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= Tropical desert = Tropical deserts are located in the regions between 5 and 30 degrees of latitude. The environment is very extreme, with the highest average monthly temperature on Earth. The rainfall is sporadic, it may not have measurable precipitation at all in a few years. Under the extreme environment and climate conditions, most topical deserts cover with sand and rocks, they are too flat and no vegetation to block out the wind. The wind may erode and transport sand, rocks and other materials, this process is known as eolian processes. The landforms under the wind erosion are various and the characteristics vary greatly in size. The representative landforms including depressions and pans, Yardangs, inverted topography and ventifacts. There is no significant population could survive in the tropical deserts since extreme aridity, heat and so little vegetation, only some specific flora and fauna which have special behavioral and physical mechanisms could be supported. Although the tropical deserts are always considered to be harsh and barren, they are hotbeds of natural resources and plays a significant role in economic development.

Geographical distribution
The tropical deserts are located in both continental interiors and coastal areas between Tropic of Cancer and Tropic of Capricorn. The representative deserts including the Sahara Desert in North Africa, Australian Desert, Arabian Desert and Syrian Desert in Western Asia, Kalahari Desert in Southern Africa, Sonoran Desert in United States and Mexico, Mojave Desert in United States, Thar Desert in Indian and Pakistan, Dasht-e Margo and Registan Desert in Afghanistan and Dasht-e Kavir and Dasht-e Loot in Iran.

Controlling factor
Tropics form a belt around the equator from latitude 3° north to latitude 3° south which is called Intertropical Convergence Zone, the tropical heat generates unstable air in this area, and the air masses become extremely dry due to the loss of the moisture during the process of tropical ascent. (Thornthwaite, 1948). Another significant controlling factor of tropical desert climate is the presence of subtropical high pressure during all the year, it has dry and hot descending air cells which is named as Hadley Cells. Specifically, the Hadley Cells dry out the air and inhibiting condensation. Additionally, as the distance from moisture source increasing, the aridity increasing (Claro, Oliveira, Rico-Gray., 2009).

Temperature
Tropical deserts have the highest average daily temperature on the planet, this is because both the high energy input during the day and the loss of heat at night are large, this phenomenon causes an extremely large daily temperature range. Specifically, the temperatures at low elevation inland desert could reach 40 to 50 °C during the day, while temperature could drop to approximately five degree centigrade at night, the range of daily difference is around 30-40 degrees centigrade (Nano & Pavey, 2006).

There are some other reasons for temperature changing significantly in the tropical desert, for instance, lacking water and vegetation on the ground could enhance absorption of the heat from insolation, the subsiding air of dominant high pressure and cloud-free sky can also lead to large amounts of insolation; the cloudless sky enhance day temperature to escape rapidly at night. (Nano & Pavey, 2006).

Precipitation
The precipitation is very irregular in the tropical deserts. The average annual precipitation in low latitude deserts is less than 250 mm. Relative humidity is very low, only 10% to 30% in interior locations. Some deserts do not have rainfall all year-round, this is because they are located far from the ocean. Furthermore, high-pressure cells and high temperature can also increase the level of aridity (Nano & Pavey, 2006).

Wind
Wind contributes to aridity greatly in the tropical deserts. If the speed of the wind over 80km/h, it is possible to generate the dust storms and sandstorms and erode the rocky surface (Costa, 1996). Therefore, wind plays an important role in shaping the various landforms, this phenomenon is known as the eolian process. There are two types of eolian process which are deflation and abrasion.

Firstly, deflation may cause lightly lowering of ground surface and develop deflation hollows, plains, basins, blowouts, wind-eroded plains and parabolic dunes (Hesp, 2005). Secondly, the eolian process leads to abrasion which forming the special landforms with a significant undercut (Hesp, 2005).

Landform
The landforms of tropical deserts are various due to the different kinds of eolian processes, the major landforms including dunes, depressions and pans, Yardangs, inverted topography.

Dunes
There are various kinds of the dune in the tropical desert, specifically, the representative dunes including dome duns, transverse dunes, barchans, star dunes, shadow dunes, linear and longitudinal dunes (Cooke, Goudie and Warren, 1993).

Depression
The desert depression has polygenetic factors such as wind erosion, broad shallow warping and block faulting, stream erosion, karst activity, salt weathering mass wasting, and zoogenic processes; the representative example is the large enclosed basin in Africa such as Farafra, Baharia, Dakhla, Qattara, Siwa and Kargha (Laity, n.d.).

Pans
The pans are widespread in Southern and Western of Australia, southern Africa and the High Plains of the United States deserts. The determines of pans including a vegetation-free surface and low humidity, low water table and poorly consolidated sediments, a huge amount of fine-grained sandstones and shales. Moreover, feedback mechanisms playing a significant role during the process of enlargement of the pan, salts are left as water accumulates in depression which retards the weather sediments, and the growth of vegetation in the future, this affects both erosional processes and depositional processes in pans (Laity, n.d.).

Yardangs
As the most impressive landform of eolian erosion, Yardangs could be observed on the orbital or aerial images of Mars and Earth. The development of Yardangs occur in most arid regions, it is dominated by wind processes. The classic forms are streamlined and elongated ridges; it also appears as flat top or stubby and short appearance, the length-to-wide ratios are from 3:1 to 10:1, it is determined by the wind direction, time of exposure to the wind and rock material (Laity, n.d.).

