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 sedimentYardangs- 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 factorsDune 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