Deserts

Deserts Dry places that have a dry climate-the yearly precipitation is less than the evaporation loss Dry regions cover 30% of the Earth’s surface Concentrated in sub-tropics, mid-latitudes, and polar regions

Meteorology Principles Warm, less dense air rises Rising air cools Moisture carrying capacity of air decreases Precipitation occurs Cold, dense air sinks (moisture depleted) Sinking air warms Moisture carrying capacity increases Warm air can hold more moisture than cold air

Low-Latitude Deserts Types of Deserts Along the equator is a Low pressure system, air rises Along 30o N and S - High Pressure systems exist (Subtropical high-pressure belt, air sinks and warms As air mass warms it can hold more moisture – any available water evaporates

Cold-Water Coasts Cold dry air from ocean moves over warm dry land Land warms air mass As air mass warms it can hold more moisture – any available water evaporates

Rain Shadows – Orographic Uplift Warm, humid air masses move off of ocean As the mass approaches a mountain it begins to rise Rising air cools Moisture carrying capacity decreases – precipitation occurs on the windward side Cold, dry air mass moves over mountains and descends on the leeward side Descending air warms and picks up moisture Dry region is a Rain Shadow – the process is orographic uplift

Desert Locations

Weathering in Arid Climates Mechanical Weathering is dominant Lack of moisture and organic acids limit chemical weathering Chemical weathering does occur, but the it takes longer

Water and Deserts Ephemeral streams are present in desert areas Deserts lack vegetation Rainfall is infrequent, but intense when it occurs Runoff is high – get flash floods One storm can do an enormous amount of erosion (triggering mass wasting and transporting sediment) Water does the majority of erosional work in the deserts

Wind Transport of Sediment Similar to stream transport – both air and water are fluids Wind systems have both a bed load and suspended load Bed Load Primarily sands, can be gravels and larger particles Particles move by saltation (sands) or creep (rolling or sliding – particles larger than sand) Causes abrasion – mechanical weathering Suspended Load Primarily silt with some clay Wind has a capacity and competence

Wind Erosion Insignificant in erosional processes Two mechanisms Deflation Loose particles of sand and dust are lifted from the surface Abrasion Sandblasting of particles

Desert Pavement Deflation occurs leaving behind coarse particles to large to carry

Daily Questions Identify and appropriately place 10 characteristics in the Venn Diagram. Wind Action Glacial Processes Stream Processes Transport sediment in direction of movement Can transport large boulders Form dunes Occur most frequently at high latitudes Few associated plants and animals Occur on at least five continents Features formed have generally diminished in size over the last few decades Commonly found at low elevations More likely to occur at high elevations Preserve a record of past climates Characterized by the presence of snow and ice Associated with erosion and deposition Material is not sorted Material is sorted Evaporation exceeds precipitation Cross beds form in deposits Faster moving Controlled by climate patterns Involves water Has a dissolved load Have a competence and capacity 5 15 3 8 14 16 18 1 6 10 12 21 4 7 9 11 2 13 19 17 20