36 
HINTS TO TKAVELLEKS. 
moisture of the air, the fall of rain, the occurrence of thunderstorms, etc. 
It is also to be noticed in the flow of rivers in mountainous regions where 
the streams take their rise from glaciers or snow, the rapid melting of 
which by the heat of the sun causes the volume of water to increase 
greatly in the afternoon, while the cessation or reduction of the rate of 
melting at night diminishes the volume of the river in the morning and 
forenoon. 
Periodic changes of greater amount but similar in kind are produced 
by the alternation of the seasons, the difference between the mean values 
of the months in which the phenomena are at a maximum and minimum 
respectively being termed the annual range. With regard to temperature, 
very moderate changes occur in the tropical zones where the altitude of 
the sun is always great, and the length of day and night varies little 
with the season (for the most part less than 5 Fahrenheit degrees); but 
in the temperate and frigid zones there are strongly marked annual 
changes. As in the case of daily range, proximity to the sea is a con¬ 
trolling factor in the annual range of temperature. To take a very 
characteristic instance, the annual range between the mean temperature 
of July and January is about 23 Fahrenheit degrees in the Lofoten 
islands on the margin of the Atlantic, while it is 120 Fahrenheit degrees 
at Verkhoyansk in the same latitude, but in the centre of the Asiatic 
continent. 
The extreme months for air temperature are January and July in 
almost every part of the world, the maximum occurring north of the 
equatorial belt in July, and south of it in January. 
The annual changes in barometric pressure and wind are equally 
marked. The belt of low pressure which lies nearly under the vertical 
sun moves northward over the surface of the globe in the northern 
summer, coming to its most northerly position in July; returning south¬ 
ward after the sun, it reaches its most southerly position in January. 
This belt of low pressure is also a belt of calms, known by sailors as the 
Doldrums, and it is a belt of frequent rains, so that as it approaches and 
passes over a place there is a rainy season, followed by a dry season 
when it retires. Near the mean position of the belt of low pressure, 
where it passes over a place twice in the year, there are two rainy 
seasons. The low pressure belt is bordered to north and south by 
belts of high atmospheric pressure, from which the trade-winds blow 
