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Meteorology   


Now since most rain falls in those regions towards which and from

which the sun turns and these are the north and the south, and since

most evaporation must take place where there is the greatest rainfall,

just as green wood gives most smoke, and since this evaporation is

wind, it is natural that the most and most important winds should come

from these quarters. (The winds from the north are called Boreae,

those from the south Noti.)

The course of winds is oblique: for though the evaporation rises

straight up from the earth, they blow round it because all the

surrounding air follows the motion of the heavens. Hence the

question might be asked whether winds originate from above or from

below. The motion comes from above: before we feel the wind blowing

the air betrays its presence if there are clouds or a mist, for

their motion shows that the wind has begun to blow before it has

actually reached us; and this implies that the source of winds is

above. But since wind is defined as 'a quantity of dry evaporation

from the earth moving round the earth', it is clear that while the

origin of the motion is from above, the matter and the generation of

wind come from below. The oblique movement of the rising evaporation

is caused from above: for the motion of the heavens determines the

processes that are at a distance from the earth, and the motion from

below is vertical and every cause is more active where it is nearest

to the effect; but in its generation and origin wind plainly derives

from the earth.

The facts bear out the view that winds are formed by the gradual

union of many evaporations just as rivers derive their sources from

the water that oozes from the earth. Every wind is weakest in the spot

from which it blows; as they proceed and leave their source at a

distance they gather strength. Thus the winter in the north is

windless and calm: that is, in the north itself; but, the breeze

that blows from there so gently as to escape observation becomes a

great wind as it passes on.

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