north- 

 east, a circumstance which justifies the presence of dust 

 particles brought from the Sahara. In some respects the 

 wind partakes of the nature of a Fohn. It is exceedingly 

 dry and blows from elevated to lower regions. Mr. Braby 

 points out that its direction necessitates there being an area 

 of high barometric pressure to the north and of low pressure 

 to the south. The area around the northern tropic, however, 

 seems scarcely far enough from the equatorial regions to be 

 the seat of a well-defined winter anticyclonic system, such as 

 prevails in the interior of Asia at that season. The equatorial 

 low-pressure belt is, however, now at its southern limit, and 

 this combined with the somewhat lower winter pressure to the 

 north appears to be sufficient to establish a north-easterly 

 wind along the Guinea Coast. 



HURRICANES OF THE WEST INDIES.— The opening 

 of the Panama Canal in the near future will no doubt bring 

 about a change in the long-established sailing routes and turn 

 much of the tide of commerce of the Atlantic and the Pacific 

 towards the Isthmus of Panama. The convergence of new 

 routes to the Caribbean Sea and the Gulf of Mexico will 

 necessitate the crossing of a wide area swept at intervals 

 during several months of the year by the severest type of 

 storm known to the mariner, viz., the West India hurricane. 

 With the view of giving information on this subject Professor 

 O. L. Fassig has tracked all the hurricanes which have 

 occurred in the West Indies during the thirty-five years 1876- 

 1910, and the results have been printed in a bulletin published 

 by the U.S. Weather Bureau. 



The hurricane belt may be defined as the area embracing 

 the Caribbean Sea, the Gulf of Mexico, and the West India 

 Islands. Within this area the points of origin are distributed 

 with a fair degree of uniformity, although belts of varying 

 frequency are clearly discernible. There is a well-marked 

 main path of greatest frequency through the northern half of the 

 Caribbean Sea, extending almost due east to west between 

 the Windward Islands and Jamaica; taking a north-west 

 crossing through the Yukatan Channel and across the western 

 end of Cuba, the path recurves in the eastern portion of the 

 Gulf of Mexico and across the Florida Peninsula into the North 

 Atlantic, with a north to north-east trend. There is a 

 secondary path, not so well defined, extending from the 

 northern group of the Windward Islands in a west-north-west 

 direction across the Bahama Islands and recurving east of 

 Florida in the North Atlantic Ocean. Between these two 

 paths lie the Greater Antilles — Cuba, Jamaica, Haiti, and 

 Porto Rico. Of these islands Porto Rico and Haiti are 

 comparatively free from the devastating winds near the 

 hurricane centres ; the western half of Cuba is crossed in 

 the recurve of a large percentage of the storms of the 

 Caribbean Sea, or those of the main branch referred to 

 above. These two paths coincide very closely with the two 

 branches of the great equatorial current of the North Atlantic 

 Ocean, the main stream of which passes through the 

 Caribbean Sea and the Yukatan Channel into the Gulf of 

 Mexico and out into the Atlantic again through the narrow 



channel between Havana and Key West. Here it meets the 

 northern branch of the equatorial current, which is more in 

 the nature of a wide surface drift of equatorial waters passing 

 through the Bahama group of islands, forming, later in its 

 course, the eastern portion of the Gulf Stream. 



The normal track of hurricanes for the entire season 

 resembles a parabola in form. The first branch extends in a 

 direction west by north, between the parallels of 18° and 

 20° N. lat., to the centre of the hurricane area (23° N. and 

 70° W.), then north-westward and north; recurving over 

 Central Florida, the trend is north-eastward over the Atlantic 

 along the second branch of the parabola. 



The geographical centre of origin for the entire season is 

 just off the north-west coast of Haiti. The average point of 

 recurve is in the centre of the Florida Peninsula. The 

 advance of the season is marked by a slight increase in the 

 latitude of the point of recurve. The movement of hurricane 

 paths from south to north and return southward coincides 

 very closely with the movement of the trades and the 

 equatorial belt of calms. 



Conditions favourable for the formation of hurricanes in 

 the West Indies begin in the month of June, but do not 

 become well developed until the month of August. From 

 August to the close of October is the real period for these 

 storms. The mean daily movement of hurricanes for the 

 entire season is about three hundred miles, or about 12-5 miles 

 per hour. After passing the recurve there is a considerable 

 increase in the velocity, especially in the latter part of the 

 season. The average period of duration of hurricanes while 

 within the zone below the latitude of 30° N. is about 

 six days ; about three days are spent in moving westward 

 along the storm's path and two days in the recurve. After 

 the recurve the storm enters into the region of temperate 

 zone cyclones, and may continue its existence for many days, 

 sometimes crossing the entire expanse of the North Atlantic 

 and even into Europe. 



HURRICANE, TYPHOON, AND CYCLONE.— These 

 terms are applied to the same type of storm, but, as Mr. O. L. 

 Fassig has pointed out in the paper quoted above, they are the 

 same in essential character. To the meteorologist they are 

 all " cyclones " or storms in which the surface winds blow 

 toward a central area of low barometric pressure at angles 

 varying between 0° and 90°. This broad definition includes 

 not only the intense storms of the Indian Ocean and the Bay 

 of Bengal, originally called " cyclones," the hurricanes of the 

 West Indies, and the typhoons of the Pacific, but also the 

 temperate-region storms usually referred to as " barometric 

 depressions," " storm areas," or simply " lows," and the 

 tornadoes of central valleys of the U.S.A., and waterspouts over 

 the seas in all parts of the world. In all storms of this class the 

 surface winds blow more or less spirally inward toward an 

 area of minimum atmospheric pressure, then upward and out- 

 ward at elevations varying with the extent and intensity of the 

 storm. The term " hurricane " is restricted to cyclones which 

 have their origin and field of action within well-defined limits, 

 embracing the West Indies and neighbouring waters of the 

 North Atlantic. The storms occurring in tropical regions of 

 the Western Pacific are called "typhoons." In the Indian 

 Ocean they retain the name originally given them by the 

 early English mariners, namely, cyclones. It is only in 

 comparatively recent years that the term " cyclone " was given 

 the broader, and at the same time more technical, definition to 

 include all so-called " revolving " storms. The temperate- 

 region cyclone covers a greater area than the tropical 

 variety, the diameter of a well-developed storm of the middle 

 latitudes being over one thousand miles, and occasionally 

 covering more than half the area of the United States ; the 

 cyclone of the tropics is generally not over three hundred to 

 four hundred miles in cross-section, but probably penetrates 

 to a greater height into the atmosphere than the extra-tropical 

 cyclone. Tropical storms are accompanied by a greater fall 

 in the barometer, resulting in more destructive winds and 

 heavier rainfall than in the temperate-region cyclone, where 

 the barometer falls with a more uniform gradient from the 



