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TROPICAL CLIMATOLOGY 



time that this wind came from the Sahara, but all evidence is against this, 

 and the present idea is that it is a local wind which is produced by a high 

 south-east and a low north-west pressure, and attains its high temperature 

 as well as its dryness in its descent from the summits of the Alps on their 

 northern side, and less typical on the southern aspect when the pressure is 

 high in the north-west and is low in the south-east. 



Foehn-like winds occur in Trebizond, and on Lake Urmia, in Persia, where 

 it is called samum, and blows down the eastern side of the New Zealand Alps 

 on to the Canterbury Plains. 



It also occurs at Resht, on the southern shores of the Caspian Sea; also in 

 South Georgia, as well as in the Andes and at Kanazawa in Japan. 



We have, above, given the usual explanation of the sirocco, but it must 

 be noted that it is probably a ' foehn ' wind, for Bridone, many years ago, 

 noted that in Sicily it could not possibly come from the Sahara, because it 

 would have been most violent on the south coast, whereas it is actually at its 

 greatest violence on the north coast, especiall}?- at Palermo. 



The ' sirocco di Levante,' between Pylos and Kyporissia, is also a ' foehn ' 

 wind, as is the sirocco of Algiers, and probably the solano of Spain and the 

 vent d'Espagne on the northern aspect of the Pyrenees. 



Cyclonic Storms. — Besides ordinary winds, cyclonic disturbances called 

 storms occur. Cyclonic storms receive different names in various parts of 

 the world, being called cyclones in India, Ceylon, and Mauritius, hurricanes 

 in the West Indies, typhoons in the China Seas. Mild cyclonic storms met 

 with on the West Coast of Africa and elsewhere are called tornadoes. 



Electrical Conditions. 



Silent electrical discharges are frequent in the tropics, and the 

 acidity which they produce is supposed to be the cause of devitrifica- 

 tion, which is so trying to persons working with all except the best 

 microscopical and other lenses. 



Thunderstorms of astonishing violence are frequent in many 

 parts of the tropics, and generally cool the atmosphere for the time. 



It is a popular belief that the electrical condition of the atmosphere 

 has some influence upon life, but if this is so then it is an untrodden 

 field, as far as we know, for we have been unable to find any definite 

 scientific observations which can confirm or refute the popular 

 belief. Hahn considers that there is no indication that atmospheric 

 electricity plays a notable part in climatology. 



Sun's Rays. 



Sunlight contains heat rays (red), light rays (yellow), and chemical 

 rays (blue, violet, and ultra-violet); that is to say, rays extending 

 from the infra-red to as far as 291 ^[jl in the ultra-violet (Gibbs), 

 but is much influenced by latitude, longitude, altitude, and the daily 

 and hourly varying local meteorological conditions. 



These rays reach the earth, according to Langley, in the following 

 percentages: — 



Ultra-violet 

 Violet 

 Blue 



Greenish-blu e 

 Yellow 

 Red 



Infra-red . . 



42 

 48 

 54 



70 



