Discovery of dune systems on other planetary bodies (Mars, Venus, Saturn’s moon Titan, and Pluto) have demonstrated the universal nature of the processes of aeolian sediment transport and bedform development despite a wide range of atmospheric properties and sediment composition. Current and future spacecraft missions, especially by landers and rovers, will provide further insights into planetary dune systems.

The orientation or trend of dunes with respect to wind direction has long been a focus of dune research, especially for dunes that form in multidirectional wind regimes. The concepts of the gross bedform-normal transport (GBN) and the bed instability and fingering modes provide analytical solutions for understanding how dunes are oriented to sand-transporting winds and indicate the importance of sand supply for dune morphology in multidirectional wind regimes. They help to explain complex dune patterns and the existence of superimposed bedforms on megadunes.

Desert dunes occur in a variety of morphologic types, each of which displays a range of sizes (height, width, and spacing). This chapter discusses how they may be classified, describes the morphology of the major dune types, and briefly discusses the conditions of sand supply and wind regime in which they occur.

Changes in the boundary conditions that govern dune systems are manifested in many different ways at a landscape scale. They include changes in dune activity, reorientation of dune trends, and formation of new areas of dunes on timescales that range from interannual to millennial. This chapter focusses on annual to decadal timescales to emphasize the importance of the processes involved, while long-term (Quaternary) changes are discussed in Part VI, Chapter 16.

Conceptual and technological advances have provided many new insights into desert dune processes and dynamics. Future prospects for desert dune studies will involve increasingly sophisticated models based on a wide variety of field and remote sensing datasets. Greater integration of research with environmental management in drylands will bring benefits to all.

Most dune sand is transported close to the surface by saltation and reptation of grains. The shearing action of the wind is resisted by grain size and shape, bed slope, moisture content, and surface roughness elements (especially the presence of vegetation). The relations between sand flux, particle size, aerodynamic roughness, slope, and wind shear velocity are complex, especially on dune surfaces, and may depart significantly from estimates derived from transport equations. The formation of wind ripples is intimately associated with the transport of sand by saltation and reptation, but the relationships between grain transport processes and the formation and development of ripples are complex, leading to a self-organizing pattern that responds rapidly to changes in sand flux and wind speed.

Dune sediments provide a record of dune accumulation and can therefore provide important information on the processes of dune formation and development. The mineral composition of dune sands can in addition indicate sand sources and transport pathways. Studies of dune sedimentary structures have yielded a wealth of information on the fundamental processes of dune accumulation and shown that dunes are composed of varying proportions of wind ripple, grainfall, and grainflow deposits, the proportions of which vary between dune types.

Quaternary paleodune systems are widespread on the margins of active dune fields and sand seas and in semiarid areas. Despite issues with their visualization and interpretation, luminescence ages for periods of dune accumulation and stability can provide the basis for comparison with other proxy datasets to understand the forcing factors that determine dune activity and stability. These include changes in precipitation and vegetation cover and wind strength and direction.

Sand seas and dune fields are comprised of dunes of different morphological types and sizes, areas of sand sheets, interdune deposits (including non-aeolian sediments), as well as extra-dune fluvial, lacustrine, and marine sediments. They represent the primary depositional sink of aeolian sand-transport systems. The greatest concentrations of desert sand seas and dune fields occur in mid-latitude arid regions, and the subtropical deserts and contain considerable volumes of aeolian sand. Sand seas and dune fields are generally located in topographic lows and in areas where wind energy and sand-transport rates are lower than those in adjacent sand source areas.