Editing Dune (section)

====Seif or longitudinal dunes====
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Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The dunes lie generally parallel to each other<ref name=Mangimeli/><ref name=dkp/> The two slip faces make them sharp-crested. They are called ''seif'' dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations).

Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along the resultant direction of sand movement (hence the name "longitudinal").<ref name=seif_terminology>{{cite book |doi=10.1007/978-1-4614-9213-9_460-2 |chapter=Longitudinal Dunes (or Linear Dunes) |title=Encyclopedia of Planetary Landforms |pages=1–11 |year=2014 |last1=Radebaugh |first1=Jani |last2=Sharma |first2=Priyanka |last3=Korteniemi |first3=Jarmo |last4=Fitzsimmons |first4=Kathryn E. |isbn=978-1-4614-9213-9 }}</ref> Some linear dunes merge to form Y-shaped compound dunes.<ref name="USG Types of Dunes"/>

Formation is debated. [[Ralph Bagnold]], in ''[[The Physics of Blown Sand and Desert Dunes]]'', suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by [[vortex|vortices]] in a unidirectional wind.<ref name=Mangimeli/> In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes.

<!-- The first image (Rub' al Khali) would be better rotated 90 degrees clockwise to match it with the contrasting image to the right --> 
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File:Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1).jpg|Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1). Most of these dunes are seif dunes. Their origin from barchans is suggested by the stubby remnant "hooks" seen on many of the dunes. Wind would be from left to right.
File:ISS-31 Linear dunes in the Great Sand Sea in southwest Egypt.jpg|Large linear seif dunes in the [[Great Sand Sea]] in southwest [[Egypt]], seen from the [[International Space Station]]. The distance between each dune is 1.5–2.5&nbsp;km.
File:Longitudinal_dune.jpg|The average-direction-longitudinal model of seif dune formation
File:Tranverse dune.jpg|alt=Transverse dune with wind blowing across crest|By contrast, transverse dunes form with the wind blowing perpendicular to the ridges, and have only one slipface, on the lee side. The stoss side is less steep.
File:Cross-bedding.gif|alt=Animation of wind pushing transverse dunes along. The sand blows from the stoss side down onto the less side, where it is buried by the next layer. The dune thus moves, and a cross-section through it shown diagonal cross-bedding|Transverse dunes lie perpendicular to the wind, which moves them forwards, producing the [[cross-bedding]] shown here.
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Seif dunes are common in the Sahara. They range up to {{convert|300|m|ft|abbr=on}} in height and {{convert|300|km|mi|abbr=on}} in length. In the southern third of the Arabian Peninsula, a vast [[Erg (landform)|erg]], called the [[Rub' al Khali]] or Empty Quarter, contains seif dunes that stretch for almost {{convert|200|km|mi|abbr=on}} and reach heights of over {{convert|300|m|ft|abbr=on}}.

Linear [[loess]] hills known as [[paha (landform)|pahas]] are superficially similar. These hills appear to have been formed during the last [[Quaternary glaciation|ice age]] under [[permafrost]] conditions dominated by sparse [[tundra]] vegetation.