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. 9-2 (a)

Cyclones and Fronts--See Fig. 9-2

• Since air goes around the Low the way it does, it promotes advection of warm air northward ahead of the Low and cold air southward 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 9 to 120 miles wide, quite narrow when compared to the length of the air masses

Fronts

• An air mass on one side of the front may __________ toward an air mass on the other side of the front
• 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. 9-3

Fronts--See Fig. 9-3

• 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 becomes 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. 9-5

Fronts--See Fig. 9-5

• 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

Dryline--Fig. 9-10

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

• Mult. Choice Question

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. 9-11

Cyclone Life Cycle--Fig. 9-11 (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. 9-11 (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. 9-11 (c)

Cyclone Life Cycle--See Fig. 9-11 (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. 9-11 (d)

Cyclone Life Cycle

• The occlusion began _______ the Low center, 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. 9-11 (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. 9-11 (f)