Download Weather & Climate Chapter 9

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
to
sea 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
(1)
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)
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 midlatitude 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