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CHAPTER 9

CHAPTER 9. WEATHER PATTERNS. Front. Zone of conflict between air masses of differing temperature / humidity

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CHAPTER 9

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  1. CHAPTER 9 WEATHER PATTERNS

  2. Front • Zone of conflict between air masses of differing temperature / humidity • Names was coined by Norwegian meteorologists during WWI … possibly because the oscillations and “flare-ups” in the boundary areas of air masses reminded them of the long battle lines of Europe (Miller and Anthes)

  3. Norwegian Cyclone Model Links cloud pattern, precipitation [duration, type, intensity], wind, barometric activity, visibility, temperature, and air quality to sea level pressure, fronts, wind direction and air flow aloft

  4. Frontal Weather Weather occurring at a front depends on: (1) amount of moisture available (2) stability of the air forced upward by air mass contact (3)the slope of the front (4) speed of frontal movement (5) upper air flow

  5. Frontal Weather, cont Passing Through a Front (1) temperature change (2) temperature-dew point spread change (3) wind change (speed; direction; shear; eddies) (4) pressure change

  6. Aside Frontal weather may not always follow the neat explanations presented. Like all weather components, they react in relative ways defined by interacting environmental factors

  7. Fronts • There are four types of fronts • Warm front – advancing warm air replaces cooler/cold air --- N.H. they occur on the east side of low-pressure cells and are followed by cold fronts --- they move at about 15 mph (about ½ rate of cold front) ---very shallow frontal surface (friction on cold air; air erosion)

  8. Fronts, cont • Warm front, cont --- warm front weather may extend hundreds of miles in advance of the front (clouds up to 48 hrs and 1000 mi ahead) --- generally not an extensive precipitation producer. Esp. where front has stable warm air rather than unstable warm air

  9. Fronts, cont Cold Front – cold air replacing warmer air --- frontal surface very “wedge-like” (faster moving cold air; friction) --- In N.H. tends to lie NE-SW and move E or SE --- advances at about 20 mph – faster in winter

  10. Fronts, cont (2) Cold Front, cont --- front may extend several hundred miles horizontally, but steepness of front means frontal weather is restricted to a narrow band --- steep front results in abrupt lifting of warm air --- weather is frequently stormy and violent (esp. if warm air overrunning front is unstable), though brief

  11. Fronts, cont Squall line - May precede fast-moving cold fronts where high altitude winds above the front prevent warm air from rising… 100 to 150 mi in advance of the front strong leading winds force warm air to rise explosively - An unbroken line of black, ominous clouds 40,000’ into the air - May be extremely turbulent (may be excess of hurricane speed winds) and bring torrential rains

  12. Quotes and People From Tornado Tuesday (all from Leaf-Chronicle/TV5) - “It’s a war zone.” (Mont Co Sheriff’s Dept) • “I hit the wall, and then I hit the ceiling, and I must have blacked out because I don’t remember landing.” (Jessica Dover, 15) • “I think I messed up my four-wheel drive trying to get there.” (Dave Dover, Jr) • A tumultuous past 12 hours for the fine people of Dickson County” (TV5)

  13. “In addition to shutting off her power Moore said the storm took out a section of a fence surrounding her backyard pool.” • “We’re sitting in the closet, my two girls and our dog. We have our flashlight. I called my husband to see if we need to get in the basement. … We keep running out and getting cheese.” (Tracy War man) • “There are multiple homes in that area [Sango] either damages or destroyed. We’re urging those people to stay home” (Det. V. Lewis)

  14. Fronts, cont (3) Occluded Front - Occurs when a cold front overtakes a warm front and is forced aloft forming a secondary front with the air over which warm air is passing --- can be a: cold occlusion – air behind the cold front is colder than cool air it is replacing warm occlusion – air behind cold front is warmer than cold air it is replacing

  15. Fronts, cont • Resulting weather can be complex depending on occlusion type and where in the occlusion process you are • Precipitation may occur at anytime during the occlusion … most common when warm air of the occlusion is being forced to rise

  16. Fronts, cont Stationary Front • Brought about when the flow on both sides of a front are parallel to the front --- front does not advance (stalls for a time) --- winds are generally parallel in the contrasting air masses --- can be one of the most boring weather patterns, or contributor to the creation of monsters

  17. Mid-Latitude Wave Cyclone • Recurrent patterns of migrating mid-latitude low pressure centers • Explained as a factor of the polar front theory of the Norwegian Cyclone Model … under the Model, an explanation of the behavior of cP and mT air masses • When they develop, they do so in a sequence pattern of change each lasting 12-24 hrs [following text diagrams, p. 255]

  18. Mid-Latitude Wave Cyclone, cont Formation • Two air masses of differing characteristics have winds blowing parallel, but different directions (a) • When conditions are favorable (ex: meandering of high altitude air) cold air will begin to push under warm air and warm begins to “spread” over the cold – a “wave” is formed distinct fronts are being delineated (b)

  19. Mid-Latitude Wave Cyclone, cont - This “wave” can have two results --- action of warm air creates lower pressure and fuels further cold air invasion --- the co-mingling of the air masses can slow/die and a stationary front returns

  20. Mid-Latitude Wave Cyclone, cont Occlusion • Because it is generally faster moving, the cold front begins to “close up” on the warm front separating more warm air from the ground (c) and intensifying the ground-level low • Finally, a fully developed cold occlusion forms with a low pressure center at its core (d; e) --- period of max. winds/ storm

  21. Mid-Latitude Wave Cyclone, cont • The pressure gradient erodes. Any remaining pressure difference disappears. (e) • A new frontal line is established at the boundaries of the cold and warm air masses

  22. Mid-Latitude Wave Cyclone, cont The air masses have not been stationary during cyclonic development and disappearance. Westerly winds have moved the air masses from the SW to NE. As the low developed and cold air wrapped around and under the warm air, a warm front has moved eastward, followed closely by a cold front

  23. Mid-Latitude Wave Cyclone, cont Lowering air pressure precedes the warm front with stormy / generally bad weather to in turn be followed by higher pressure air and clearing and cooling. Warm fronts, closely followed by cold fronts move endlessly across NA.

  24. Mid-Latitude Wave Cyclone, cont Typical Mid-latitude Cyclone Source Regions (1) Gulf of Alaska Low (2) Alberta Clipper (3) Colorado Low (4) Gulf Low (5) Hatteras Low

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