1. MET 102 Pacific Climates and Cultures Lecture 15: Air Masses & Fronts MET 102 Pacific Climates and Cultures

2. Discussion Questions – Clark 2011 What are the different general types of air masses? What are the different general types of air masses? Maritime Tropical (mT) Maritime Polar (mP) Continental Tropical (cT) Continental Polar (cP) Based on previous information, what type of air mass do you think dominates the Pacific Islands? Why? Based on previous information, what type of air mass do you think dominates the Pacific Islands? Why? The Pacific Islands are dominated by Maritime Tropical (mT) Air masses since they are surrounded by ocean and are located within the tropics. What are the 5 different types of fronts? What are the 5 different types of fronts? Warm Front, Cold Front, Stationary Front, Occluded Front, Dryline What two types of fronts are associated with a cyclone? What two types of fronts are associated with a cyclone? Warm Front and Cold Front

3. What is an Airmass? Air MassAir Mass –really big blob of air with similar properties –Usually 1600 km (1000 miles) across –Several km thick Change in weather when one air moves out and a new air moves in FRONTAL PASSAGE –Also known as a FRONTAL PASSAGE –Brings changes in T, P, RH and Winds large regions very similar propertiesAir masses form when they move over large regions that exhibit very similar properties

4. Airmass Source Regions PolarTropicalPolar and Tropical regions tend to exhibit such properties and therefore are good source regions. mid-latitudesThe mid-latitudes tend to be strongly varies and therefore are not good source regions

5. Types of Airmasses PolarPolar (P) air masses form over northern Canada and the Arctic. COLD –The tend to be COLD. (Artic (A) and Antartic (AA) are special designations for those regions) TropicalTropical (T) air masses form to the south of the United States. –These tend to be WARM. (Equatorial (E) is a special designation for the Equator) MaritimeMaritime (m) Air masses form over ocean (either the Pacific, North Atlantic, subtopical Pacific, or Gulf of Mexico). HUMID –These tend to be HUMID. ContinentalContinental (c) air masses form over land (northern Canada or northern Mexico). DRY –These tend to be DRY. Air masses are designated by two-letter combinations describing whether they are m or c, P or T. –Describes basic info about T and RH.

6. Airmass Designations All the possible combinations are: – mP – mT – mE – cP – cA – cT

7. Continental Polar Also continental Arctic (cA) continental Antarctic (cAA) Generally cold and dry Generally cold and dry During winter, cooled by the land, creating stable air mass. Mostly cloud free Mostly cloud free In summer, warmer and more humid than in winter Tend to remain farther north and therefore do not influence as much of North American as during winter

8. Maritime Polar Humid and coolHumid and cool Generates low level stratus clouds, which maintain instability through cloud-top radiative cooling This leads to somewhat unpleasant showers in Pacific NW, and to much of Western US winter weather.

9. Continental Tropical Hot and dryHot and dry, forming over the desert regions of North Mexico and South West US. unstable aircloud-freeIntense surface heating creates very unstable air, but generally cloud-free because of extremely dry conditions thunderstormIf however, some moisture is acquired, intense thunderstorm can develop.

10. Maritime Tropical Warm and moist Can be unstable because of high heating from warm waters  perfect for precipitation. Can trigger Thunderstorms.

11. Polar Front Theory Norwegian Cyclone Model Also called the Norwegian Cyclone Model Discovered by Norwegian scientists during World War I Theory states that Mid-Latitude Cyclones (MLCs) Theory states that Mid-Latitude Cyclones (MLCs) Develop in conjunction with the Polar Front Cold equatorward moving air collides with warm poleward moving air. FRONTS The collisions create FRONTS! In the upper atmosphere polar front is continuous, at the surface it is DISCONTINUOUS.

12. Pacific Mid-Latitude Cyclones

13. Fronts Boundaries Boundaries surfaces that separate air masses of different densities (think temperature) Can be combinations of warm, cold, dry, moist… Usually 15-200 km wide bands but narrow Represented by narrow lines on a weather map Warm Warm Cold Cold Stationary Stationary Occluded Occluded Dry lines Dry lines Dry Line

14. Fronts overlies Warmer air overlies cooler air (it’s less dense) Ideally the fronts move in approx. the same direction. travels air masses The FRONT is the barrier that travels with the air masses No matter which air mass is moving faster the warm air ALWAYS moves above cold air. No matter which air mass is moving faster the warm air ALWAYS moves above cold air. Overrunning Overrunning – describes warm air moving over cooler air.

15. Warm Fronts warm When temperatures change from cold to warm after a frontal passage. Symbol Gradual Slope Moisture StabilityMoisture content and Stability of the warm air mass determine the amount and type of precipitation.

16. Warm Fronts As a warm front approaches you see clouds in a certain order: “Contrails” too 1000 km 300 km

17. Warm Front Summary HORIZONTAL Lifting associated with warm fronts has a large HORIZONTAL component Summary gradual slope (1 km vertical : 200 km horiz.) Slow rate of advance EASTSOUTHWEST Winds shift from EAST to SOUTHWEST Travel at 25-35 km/h (15-20 mph) RISE Temperatures gradually RISE LARGE LONG Tend to produce light-moderate precipitation over a LARGE area for a LONG time.

18. Cold Fronts When temperatures change from warm to cold after a frontal passage. Symbol Slope is steep!

19. Cold Fronts Friction Friction slows the surface position of the front compared to it’s position aloft. Travel at 35-50 km/hr (20-35 mph) More VIOLENT in nature than warm fronts

20. Cold Fronts As a cold front approaches you see clouds in a certain order: 1) Maybe Altocumulus 2) Cumulus 3) Towering Cumulus

21. Cold Fronts Summary VERTICAL Lifting associated with cold fronts has a large VERTICAL component due to: Steep slope (1 km vertical : 100 km horizontal) Twice as steep as a warm front. Fast rate of advance (35-50 km/h (20-35 mph)) Wind directions Southwest Northwest Winds shift from Southwest to Northwest SMALL SHORT Tend to produce heavy precipitation over a SMALL area for a SHORT time.

22. Weather Behind a Cold Front behindSUBSIDING Weather behind cold fronts is usually characterized by SUBSIDING air usually a continental polar (cP) air mass usually cloudless generally stable which limits cloud development

23. Stationary Front A front between warm and cold air masses that is moving very slowly or not at all. Air flow is parallel to the front Symbol

24. Occluded Front Symbol Warm air becomes suspended over the cold and cool air. Weather near an occluded front is complex

25. Occluded Front Cold-type occluded front Cold-type occluded front In the Rockies Weather resembles cold fronts Cold front air is colder than the cool air it is invading. Warm-type occluded front Warm-type occluded front Pacific Coast Milder maritime air invades cP air

26. Dry Lines A narrow zone where there is an abrupt change in moisture. When dry continental (cT) air converges with humid maritime tropical (mT) air. Denserless dense Denser cT air acts to lift the less dense mT air. Seen commonly in the Desert Southwest