Off Topic: Planet Earth – Climates and Deserts

The following post is an off topic discussion of planet earth, which will consists of miscellaneous topics involving climate types and deserts.

We can begin our study of the planet earth by discussing different types of sand dunes. Dunes are found wherever sand is blown around, as sand dunes are the construct of Aeolian processes where wind erodes loose sand. There are five main types: Barchan, Star, Parabolic, Tranverse and Longitudinal, though these sometimes go by other names. These dunes are  the product of wind direction. With Barchan dunes, the wind is predominantly in one direction which leads to the development of a crescent shape dune. The shape is convex and the “horns” point in the direction of the wind direction. The other two types of dunes where the wind is in one direction are parabolic and transverse dunes. Parabolic dunes are similar to Barchan dunes, although the “horns” point opposite to the direction of the wind. The key defining feature of this dune type is the presence of vegetation and the fact that they are effected by “blowouts” which is erosion of the vegetated sand.

dunes

As shown above, transverse dunes are also quite similar to barchans, but have wavy ridges instead of a crescent shape. The ridges are at right angles to the wind direction. Sand dunes which are formed by wind going in multiple directions are either Linear/Longitudinal or Star dunes. Star dunes are the result of wind moving in many directions whereas Longitudinal dunes are formed where wind converges into a single point, forming parallel lines to the direction of the winds.

An important term concerning dunes is “saltation”. Saltation is the rolling and bouncing of sand grains due to wind. The distinction between saltation, creep and suspension is that saltation forms a parabolic shape, though these are all wind-based processes.

aeolian

Within hot deserts (as opposed to cold deserts), it is common to find structures such as mesas and buttes.

plateau

From left to right in the image, the difference between each type of landform is apparent. The pinnacle (or spire) is the most narrow. It is important to note that all of these desert structures are formed by not only wind, but also water (and heat). In addition, a desert surface is generally made of sand, rock and mountainous formations.

An important feature of deserts is desert pavement. This sheet-like rock formation of rock particles formed when wind or water has removed the sand, which is a very slow process. There are several theories as to why desert pavement exists including intermittent removal of sand by wind and later rain or possibly by shrinking and swelling of clay.

pavement

Another concept of sand erosion is deflation, defined as the release of sand from soil by wind.

An important characteristic of deserts is the extreme temperature of the region. During the day, (hot) deserts are hot, as all of the heat from the sun is reflected by the sand in the ground. This raises the temperature of the ground due the lack of water near the surface. If water was present near the surface, most of the heat would go into evaporating the water. However, even hot deserts are cold at night because the dry surface does not store heat as well as a moist surface. Since water vapor is a greenhouse gas (and there is little water vapor in the air in a desert), infrared radiation is lost to outer space which contributes to the cold night temperatures.

Ventifacts, pictured below, are stones shaped by wind erosion. They are commonly found in arid climates with very little vegetation and feature strong winds. This is because vegetation often interferes with particle transport.

ventifact

An inselberg, as its Germanic name implies, is a type of mountain that is isolated and tends to be surrounded by sand. The area around the inselberg tends to be relatively flat, another defining characteristic of the structure. The word “insel” refers to island, which reinforces this concept.

saustralia-inselbergs-1024x768

A playa lake is a temporary body of water which also referred to as a dry lake. They are created whenever water ends up in a depression, however when the evaporation rate is quicker than the incoming water, the lake dries up. This tends to leave a buildup of salt.

An interesting piece of information about deserts is that they tend to located at 30 degree latitudes in both the north and south hemispheres. At the equator there is a low pressure zone due to direct sunlight, however at the 30 degree points there is high pressure, which leads to dry weather. At the equator, the climate tends to be relatively stable and has heavy rainfall. The sinking of air is what leads to these deserts, so in that way high pressure regions are very important to the development of deserts. The world’s largest hot desert (H climate) is the Sahara and the largest cold desert (K) is Antarctica. The major difference between a BW (Arid) climate and a BS (semiarid) climate is the amount of precipitation. Less than ten inches indicates arid climate and generally 10-20 inches indicates semiarid.

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s