O come with me and be my love, says Marlowe's Passionate Shepherd to one whom we presume to be his lady friend
And we will all the pleasures prove
That hills and valleys, dales, and fields,
Woods or steepy mountains yield.
Now, should this passionate pair decide to build their love nest in one of the many places suggested by the shepherd, they would be ill advised, for a start, to site it on a "steepy" mountain exposure to the wind can often make such locations far from pleasant. But valley sites, as well, have disadvantages and they are meteorologically more complex.
The microclimate in a valley is intimately bound up in the topography. In clear weather when there is little general wind, for example, the temperature on sloping ground drops rapidly at night, and cools the air in contact with it. This cold air, being denser and heavier than any warmer air around, begins to run downhill, just as water might, following whatever natural drainage channels it can find. In due course these gentle cold streams from the slopes converge to run down the main spine of the valley like a mighty river several hundred feet in depth, providing an often strong, and always chilling, nightly breeze.
The opposite happens during the day. After dawn, the morning sun begins to warm the ground, band the heated air resting on the slopes begins to slide uphill. In due course, as the whole area becomes warm, there is a general tendency for a warm breeze to blow up the valley. Thus, in a valley opening south, the front of a house that faces down the valley will have the advantages of both sunshine and a cooling breeze by day, while the less important rear portions of the house bear the brunt of the chilling night time winds.
Aspect is also critically important. Ground that slopes southwards towards the mid-day sun receives more radiant heat per unit area than level ground in the latter case, the sun hits the ground obliquely, and the same "bundle" of rays is spread over a larger area than if the surface were inclined to accept the impinging radiation at right angles. As a rough rule, a south facing surface with a slope of 45 gets about 50 per cent more energy than horizontal ground, while a north facing slope receives only about half. The aspect and slope of a site, therefore, govern the warmth and dryness of the soil, and one must weigh the advantages of early fruit, vegetables and flowers against a tendency for accentuated drought during rainless spells.