Deserts Flashcards
what is a desert
an area/region, lacking in moisture, often exposing expanses of barren rock and/or sediment + hosting climatic condition hostile to life
what are teh 3 main types of deserts
Subtropical Deserts
Polar Deserts
Mid-Latitude Deserts
- rainshadow
- continental interior
what are semi-arid regions ( dry but no quite deserts) referred to as?
“steepes”
How does atmospheric circulation work?
Due to Earth’s rotation, convection cells are a little more complicated
Air flow is deflected due to Coriolis force
The convection pattern is broken up into 3 types of circulation cells
what are hadley cells
On either side of equator to 30 latitude (N + S). Equator marks zone of low pressure where air rises
what are polar cells?
Centred at poles (90 latitude) where cold air sinks to produce polar high pressure area
what are ferrell cells
btwn Hadley + Polar cells (circulation driven mostly by movement from Hadley + Polar cells
where do subtropical deserts occur?
In general, subtropical deserts occur under high-pressure belts btwn Hadley + Ferrell cells (where dry air flows downward)
Opposite to conditions in low pressure belts in which moist air rises, moisture condenses into droplets (when cooled) + falls as precipitation (very wet climate)
where do polar deserts occur?
Polar deserts occur under high-pressure regions (where dry air flows downward)
Sub-tropical deserts, which are hot
Polar deserts at the poles are very cold
Polar deserts occur both near North Pole and South Pole (Antarctica)
what are rainshadow deserts?
Deserts can develop at mid-latitudes due to the “rainshadow” effect of mountain
As moist air from the ocean meets a mountain barrier, it is forced to rise over the mountain
Moisture condenses to form clouds + falls as precipitation on windward side of the mountain
A rain shadow desert is produced on the leeward side of the mountains (as moisture supply has been spent)
Called “rainshadow” deserts bc they occur in “rainshadow” areas of mountain belt
when can large deserts develop in mid-latitudes
if there is a very large continent at that latitude
what are physical components of deserts
water
Tends to be scarce for much of the time
But when it rains, it really rains (torrential downpours)
Flash floods common, produce ephemeral (short-lived) streams
wind
Can be little wind
But when wind blows, it is often severe
As w/ the case w/ water, the lack of anchoring vegetation means that wind has strong erosive power
how is sediment transported by wind
Bedload (at land surface) via rolling, sliding, saltation
Suspended load (suspended in air)
what is the process of deflation in deserts
Wind selectively picks up light/small particles + carries them away in suspension (“winnowing”)
The leaves a residue of larger fragments, forming a desert pavement of larger stones
The pavement can protect the underlying material from further erosion
what is the process of a blowout
produced by deflation are shallow depressions appropriately called blowouts
Within a blowout, small patches of soil anchored by vegetation can remain as raised humps
what is the process of abrasion
Sediment particles carried in suspension can polish solid surfaces of exposed rock outcrops + fragments via abrasion
Rock that have been worn smooth + polished by this sandblasting process are called ventifacts
what are aelian deposits
accumulations of sediment composed of sediment particles transported + deposited by wind
what is loess
Windblown silt + clay (associated w/ arid regions + dry conditions associated w/ glaciations) Thick accumulations of loess can be deposited far from their desert or glacial source areas
what is sand
Sand-sized particles or resistant minerals. Commonly moulded by wind into dunes
what are crossbedding in dunes
Many dunes have asymmetric profile w/ gently inclined upwind (windward) slope + steep leeward (down-wind slope)
Sand is transported (by wind) up the gentler windward slope by saltation, then avalanches down the steep “slip” face of the down-wind (leeward) slope
As the process continues, the dunes migrate downwind, producing inclined patterns called crossbeds (crossbeds can also produced by ripples in a water current)
Crossbeds always slant in the down-current direction (so are valuable flow direction indicators)
what factors do the shapes of dunes depend on?
sand supply, wind strength (and speed), presence/absence of vegetation + various other factors
what are barchan dunes?
crescent-shaped dunes w/ their tips pointing downwind. Develop on level surfaces w/ low sand supply
what are transverse dunes?
ridge-like, oriented perpendicular to wind direction. Develop where sand supply is very high
what are parabolic dunes
Crescent-shaped dunes w/ tips pointing upwind. Develop where the land surface is stabilized by vegetation + sand supply is high (depressions btwn the tips are blowouts)