Inverted topography
Inverted topography forms where the areas had low elevation previously such as deltaic distributary systems and river systems, they left standing higher relief due to relative resistance to wind erosion. The inverted topography is frequently appeared in Yardangs fields, the examples including raised channel in Egypt, Oman, China and Mars (Laity, n.d.).

Biogeography
The environment in tropical deserts is harsh as well as barren, only some special plants and animals which have special behavioral and physical mechanisms could live.

Biological adaption to aridity
For the flora, the general adaptations including transferring the leaves into spines in order to protect themselves. Since leaves reduced, the development of the stem is a major photosynthetic structure, it is also responsible for storing water. A common example is cacti which has specific means to store and conserve water, and fewer or no leaves which are certain to reduce transpiration (Marietta College, 2014). In addition to the protection from spines, chemical defences are also very general. Desert plants can grow slowly as less photosynthesis taking place, this is an advantage for the plants to make greater investments in defence (Marietta College, 2014). Another adaption is the development of long roots, these roots are extremely long, they allow the flora to acquire moisture at the water table. Furthermore, some desert plants using behavioural adaption, for instance, some flora can live for only one season or one year, and the desert perennial could survive by staying dormant during the extremely dry periods; when the environment is getting relative moisture, they become available again (DesertUSA, n.d.).

For the fauna, the easiest way is many animals choose to stay out of the tropical deserts as much as possible to avoid the heat and aridity. As a result of scare water, most animals get the water from eating succulent plants and seeds, or the body and blood tissues of their prey. They also have specific ways to store water and prevent water from leaving the bodies. Some animals live in burrows under the ground which is not too hot, and relatively humid, they stay in such burrow during the hot daytime, and only come out to seek food at night, the examples of these animals including kangaroo rats and lizards. Moreover, there are some animals such as wolf spiders and scorpions, they have a thick outer covering which prevents moisture loss as much as possible. It has also been found that animals in tropical deserts can concentrate urine by kidneys to excrete less water (Marietta College, 2014).

Flora
The representative desert plants including barrel cactus, brittlebush, chain fruit cholla, creosote, crimson hedgehog, cactus, common saltbush, desert ironwood, fairy duster, Joshua tree, Mojave aster, ocotillo, organ pipe cactus, pancake prickly pear cactus, paloverde, saguaro cactus, soaptree yucca, cholla guera, Triangle-leaf bursage, tumbleweed, velvet mesquite (“Blue planet biomes”, 2000).

Fauna
The representative fauna in tropical desert including armadillo lizard, banded Gila monster, bobcat, cactus wren, cactus ferruginous pygmy owl, coyote, desert bighorn sheep, desert kangaroo rat, desert tortoise, javelina, Mojave rattlesnake, cougar, Sonoran Desert toad, Sonoran pronghorn antelope, thorny devil (“Blue planet biomes”, 2000).

Natural resource
It has been found that rich mineral resources are located in the tropical deserts and some of them are unique. The representative mineral including borax, sodium nitrate, sodium, iodine, calcium, bromine, and strontium compounds. These minerals create when the water is evaporated in desert lakes (Dorling, 2019).

Borax
Borax is a natural cleaner and fresher, it is also known as a detergent booster, the boric acid is derived from the borax, it can be used in the manufacture of agricultural chemicals such as herbicide and insecticide, and also have a widely use in fire retardants, glass, ceramics, water softeners, pharmaceuticals, paint, enamel, cosmetics and coated paper. Billions of dollars of borax have been mined in the northern Mojave Desert since 1881 (Dorling, 2019).

Sodium nitrate
Sodium nitrate formed through water evaporation in desert areas. The richest cache of sodium nitrate is located in South America, approximately 3 million metric tons are mined during the War I. it was the earliest food preservatives, for instance, it was used for curing fish and meat to produce bacon, ham, sausage and deli meats. Moreover, it is also used for the manufacture of pharmaceuticals, fertilizers, dyes, explosives flares and enamels (Dorling, 2019).

Fossil fuels
Natural gas and oil are complex hydrocarbons, they have formed millions of years ago from the decomposition of animals and plants. They are the primary energy source in the world and existing as viscous or solid, liquid, and gaseous form. The five largest oil fields are in Saudi Arabia, Iraq and Kuwait. The largest petroleum-producing region in the world is the Arabian Desert (Dorling, 2019).

Metallic minerals
Most major kinds of mineral which deposits formed by groundwater are located in the deserts, for example, some valuable metallic mineral such as gold, silver, iron, zinc, and uranium are found in Western desert in Australia. This is because of the special geologic processes, and climate factors in the desert can preserve and enhance the mineral deposit (Dorling, 2019).

Gemstones
The tropical deserts are a treasure trove of semi-precious gemstones, the examples including chalcedony, opal, quartz, turquoise, jade, amethyst, petrified wood, topaz. The precious gemstones such as diamonds which is used in jewellery and decoration. Although some gemstones can also be found in the temperate zone throughout the world, turquoise can only be found in the tropical desert, it is a very valuable and popular opaque gemstone, with an exquisite blue-green or sky-blue colour and very exquisite veins (Dorling, 2019).