What’s the Weather Like Outside?

  • Well, if a Low is coming toward you, it’s about to turn foul

Passage of Cyclone—Fig. 9-12

Weather With a Cyclone--See Fig. 9-12

  • You notice clouds are common ahead of both the cold front and warm front, plus cloudiness is wrapped all around the Low

Weather With a Cyclone--Fig. 9-21 (b)

Weather With a Cyclone--See Fig. 9-21 (b)

  • When a cyclone is mature (occluded front has already formed), the cloud structure often resembles a comma on satellite

Weather With a Cyclone--Fig. 9-11

Weather With a Cyclone--See Fig. 9-11

  • Not surprisingly, the precip is found in the same areas as the cloudiness

Weather With a Cyclone--Fig. 9-12

Zonal Flow

  • True-False Question

 

 

 

 

 

 

Zonal Flow

  • True-False Answer
  • When the flow is zonal, the wind is nearly ____________, and mid-latitude cyclones ________ to develop

 Cyclogenesis

  • When a wave forms aloft, instead of zonal, the flow is __________--so there are _______ and _______
  • This upper-level wave, so important to instigating cyclogenesis, can be generated by surface factors such as ____________, land-sea _______ contrasts, or ocean current influences

Cyclogenesis

  • Surface Lows often form just ahead of an upper level _______ and centered ______ the jet stream

Cyclogenesis—Fig. 9-13

Airflow Aloft

  • Recall that Low (High) pressure systems cause surface _____________ (_____________) as air flows to the Low (from the High)
  • How could the pressure in the Low go down if air is coming into it? Adding air should make the pressure rise

Airflow Aloft

  • Something has to take out the incoming air to maintain the low pressure
  • ____________ aloft is needed
  • Since ____________ is air spreading out, ____________ above the Low takes air out of the column

Airflow Aloft

  • If ____________ aloft exceeds surface ____________, the Low will ________ (central pressure will lower)
  • If it is weaker than surface _____________, the Low will _____ (central pressure will rise)

Airflow Aloft

  • ____________ aloft occurs downstream of (ahead of) the _____
  • This is one reason surface Low development is favored ahead of the upper-air _______ (Fig. 9-13)

Airflow Aloft--See Fig. 9-13

  • Convergence occurs as air approaches the trough, so surface Highs are favored behind the upper-level trough

Airflow Aloft—Fig. 9-14

The remaining notes will not be covered in lecture, but you should read them and the relevant pages of the book. A few questions on Test #4 may come from this material. Look for hints as to the most relevant material.

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Patterns of Movement

  • Cyclonic disturbances that affect the U.S. often originate in the Pacific Ocean
  • As they traverse the Rockies, they sometimes dissipate
  • However, they can and do reform lee of the Rockies

Cyclogenesis Areas--Fig. 9-15

Typical Cyclone Paths--Fig. 9-16

Patterns of Movement

  • Some redevelop in Canada and move southeast to the Great Lakes before turning northeast
  • In winter, these are often referred to as Alberta Clippers because they form in the province of Alberta and move rather quickly--bringing cold air and some snow to the Upper Midwest

Patterns of Movement

  • The most common area for Midwestern Lows to form is E Colorado
  • These Lows could be reforming from Pacific disturbances, or they could be brand new formations due to unstable Gulf of Mexico air advecting in

Patterns of Movement

  • The Colorado Lows generally move SE at first, before taking an easterly path for a bit
  • Then they could continue eastward, or they could swing to the NE, or even N
  • In winter, when they track east to SE Kansas, and then turn NE through Missouri, that’s when Iowa can get big snowfalls

Patterns of Movement

  • Other cyclones can form farther south by New Mexico and Texas
  • These usually move eastward and pass to the south of the Midwest
  • Cyclones can form east of the Appalachians and turn north along the Atlantic seaboard and cause a "Nor'easter"

Cyclone Migration

  • The 500 mb level is often called the "steering level" for mid-latitude cyclones
  • The cyclone path tends to be directed by the 500 mb wind directions
  • The speed of the cyclone is generally about half as fast as the 500 mb wind speeds

Cyclone Migration--Fig. 9-17

Anticyclonic Weather

  • Winter anticyclones often move southeast from Canada to bring cold air to the United States after a cyclone has passed
  • If the anticyclone came from the Arctic, it would likely be a cA air mass and bring a cold air outbreak to the country

Anticyclonic Weather—Fig. 9-18

Blocking Highs

  • Notice in the last picture, the cyclone and fronts to the west were being blocked by the High to the east
  • When Highs are strong and cover a large area, they often will not move for weeks and are called Blocking Highs

Blocking Highs

  • Once in place, the stagnant Highs block the eastward migration of cyclones, forcing them to move poleward instead
  • Thus, areas under the High get dry conditions, while other areas continually get Lows in the area and receive a lot of precip

Blocking Highs

  • Also, since Highs inhibit vertical motion, and cause light horizontal winds, air pollution episodes often occur in large cities affected by blocking Highs
  • The pollutants released cannot rise up and disperse and instead build up near the surface causing humans to breathe them in