CHAPTER 9

WEATHER PATTERNS

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)

Norwegian Cyclone Model

Links cloud pattern, precipitation [duration, type, intensity], wind, barometric activity, visibility, temperature, and air quality

tosea level pressure, fronts, wind direction and air flow aloft

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

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

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

Fronts

- There are four types of fronts(1) 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)

Fronts, cont

(1) 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

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

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

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

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)

- “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)

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

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

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

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]

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)

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

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

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

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

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.

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