Weather Patterns
- Chapter 9
- Mult.
Choice Question
Mid-Latitude Cyclone
- Mult.
Choice Answer:
- When _____ traverse the U.S. they
are called mid-latitude cyclones
- There are usually one or more
_______ extending from the Low center
Cyclones and Fronts--Fig. 9-3
Cyclones and Fronts--See Fig. 9-3
- Since air goes around the
Low the way it does, it promotes advection of warm (cold) air northward
(southward) ahead of (behind) the Low
- So warm fronts are usually
found ahead of the Low and cold fronts behind it, at first
Fronts
- Before we go further with the
polar-front theory, let’s talk about the different types of fronts
- Fronts indicate boundaries
between air masses of different densities
- These boundaries are
generally from 15 to 200 km wide, quite narrow when compared to the length
of the air masses
Fronts
- One of the air masses may
__________ toward the other
- This causes air from the
less dense (________) air mass to lift up over the denser (________) air
mass--this process is called "_____________"
- When the _______ air is
lifted, it can generate clouds and precipitation, which is why precip can be found near fronts
Fronts
- A warm front is indicated
when an area was occupied by _____ air that is moving _____ and is being
replaced by _____ air
- So the warm front position
is dictated by the movement of the retreating _______ air, not the
advancing _______ air
Warm Front—Fig. 9-4
Fronts--See Fig. 9-4
- The warm air overruns the
cooler air on a gentle slope, so on the cool side of the warm front you
generally get widespread cloudiness, and precipitation is often light to
moderate, over a widespread area, and could last a while
- Thunderstorms can form during
the warm months, when the overrunning warm air is very unstable
Fronts
- By contrast, a cold front is
a boundary between a colder air mass ___________ on a warmer air mass
- The slope of the cold front
is ________, so warm air is forced to rise more ____________ along it
Cold Front—Fig. 9-6
Fronts--See Fig. 9-6
- Since upward motion is more
vigorous along a cold front, heavier precipitation is usually found along
it than along a warm front
- However, the precip is usually of shorter duration and not as
widespread
Fronts
- A stationary front is
indicated when the _______ air mass is not moving toward or away from the
_______ air mass
- Thus, the boundary between
the air masses does not move much—_____________
Fronts
- There can still be
precipitation along stationary fronts, usually similar to what falls ahead
of a _____ front
- If the stationary front does
not move for several days, the same areas could get rain every day
resulting in __________
Fronts
- Usually the _____ air
behind the cold front is advancing _______ than the _____ air ahead of the
warm front is retreating
- This results in the _____
front moving faster than the _____ front
- Eventually the _____ front
catches up to the _____ front forming an __________ front
Fronts
- The _____ front catches up
to the _____ front first at positions close to the Low (winds are _______
closer to Lows and there is _____ distance between the fronts there)
- Precipitation with occluded
front is generally like what occurs along the _____ front
- Since it takes time for
occlusion to occur, by the time it happens, the Low is usually at its
__________ stage and will weaken soon
Fronts—Fig. 9-8 (a), (b), (c)
Fronts
- Most often, the _____ air
behind a cold front is associated with a ___ air mass, and _____ air ahead
of the cold front is associated with a ___ air mass
- _____ air ahead of warm
fronts is often a more moist version of ___, but it's sometimes ___ as
well if near the East Coast
Fronts
- A dryline
is indicated when air masses of like temperatures but different amounts of
________ collide
- Hot, dry air is ________
the dryline, with very warm, moist air ______ of
it
- Thus, usually a ___ is
behind the dryline, with a ___ ahead of it
Fronts
- Drylines
are thus mostly spring and summer ______ ______ phenomena
- The dry air ________ the dryline is __________ than the moist air ________ of it
[avg. molecular weight of dry air (29) is greater than molecular weight of
water (18)]
- The ______ air then lifts
the ______ air, resulting in upward motion and precip
Fronts
- Drylines
are associated with vigorous lines of thunderstorms and often provide the
right environment for tornado formation
- A couple times a year, a dryline can move east far enough to reach Iowa and prompt a
severe weather outbreak
Dryline--Fig. 10-11
Cyclone Life Cycle
- The _____________ theory is
an idealized model of how a mid-latitude cyclone develops
- In real life, it doesn’t
necessarily happen this way, but it is often very _________
Cyclone Life Cycle
Cyclone Life Cycle
- Mult.
Choice Answer:
- Often, two air masses of different
___________ are next to each other, but at first neither is advancing
toward or away from the other
- Thus, we may start out with
a ___________ front
Cyclone Life Cycle—Fig. 9-10
Cyclone Life Cycle--Fig. 9-10 (a)
Cyclone Life Cycle
- We need some sort of
disturbance to come along and kick-start things
- This is often a
"_____" of instability aloft that moves in and causes movement
of air to begin and __________ to fall in the center
- There is usually ______________
aloft associated with this wave
Cyclone Life Cycle—Fig. 9-10 (b)
Cyclone Life Cycle
- The cyclonic circulation is
soon established, with _____ air moving south _______ the Low (cold front
forms)
- While, ______ of the Low,
_____ air is replacing _____ air that has retreated (warm front forms)
- Meanwhile, the disturbance
has __________ the Low
Cyclone Life Cycle—Fig. 9-10 (c)
Cyclone Life Cycle--See Fig. 9-10 (c)
- At this time, warm air has
been lifted over cold air along both fronts, so we have clouds and some
precipitation forming
Cyclone Life Cycle
- As time goes on, the _____
air coming around the back side of the Low catches up to the front edge of
the _____ air (the _____ front catches up to the _____ front)
- Then the __________ begins,
and the cyclone is at its most ________ stage
Cyclone Life Cycle—Fig. 9-10 (d)
Cyclone Life Cycle
- The occlusion began ________
the Low, and now it continues along the _______
- This effectively lifts all
the ______ air off the ground
- So our steep surface
_____________ gradient we used to have no longer exists
Cyclone Life Cycle—Fig. 9-10 (e)
Cyclone Life Cycle
- Once that surface ______
gradient is gone, there is no more ______ surface air to lift
- The _______________ aloft
weakens as well
- Now the Low struggles to
strengthen further and may __________, having done its job of eliminating
____________ gradients and getting every place to be similar ________ at
the surface
- Now there is __________
Cyclone Life Cycle—Fig. 9-10 (f)