1. Fronts and Air Masses

2. Air Masses & Fronts Air Mass = large body of air whose temperature and moisture is similar at a given height (can cover thousands of square miles). Air Mass = large body of air whose temperature and moisture is similar at a given height (can cover thousands of square miles). Air masses originate in source regions that are large and uniform Air masses originate in source regions that are large and uniform –Continental (c) = Dry –Maritime (m) = Wet –Polar (P) = Cold –Tropical (T) = Warm cP = cold & dry mP = cold & wet cT = warm & dry mT = warm & wet

4. General Mass Features When an air mass is colder than the underlying surface When an air mass is colder than the underlying surface –Warming from below –Increased low level lapse rate Atmospheric instability Atmospheric instability –Increased convection & mixing near the ground –Good visibility, cumulus clouds, showers When an air mass is warmer than the underlying surface When an air mass is warmer than the underlying surface –Cooling from below –Reduced low level lapse rate Atmospheric stability –Little vertical mixing –Accumulation of dust, smoke, and restricted visibility –Stratiform clouds, drizzle or fog

5. Fronts A Front is the transition zone between air masses different characteristics Fronts are recognizable by: – –sharp change in temperature – –shift in wind direction – –change in clouds – –change in moisture content (dew point) – –direction change of isobars (A line of equal barometric pressure on a weather map)

6. Isobars

7. Types of fronts Front - boundary, transition zone between two different air masses Front - boundary, transition zone between two different air masses –the two air masses have different densities. Frequently, they are characterized by different temperatures and moisture contents –front has horizontal and vertical extent –frontal boundary/zone can be 1-100 km wide!! –types of synoptic-scale fronts: stationary fronts stationary fronts cold fronts cold fronts warm fronts warm fronts occluded fronts occluded fronts

8. Stationary Fronts has little/no movement denoted by alternating cold/warm frontal symbols Associated weather: – –clear, partly cloudy, cloudy, light precip – –usually nothing severe

9. Cold Fronts Zone/boundary between warmer, more moist, unstable air (usually mT) being replaced by colder, drier, more stable air (usually cP). Location of cold front: – –leading edge of sharp temperature change – –moisture content (dew point) changes dramatically – –wind shift (direction and speed) – –pressure trough (pressure tendency is useful!!!) – –often cloudy/showers/thunderstorms/sometimes severe What does a vertical cross section through a front look like??

11. Warm Fronts Zone/boundary between advancing warmer, more moist air (usually mT) and cooler, drier air (usually mP) average speed is about 10 knots What does a vertical cross section through a front look like??

13. Occluded Fronts Why they form There are two types of occluded fronts: – –cold occlusion – –warm occlusion

14. Cold Occlusion cold front "lifts" the warm front up and over the very cold air Associated weather is similar to a warm front as the occluded front approaches once the front has passed, the associated weather is similar to a cold front most common type of occluded front vertical structure is often difficult to observe

15. Warm Occlusion cold air behind cold front is not dense enough to lift cold air ahead of warm front cold front rides up and over the warm front upper-level cold front reached station before surface warm occlusion