In forming an opinion of the characters of any given climate the temperature of the air is alone of any great importance, the data afforded by the sun thermometer and that used for determining radiation being of comparatively little interest, so that the following data are the most important:--
(1) The mean temperature of each month. This can only be given accurately by self-registering instruments, but in the absence of these is usually taken as
max. t. + min. t. + t. at 9 hours + t. at 21 hours -------------------------------------------------- 4
(2) The mean monthly daily range of temperature, or what is practically as valuable, the mean maxima and minima.
(3) The “relative humidity” of the air, or the proportion of moisture actually present to the amount that would suffice to produce saturation, for each month of the year.
(5) The number of rainy days in each month.
(6) The average condition of the sky, whether clear or overcast, in each month.
(7) The amount and daily distribution of wind, its direction being for us of little moment.
In speaking of air temperatures it must always be understood that temperatures _in the shade_ are referred to, and in systematic scientific observations the greatest care must be taken that not only shall the instruments be thoroughly protected from the direct, but also from the reflected rays of the sun; and that means are also taken to secure a free current of air over them.
If an absolutely exact determination of the temperature of the air in any given situation be required, the observation should be taken by swinging the instrument, attached to a short cord, rapidly round the head, and by this method it is possible to secure a close approximation to the actual air temperature, even in the open and under the direct rays of the sun.
In considering the effects of climate on human beings it will be well to commence with a short consideration of the way in which they are affected by each of these elements of climate.
_Temperature._--In health the temperature of the human blood varies but little, whatever may be the climatic conditions to which we are submitted, the normal point being generally taken as 98·4° F. (37° C), though it may range half a degree or so above or below this level without prejudice to health or comfort. The mechanism by which this uniformity of internal temperature is maintained, in spite of the widest differences in the temperature of our environment, depends upon an automatic regulation of the nutritive processes going on within the system.