CEE 1030 1st Edition Lecture 18 Distribution of arid land dry regions deserts and steppes over 30 of earth s land surface Dry Climate Zones climatologists define dry climate as one in which annual precipitation is less than water loss by evaporation desert arid driest of the dry climate zones steppe semi arid marginal zone borders and deserts Types of Deserts hot and dry warm all year very hot in summer precipitation often evaporates in air rainfall very low and or concentrated semiarid summers moderately long and dry winters with low precipitation rainfall very low and or concentrated costal cool winters followed by moderately long warm summers rainfall low and concentrated in short bursts cold short moderately warm summers long cold winters with snowfall high precipitation in winter occasionally in summer What is a dry climate anywhere water loss is greater than water gain Low Latitude deserts two bands of deserts are concentrated at low latitudes between 20 30 degrees north and 20 30 degrees south latitude belts of arid and semiarid climate stretch across nearly 9300 km at the tropic of cancer and the tropic of capricorn atmospheric circulation creates to high pressure belts at low latitudes in high pressure zones dry air descends from high altitudes resulting in warm dry climate Mid latitude deserts mid latitude deserts and steppes occur in areas sheltered by mountains high mountains in path of prevailing winds produce rain shadow deserts These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Rain Shadows prevailing winds move warm moist air up windward slope of mountain temperature decreases and water vapor in air condenses and precipitates when air descends on other side it is extremely dry resulting in other side of mountain Continental interior deserts continental interior deserts form on inland of large land masses wind moving onto continent loses most moisture by precipitation as air moves further inland it has no moisture left so produces little to no precipitation Weathering in arid climates same geologic processes as in humid regions but different effects in arid conditions most weathered debris in deserts consists of unaltered rock and mineral fragments from mechanical weathering relatively little chemical weathering occurs in deserts results in clay think soils and oxidized minerals Running water most desert erosion is result of running water NOT wind deserts are characterized by dry washes dry channels of ephemeral streams that contain water only during heavy rainfall Desert floods precipitation rare in deserts usually very heavy rain sparse vegetation and poorly integrated drainage systems contribute to unhindered runoff flash floods along valley floors are common short lived rain storms can cause Desert rivers most permanent rivers crossing deserts Often in mountains combination of rapidly deplete streams Desert groundwater water table usually very deep forming oasis where it intersects surface few desert streams gain from groundwater most lose Wind in the desert although most desert erosions result from running water wind also works to erode transport and deposit sediments key differences between running water and moving air wind is less capable than water of picking up and transporting coarse particles wind is not confined to channels and can spread sediment over a large area Wind Transport wind like running water transports sediment as suspended load fine particles transported within the air saltation transport of particles through series of bounces bedload particles moved along the ground surface by wind also called creep as wind velocity increases it can transport larger sedimentary particles by saltation and suspension wind transport can result in dust and sand storms in a sandstorm about 20 25 of sand is moved by saltation Wind Erosion deflation wind lifts fine loose sediment from desert floor leaves behind larger grains wind erodes by abrasion friction and impact of particles on rock surfaces physical weathering limited in vertical extent evidence of wind abrasion wind sculpted ridges Wind erosion features blowout dune depression excavated by wind n easily eroded sediment Yardangs Great Sphinx probably began as an enormous yardang Ancient Egyptians carved existing landform into sculpture Wind Deposits wind produces distinctive depositional landforms in deserts types of wind deposits mounds and riges of sand from the wind s bed load dunes extensive blankets of slit from the wind s suspension load loess Loess deposits loess forms where windblown silt accumulates blown in from deserts or glacial outwash deposits loess deposits can be 30m thick typically without layering built up over 1000 s of years Sand dunes dune hill or ridge of wind deposited sand dunes form where wind speed slows dune shape reflects multiple factors Dune formation most dunes have an asymmetrical profile with a steep leeward slope and a gently inclined windward side initially bedload accumulates in wind shadow sand moves up windward slope by saltation accumulating at crest sand dunes do not sit in one place when leeward side of dune becomes steeper than the angle of repose then crest collapses down slip face Dune Migration sand deposited on slip face forms layers slanted in direction of wind form cross beds saltation and deposition results in direction of wind Changing Wind direction changes in the prevailing wind direction are recorded by changes in layers or cross beds Lithified dunes dunes can be lithified as sandstone indicate arid paleoclimate at time of deposition regardless of he area s modern climate slope of cross bedding and grain sizes indicate wind direction and speed at time of dune formation Types of sand dunes shapes and sizes of sand dunes vary according to multiple factors wind direction wind velocity availability of sand amount of vegetation four most common types of sand dunes longitudinal dunes barchans dunes transverse dunes parabolic dunes longitudinal dunes long ridges of sand oriented parallel to prevailing wind dunes form where sand supplies are limited barchan dunes sand dunes shaped like crescents with tips pointing downwind minimal vegetation medium wind medium sand parabolic dunes crescent shaped dunes similar in shape to a barchans dune except that tips point into wind often form along coasts Deserts and expanding desert margins are fragile environments as