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Chapter 3. Climates of the World’s Deserts. Some General Properties. Meteorological characteristics Surface characteristics Vegetation characteristics. Variety of Properties. Cold deserts and hot deserts
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Chapter 3 Climates of the World’s Deserts
Some General Properties • Meteorological characteristics • Surface characteristics • Vegetation characteristics
Variety of Properties • Cold deserts and hot deserts • Deserts with winter precipitation and deserts with summer precipitation and deserts with virtually no precipitation • Perpetually foggy deserts and deserts with near the maximum possible sunshine • Barren deserts and heavily vegetated deserts • Sand-dune deserts and deserts with rocky plains
Summary of MeteorologicalCharacteristics (Then we’ll cover physiographic and vegetation characteristics)
Daily total solar energy received at top of atmosphere on horizontal surface COLD DESERT WARM DESERT
Climate% arid landsColdest moWarmest moExamples Hot 43 10-30 >30 Central Sahara Great Sandy (Aust) Mild Winter 18 10-20 10-30 Southern Sahara Kalahari Mexico Deserts Simpson (Aust) Cool Winter 15 0-10 10-30 Northern Sahara Atacama Mojave Cold Winter 24 <0 10-30 Canadian Prairie Gobi Turkestan Deserts of China Great Basin
Coastal/Foggy Desert • Summer temperatures are not especially high, but the winters are not cold because the ocean is close • Can be very uncomfortable because of high humidity
Temperature (solid) and Relative Humidity (dashed), at Different Distances From Namib Desert Coast FOG 30 km from coast 5 km from coast Far inland
Seasonality of Precipitation (some/none of each)
Rule of Thumb • Summer precip on Equatorial side • Winter precip on mid-latitude side
Desert Temperature • Why is it high? • Lack of evaporation from surface • Lack of vegetation (transpiration) • Not much cloud cover
Desert Winds • Subtropical deserts - High pressure areas are generally characterized by weak winds - Thunderstorms can produce high winds - Mid-latitude disturbances (lows) with high winds can penetrate into deserts • Cold deserts in mid-latitudes, experience mid-latitude cyclones (lows) that can have strong winds • Lack of vegetation – near-surface winds higher • Winds are very desiccating – high temperature coupled with high winds
Desert Humidity • Relative humidity can be as low as a few percent, or as high as 100% (foggy coastal desert) • Specific humidity • Compared to hot humid tropics, desert air is drier • But desert air of 95 F and 15% RH has more water vapor than does saturated air at 32 F in a winter storm
Humidity Calculations • Use the table on the previous slide to find vapor pressure, saturation vapor pressure, and relative humidity for: • Temperature = 38 C, Dew point = 7 C • Temperature = -1 C, Dew point = -4 C • Which location is more humid? • It depends on how you define humidity!!!!
Desert Humidity • Compare the relative and absolute humidity of a desert and winter storm • Desert • 95 F and 15% RH • Find dew point temperature for these conditions • Winter storm • 32 F and 100% RH • Find dew point temperature for these conditions • Which location is more humid?
General Physiographic Characteristics(Why do we care in a meteorology course?)
Types • Sand “sheets” and sand dunes • Bare flat rock • “Desert pavement” – matrix of pebbles cemented together on the surface • Salt flats • Large rocks and mountains • Clay plains
Vegetation Types • Small trees • Shrubs • Succulents • Grasses • Herbs • Lichens
Vegetation Types That are Adapted to Desert Conditions • Phreatophyte – long roots • Xerophyte – mechanisms for conserving water • Halophytes – adapted to saline soils • Psammophytes – grow in sandy soils • Therophytes (annuals) – seeds remain dormant in soil during dry season or dry years (contrast with perennial)
Landscape Types in Terms of Vegetation • Steppes – grassland without trees, generally in midlatitudes • Savannas – also open grassland, but there are scattered shrubs and trees (subtropical, representing a transition between tropical forests and grassland of arid areas)
