seif dune
seif

The wing will continue to grow under both wind conditions, thus producing a seif dune.

Seif dunes may thus form long rows whose parallel ridge lines follow the wind's direction.

Seif dunes are thought to develop from barchans if a change of the usual wind direction occurs.

Seif dunes are sharp-crested and are common in the Sahara.

The formation of the seif dune is less well understood, but transition forms, which can be connected into a series, have been observed by Bagnold.

When there are two directions from which winds regularly blow, a series of long, linear dunes known as seif dunes may form.

In the sheltered troughs between highly developed seif dunes barchans may be formed because the wind is unidirectional.

Common in the Sahara desert are seif dunes: here the air currents form sand dunes parallel to the prevailing direction of the wind.

The exposures of Permian sands exhibit complex cross-bedding that is believed to represent ancient seif dune deposits.

Seif dunes are formed when strong winds blow from a quarter other than that from which the prevalent winds, responsible for the general sand drift, arrive.

In addition, Bagnold has observed that barchans may be formed in the troughs between series of seif dunes, in the manner shown in Fig. 1 1.5.

On a seif dune the slip face develops on the side facing away from the strong wind, while the slip face of a barchan faces the direction of movement.

Seif dunes are larger: in the Egyptian Sand Sea they attain a height of loo metres (330 ft) and dunes twice this height have been reported from southern Iran.

Longitudinal dunes (also called Seif dunes, after the Arabic word for "sword"), elongate parallel to the prevailing wind, possibly caused by a larger dune having its smaller sides blown away.

The seif dune differs from the barchan, therefore, in that the slip faces are on the side away from the strong wind and not facing the direction of advance as in the barchan.

The direction of the seif dunes shows the direction of the prevailing wind, which is also responsible for the barchans in the troughs, where the effects of strong winds from another quarter are not felt.

In the southern third of the Arabian Peninsula, a vast erg called the Rub' al Khali or the Empty Quarter, contains seif dunes that stretch for almost 200 km and reach heights of over 300 m.

It will be seen that, if the maintenance of such longitudinal vortices is possible, the presence of cloud streets (long lines of shallow cumulus clouds), which have also been clearly shown by satellite photographs, and seif dunes could be explained by one single hypothesis.