They occur chiefly in connection with a change of wind, and are no doubt consequent on the meeting of two opposite currents.

Their violence, however, is moderate, compared with that of tropical tornadoes, and it is not often that they do any considerable damage to the crops over which they sweep.
One further characteristic of the flat region may be noticed.

The intense heat of the summer sun striking on the dry sand or the saline efflorescence of the desert throws the air over them into such a state of quivering undulation as produces the most wonderful and varying effects, distorting the forms of objects, and rendering the most familiar strange and hard to be recognized.

A mud bank furrowed by the rain will exhibit the appearance of a magnificent city, with columns, domes, minarets, and pyramids; a few stunted bushes will be transformed into a forest of stately trees; a distant mountain will, in the space of a minute, assume first the appearance of a lofty peak, then swell out at the top, and resemble a mighty mushroom, next split into several parts, and finally settle down into a flat tableland.

Occasionally, though not very often that semblance of water is produced which Europeans are are apt to suppose the usual effect of mirage.