9-Clouds_formation-2011-v0

9-Clouds_formation-2011-v0 - Chapter 6 Chapter Cloud...

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Chapter 6 Chapter 6 Cloud Development and Forms Cloud Development and Forms
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1. Orographic lifting, the forcing of air above a mountain barrier 2. Localized convective lifting due to buoyancy 2. Frontal lifting, the displacement of one air mass over another 3. Convergence, the horizontal movement of air into an area at low levels Four mechanisms lift air so that condensation and cloud formation can occur:
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If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude (m) at which this occurs is known as the lifting condensation level (LCL) The LCL is the level of the basis of clouds. The rate at which saturated air cools is the saturated adiabatic lapse rate (SALR), which is about 0.5 °C/100 m (3.3 °F/1000 ft).
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The upward displacement of air that leads to adiabatic cooling is called orographic uplift (or the orographic effect ). When air approaches a topographic barrier, it can be lifted upward or deflected around the barrier. Downwind of a mountain ridge, on its leeward side, air descends the slope and warms by compression to create a rain shadow effect, an area of lower precipitation.
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Example LCL LCL Lapse rate decreases: condensation warms the parcel DALR=10 DALR=10 o C/km C/km DALR=Dry adiabatic lapse rate LCL= Lifting Condensation Level Parcel is dry and compresses at DALR 1 km
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Impact on vegetation Yellow pine forest at 4,000' on west slope of Sierra Great Basin sage at 4,000' on floor of Owens Valley east of Sierra
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Convection produced by surface heating Differences in surface characteristics result in differential heating Usually shows strong diurnal cycle peaking in mid-afternoon
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Cumulus clouds develop in summer east of San Diego - combination of topography and heating 10 min later
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Fronts are transition zones in which great temperature differences occur across relatively short distances. When cold air advances toward warmer air ( cold front ), the denser cold air displaces the lighter warm air ahead of it (a). When warm air flows toward a wedge of cold air ( warm front ), the warm air is forced upward in much the same way that the orographic effect causes air to rise above a mountain barrier (b). Regional to Large Scale
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When a low-pressure cell is near the surface, winds in the lower atmosphere tend to converge on the center of the low from all directions. Horizontal movement toward a common location implies an accumulation of mass called horizontal convergence , or just convergence for short.
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The air’s susceptibility to uplift is called its static stability . Statically unstable air becomes buoyant when lifted and continues to rise if given an initial upward push. Statically stable air
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This note was uploaded on 12/27/2011 for the course GEOG 110 taught by Professor Leila during the Fall '09 term at UCSB.

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9-Clouds_formation-2011-v0 - Chapter 6 Chapter Cloud...

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