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Weather – Air Mass extremely large body of air with similar characteristics of temperature and moisture. Forms when air stagnates for long periods of time over a uniform surface Characteristic weather of an air mass is determined by the surface above which it forms
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Air masses affecting North America
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Air mass modification An air mass may be modified when advected away from the source region. The modified air mass may be warmed/cooled from below --- remember atmospheric stability! It may also take up water vapor or be dried out by passing over certain terrain etc.
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The air mass takes up characteristics from the surface of the lake
The air mass takes up characteristics from the surface of the lake. Heating from below unstable. Warming + evaporationgains moisture
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Lecture 18 Fronts Life cycle of the extratropical cyclone
Three dimensional view of the extratropical cyclone The extratropical anticyclone
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Fronts Separate different air masses Cold, warm, occluded fronts
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Sfc weather associated with a cold front
Pressure T Wind Clouds & precip
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Vertical slice through a cold front
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Sfc weather with a warm front
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Vertical structure of a warm front
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Occluded fronts Cold type Warm type
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Extratropical cyclone
Forms as a wave on a stationary front separating two different air masses Goes through a life cycle: Growing stage Mature stage Dissipating stage
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The extratropical cyclone
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Growing stage
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Mature stage
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Dissipating stage
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The extratropical cyclone
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Growing stage
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Mature stage
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Dissipating stage
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Another view of the growth and dissipation
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Cyclogenesis --- needed ingredients
Surface temperature gradient A strong jet stream The presence of mountains (or other surface inhomogenious features, e.g. coastline) Tilted pattern of rising and sinking air (in frontal zones) releases baroclinic energy
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Typical cyclone paths and areas of cyclogenesis
Ruled by surface temperature gradient and a strong jet Nor’easters Cyclones riding the pineapple express from central Pacific to south and central California Alberta clippers
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Well known cyclone tracks
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Preferred areas of cyclogenesis
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Cyclones and jet streams
Surface lows tend to move in the direction of the flow at midtropospheric level (500mb) Surface pressure drops when there is divergence of the wind in the column of air above the surface low. Speed divergence diffluence
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Divergence above a deepening low
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Three dimensional view of the cyclone
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Convergence at bottom requires divergence at top and vice versa
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Depiction of 3D airflow in cyclone/anticyclone
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The extratropical anticyclone
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High pressure systems Come at the heals of the cyclone
Highs are air masses where with mostly homogeneous temperature and humidity Highs are usually clear, dry and calm A high looks like a blob on a weather map, as opposed to the bull’s eye that the low is Highs can last for weeks (especially in summer)
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Highs Have divergence at the surface, convergence at upper level
Diverging air weakens temperature and moisture gradients by spreading out the isolines Weak gradients mean no fronts Divergence at sfc requires sinking air, dry Highly stable and cloudless
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High pressure, low visibility
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