Deserts

1. Deserts

2. 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

3. 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

4. Types of Deserts Low-Latitude 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

5. 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

6. 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

7. Desert Locations

8. 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

9. 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

10. 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

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

13. Desert Pavement • Deflation occurs leaving behind coarse particles to large to carry

15. Daily Questions 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 Identify and appropriately place 10 characteristics in the Venn Diagram. 3 8 14 16 18 1 6 10 12 21 4 7 9 11 2 13 19 17 20