1919.] Pemberton.—Weather Forecasting in New Zealand. 89 
the strength of the wind depends upon the amount of the baric gradient : 
the steeper the gradient the greater will be the velocity of the wind. 
The air being extremely mobile, it follows that when there is an 
inequality of level between different places the tendency is for a flow 
to set in from the higher level to the lower, in an endeavour to restore 
equilibrium. Consequently the greater the difference in level, as shown by 
the closeness of the isobars, the more rapid will be the flow. 
Tables have sometimes been set out for different places showing the 
gradient and the corresponding mean wind-velocities, but these can be 
taken as only approximate. There are many factors, such as local con¬ 
figuration and conditions, which also govern the wind-velocity, and there¬ 
fore the highest winds are not always found where the gradient is steepest. 
For instance, when the gradients are steeper either over the North Island 
or the South than they are about Cook Strait the strongest winds are 
invariably experienced in the Strait, and this is naturally accounted for by 
the indraft through the Strait. As the widest portion of the Strait is to the 
north-west, with a westerly wind the volume of air becomes more confined 
when it reaches the narrower limits about Wellington, and consequently 
the horizontal pressure exerted must be greater and the velocity increased. 
Thus it is found that a steep gradient over the South Island causes a 
stronger wind in Wellington than would a similar gradient over the North 
Island. 
It will therefore be seen that the actual reading of the barometer at a 
single station can give one but little indication as to the direction and force 
of the wind, or the kind of weather to expect For the purpose of antici¬ 
pating the character of the weather the larger one’s field of vision the greater 
is the probability of the forecast being verified. On this account it is 
evident that large tracts of country, such as continents, should present 
less difficulty in forecasting weather than does, for instance, New Zealand, 
whose greatest width from the west to the east coast is only 280 miles in 
the North Island, and 180 miles in the South. 
Cyclones and Anticyclones. 
When a set of weather maps is examined an endless variety of isobaric 
shapes may be discovered. In fact, it would be almost impossible to find 
two maps presenting identical characteristics in this respect. There are, 
however, really only seven fundamental types of pressure represented by 
these isobars, and of these the two principal ones are the cyclone and the 
anticyclone. 
Both these systems are approximately circular or oval. In the cyclone 
the central or inner isobar represents the lowest reading of the barometer. 
Around the centre the wind rotates in a similar direction to the hands of 
a clock, so that, should the cyclone be moving in an easterly direction, in 
front of the centre the winds would be northerly, and in the rear southerly ; 
directly north of the centre the winds would be westerly ; while to the 
south, easterly winds would prevail. These directions hold good every¬ 
where in the Southern Hemisphere, while north of the Equator exactly 
opposite directions are experienced, a law which is governed by the effect 
of the rotation of the earth on all moving bodies on its surface. 
Usually the isobars in a cyclone are very numerous, and this accounts 
for the strong winds experienced. Besides the cyclone there are other 
types of pressure which cause high winds and heavy rains, and it is a 
mistake to refer to every storm of exceptional severity as a cyclonic storm, 
without having the knowledge that the conditions were really due to a 
cyclone. 
