1919.] Pemberton. — Weather Forecasting in New Zealand. 
165 
WEATHER FORECASTING IN NEW ZEALAND. 
(Continued from page 94.) 
By B. Y. Pemberton, F.B.Met.Soc., Assistant Meteorologist, Meteoro¬ 
logical Office, Wellington, New Zealand. 
Typical Westerly Area of Low Pressure. 
Fig. 4 depicts a westerly area of low pressure, and is, no doubt, the 
northern extension of an Antarctic cyclone wedged between the rear and 
front of two adjacent anticyclones. 
The rate of movement from west to east varies considerably, and 
averages about 400 miles per day. The dimensions and forms of these dis¬ 
turbances also vary, from the commonest type with a decided /\ shape to 
those having practically straight isobars extending over thousands of miles, 
and which consequently take many days to traverse the Dominion. The 
one now referred to may be taken as an intermediate between the 
above two. Its influences were effective over the Dominion between the 
12th and 19th October, 1911. 
The trough is that portion of the “ low ” along an imaginary line passing 
through the apices of the /\, and is therefore the dividing-line between the 
falling and rising barometer. In this case the trough passed over New 
Zealand on the 18th, and the winds, which had blown with gale force from 
the north and west previous to this date, backed to southerlies, still with 
gale force, but gradually decreasing with the advent of an anticyclone. 
A characteristic effect of this type of disturbance is seen in the occur¬ 
rence of hot north-west winds in the east-coast districts in front of the 
trough. The cause of the high temperature of these winds is as follows : 
As the moisture-laden westerly winds strike the slopes of the Southern Alps 
and are forced to rise, the cooling which results from the consequent expan¬ 
sion of the air condenses the vapour, and heavy rain falls on the westward 
slopes. Latent heat is thus set free. The air descending on the lee slopes 
is dynamically heated by compression, and, with the addition of the latent 
heat previously attained, it reaches the Canterbury Plains as an exceed¬ 
ingly hot and dry wind. The name given to such a wind in Switzerland is 
the “ Fohn.” Coming in the winter-time it is a welcome change from the 
keen temperatures then generally ruling, but in the summer it has a very 
enervating effect. 
The change of wind from north-west to southerly is sometimes very 
sudden on the east coast, according to the sharpness of the /\, and this 
sudden change has earned the cognomen “ southerly burster.” A rapid fall 
of temperature takes place, and heavy rain falls, sometimes accompanying 
an electrical disturbance. When the /\ is less pronounced the change of 
wind is not so rapid, the direction remaining westerly while the trough is 
passing over the country and working gradually round to south-west. 
Complex Westerly Areas of Low Pressure. 
It sometimes happens that these westerly areas of low pressure have 
two, or even three, distinct depressions or /\s, and the chart for the 5th 
November, 1915 (fig. 5), is an example of such. The first depression passed 
