Main characteristics of a hot desert:
— It receives very little precipation throughout the year.
— The land heats up quickly in daytime while cools down quickly in nighttime. The reason for this is that there is little evaporation in the atmosphere to block sunlight during the day and at night allows heat to escape easily. 
“Deserts have been defined and classified in a number of ways, generally combining total precipitation, number of days on which this falls, temperature, and humidity, and sometimes additional factors.” – Wikipedia
THE FORMATION OF SAND DUNES
Sand dunes are probably the hot deserts most iconic feacture.
A dune is basically a hill of sand. Sand dunes are formed by the interaction with wind or water and they occur in different sizes and shapes. It takes many thousands of years for sand dunes to accumulate. In places with an abundant source of sand and strong, stable wind, dunes can reach heights over 250 m. Grains of sand travels quite slowly with a speed of about 15 m per year. Finer, dust size particles travels further. 500 tons of it ends in the atlantic every year. Sand dunes are constantly changing. 
Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter “slip face” in the lee of the wind. – Wikipedia
Through weathering processes — such as wind, water or ice – rocks and minerals are broken down into smaller grains. The sand you can touch in the Sahara origin mostly from the mineral quartz. It is the second-most-abundant mineral in Earth’s continental crust. 
“The vast majority of the existing sand grains have been reduced, either recently or in long-past ages, to something very near to their present size by the action of water and perhaps to a lesser extent by that of ice. It is possible that wind action has a visible effect on the shape of the grains once they have been reduced to such a size that the wind can move them.” 
The desert moves. As the sun heats the air in a given area, it expands and rises. As it occurs, denser and cooler air move into this area. Wind is the result. It is mainly this force which transports and moves the desert. There are in particular three ways for the wind to transport sand. It depends on the size of the grains and how strong the wind is. 
Light and fine loose material is kept in the air, which scientists call suspension. The particles need to have a diameter less than 0.05 mm and they tend to be carried up into the air and to be scattered as dust. These form the dust clouds. 
Saltation happens to medium sized particles between 0.05mm – 0.5mm. They travel through a series of bounces. These bounces is a function of the drag and lift forces exerted by the wind and are sufficient to lift some particles from the surface and are then pulled downward by gravity. 
Surface creep affects particles of sizes bigger than 0.5 mm. These objects move through rolling and creeping. 
Due to suspension, saltation and surface creeping the sand dunes moves. Much like a wave if you look upon it from a longterm perspective. Depending on the conditions in different areas in the desert, you can find a wide range of variations of sand dunes. 
It is on the backslope side of the sand dune where the stunning sand ripples are formed. Sand ripples are smaller than dunes, with widths ranging from about 1-20 cm. Because they are so small, they are not formed by the same instability that creates dunes. 
When sand is moved by the wind, it saltates as described above. As the bouncing sand hit the ground, they splash into the surface, causing other grains to bounce short distances. This is the process that creates this magnificient pattern. The distance from one ripple to the next is related to the distance grains are splashed, which is in turn related to grain size and wind speed. An interesting fact is that ripples might reflect winds from the previous hour, whereas kilometer long dunes might reflect wind patterns from millennias back. 
— Foster & Partners, UAE Pavilion Milan Expo. Reaction and Diffusion Equations. It is a mathematical equation used in the project to create a similar ripple wave pattern as true sand dunes show
— Singing Sand Dunes. New research shows that sand of grains of a certain size tend to produce resonant notes around 90Hz.
Photographs & Diagrams:
FIG. 1-3, 5:
Rosino. (2005). Erg Chebbi Marocco [photograph]. https://www.flickr.com/photos/rosino/albums/1766244 [2016-02-12]
Soni, S. (2014). Aeolian process and land form. [diagram] http://www.slideshare.net/shivamsoni2011/aeolian-processland-form-by-shivam-soni-final-ppt-2007 [2016-02-12]
All black and white diagrams are made by the author.
 Wikipedia. (2016). Desert. https://en.wikipedia.org/wiki/Desert [2016-02-12]
 Wikipedia. (2016). Dune. https://en.wikipedia.org/wiki/Dune [2016-02-12]
 Wikipedia. (2016). Patterns in nature. https://en.wikipedia.org/wiki/Patterns_in_nature [2016-02-12]
 Wikpedia. (2015). Reaction-diffusion system. https://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_system [2016-02-12]
 Desert Animals. (u.å.). Desert. http://www.desertanimals.net/desert.html [2016-02-12]
 Fenton, L. (2014). Sand Waves in the Desert. The Planetary Society [blog], February 21. http://www.planetary.org/blogs/guest-blogs/2014/0219-sand-waves-in-the-desert.html [2016-02-12]
 Soni, S. (2014). Aeolian process and land form. http://www.slideshare.net/shivamsoni2011/aeolian-processland-form-by-shivam-soni-final-ppt-2007 [2016-02-12]
 Bagnold, R, A. (2005). The Physics of Blown Sand and Desert Dunes. Dover Publications.
Youtube & Vimeo: