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 Warm air can hold more moisture than

cold aircold air

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

Types of Deserts

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 Water does the majority of erosional work in the desertserosional 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 andlarger 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 QuestionsIdentify and appropriately place 10 characteristics in the Venn Diagram.

1. Transport sediment in direction of movement2. Can transport large boulders3. Form dunes4. Occur most frequently at high latitudes5. Few associated plants and animals6. Occur on at least five continents7. Features formed have generally diminished in

size over the last few decades8. Commonly found at low elevations9. More likely to occur at high elevations10. Preserve a record of past climates11. Characterized by the presence of snow and ice12. Associated with erosion and deposition13. Material is not sorted14. Material is sorted15. Evaporation exceeds precipitation16. Cross beds form in deposits17. Faster moving18. Controlled by climate patterns19. Involves water20. Has a dissolved load21. Have a competence and capacity

1 6 1012 21

2 13 19

3 8 14 16

4 7 9 11

5 15

17 20

18