Unless one encamps in an open country the wind will seldom strike his tent steadily in a direction parallel with the ground. Rather it goes eddying and curling like driven smoke: hence the flapping and slatting of the fly-sheet.
During a squall a violent blow may fall straight down upon the roof as chough bent on snapping the poles; then, rebounding, it will try to carry the tent away and aloft, as though your shelter were an open umbrella. This action is often marked in ravines and in a glade surrounded by tall trees, but it may occur anywhere. The stanchest tents for a very windy situation are those of conical or pyramidal form, set up on or under tripods.
A fly-sheet is a perfect wind trap, especially if It orojects in the form of a porch, or is set well away from the ridge of the tent. This is one reason why I prefer a waterproofed tent without fly (except for hot climates). If a porch is wanted, rig a sheet of canvas over a separate frame in front of the tent; then, if it blows away, it will not wreck the tent too.
The flies of army tents, and of other patterns built for hard and varied service, are guyed to double-notched wall stakes, and so set rather close to the roof (see Fig. 9).