AOS 3 Final Flashcards
1
Q
What does planetary scale circulation mostly result from?
A
1) The difference of temperature between the equator and the poles (PGF)
2) The rotation of the planet (Coriolis)
3) Distributions of land and water over the planet (friction)
2
Q
Describe the polar front
A
Cold air from the pole meets warmer air from the subtropical high at the polar front
3
Q
Upper winds are well-predicted within the ________ but completely off for the _________
A
Hadley cell, Ferrel cell
4
Q
In which direction does the air in the polar jet stream
flow in the Northern Hemisphere?
A
From west to east
5
Q
Compare the polar jet stream and subtropical jet stream.
A
- Polar jet is very strong (110 mph in the winter) & effects US weather development
- Subtropical jet is much weaker & brings warm moist air to the US
6
Q
Describe rossby waves
A
- The largest of the atmospheric long waves
- three to seven Rossby waves circle the globe at any one time, and each has its own wavelength and amplitude
- Although they have preferred anchoring positions they do migrate eastward (very slowly)
7
Q
What happens when rossby waves “break”?
A
- seen as oscillations in the polar jet stream
- By moving the jet stream around, they can affect the weather
- “Breaking” Rossby waves can be considered a source of low and high pressure centers
8
Q
What happens to ice crystals in the Bergeron process
A
Ice crystals grow rapidly at the expense of supercooled droplets (via net deposition and net evaporation)
9
Q
Describe the process of precipitation.
A
- Unsaturated air rises and cools by adiabatic expansion, reaching saturation
- Presence of CCN allows for condensation, leading to formation of tiny droplets
- Droplets grow by additional condensation
10
Q
For it to rain, droplets need to fall ________ than the vertical velocity in the updraft
A
faster
11
Q
Describe the terminal velocity state of equilibrium
A
drag force equals gravitational force
12
Q
Describe Collision Coalescence
A
- Warm clouds grow
- cloud droplets collide and stick together
- Promoted by large collector drops, which have high terminal velocities so they fall faster and collide with smaller drops
13
Q
What is riming?
A
liquid water freezes onto ice crystals
14
Q
What is aggregation?
A
ice crystals collide and merge via thin coating of liquid water (ice crystals come together to form snowflakes)
15
Q
What limits the growth of ice particles by the Bergeron process?
A
- amount of supercooled water in the cloud
- time an ice particle remains in the cloud
16
Q
Describe snow crystals.
A
- single crystals
- depend on temperature and degree of supersaturation (for ice)
17
Q
Describe snowflakes. How do they differ for warm/cold clouds?
A
- aggregates of snow crystals
- mostly formed by riming in warmer clouds and form dense wet snowpack (snowballs)
- formed by aggregation in colder clouds making less desne powdery snowpack (skiing)
18
Q
What is lake effect snow?
A
- Develop as the warm lake waters evaporate into cold air
- Friction over land reduces winds and creates a lifting mechanism downwind from the lake
19
Q
Compare rain in warm and cool clouds.
A
- Cool clouds:
Most middle-latitude precipitation starts out as snow, then melts to rain - Warm clouds:
Predominant in the tropics, starts and falls as rain
20
Q
Compare graupel and hail
A
Graupel: Rimed ice crystal
Hail: Concentric layers of ice built around graupel (wet growth results in solid ice)
Both results from in-cloud processes!
21
Q
What are possible types of precipitation that could originate from a cold cloud?
A
- Rain
- Snow
- Hail
- Graupel
- Freezing rain
- Sleet
22
Q
Compare the different spatial scales
A
Global Scale
- planetary waves
- hadley cell
Synoptic scale
- extratropical cyclones
- weather systems
Mesoscale
- thunderstorms
- tornadoes
(smaller length and time scales as you move down)
23
Q
Describe the single-cell model
A
- Simplified model of the general circulation proposed by Hadley (1735)
- Based on idealized water planet with no effects of rotation (no CF, no friction)
- Deflection by Coriolis
- One single cell in each hemisphere
- First model of a thermally driven circulation
24
Q
Describe the three-cell model
A
- Improved model of the general circulation proposed by Ferrel (1865)
- Each hemisphere divided into three cells
- Not perfect, but more realistic thansingle-cell model
25
Describe surface winds
- Surface winds fairly well predicted (note that “westerlies”= from the west)
- Main differences are caused by land-water contrasts
26
What are air masses?
large volumes of air with “uniform” temperature and humidity
27
What are fronts?
boundaries between different air masses
28
What are source regions?
areas of the globe where air masses form
29
What is required for an air mass to form?
Long-term heating or cooling of large bodies of air must remain over a source region for a substantial length of time for an air mass to form (low and high latitudes)
30
Describe the basic characteristics of air masses and source regions
1. Moisture:
* Continental (c) = dry
* Maritime (m) = humid
2. Temperature:
* Tropical (T) = warm
* Polar (P) = cold
* Arctic (A) = very cold
31
Is the position of the Polar Jet Stream more or less constant during each season?
No, rossby waves can cause the jet to meander
32
Describe the Maritime Polar from the Gulf of Alaska
* Cold, humid, unstable
* The main trajectory into US depends on the position of the Polar Jet Stream
* During winter, it can enter the US through California
* One of the two major sources of (winter) precipitation in California
* Already cold and humid + adiabatic expansion = precipitation
33
Why is California's snowpack important?
- The snowpack is particularly important to California’s drought picture because when the snow melts, the water runs off and refills the state’s reservoirs
- provides roughly 1/3 of the state’s overall water supply
34
What happened in Winter of 2018 in LA?
- really dry winter for CA
- only 1 major precipitation event (jan 7-8)
- polar jet stream was basically locked in one position for all of December so there was almost no rain
- dipped to South for just a few days and there was rain
- shows jet stream position really determines if we have cold/wet or warm/dry winter
35
Describe the Maritime Polar Air Mass from Atlantic
- Cold, humid, unstable
- Nor’easters: usually the air mass reaches the coast when it is caught up in a strong extra-tropical cyclone
36
Describe the Maritime Tropical Air Masses
- develop over warm tropical waters.
- warm, moist (high dew points), and unstable near the surface
- precipitation/thunderstorm source in southeast
- only occasional precipitation during summer (bc of cold ocean current in Pacific)
37
What is the pineapple express (atmospheric rivers)?
- relatively long, narrow regions in the atmosphere that transport most of the water vapor outside of the tropics
- second major source of precipitation in California
38
What are the two main sources of precipitation in Southern California?
1) The maritime polar air mass from the Gulf of Alaska
2) Atmospheric rivers bringing moisture from the tropical Pacific Ocean
39
In terms of synoptic pattern development, what happens 7 to 10 days before a precipitation event?
7 to 10 days before event:
1) Heavy rain over far western pacific
2) moisture plume extends northeast
3) strong polar jet
4) strong blocking high
40
In terms of synoptic pattern development, what happens 3 to 5 days before a precipitation event?
3 to 5 day before event:
1) Heavy rain shifts east
2) mixture plume extends further northeast
3) split jet forms
4) block weakens and shifts westward
41
In terms of synoptic pattern development, what happens at the time of a precipitation event?
Precipitation event:
1) Heavy rain shifts
2) deep tropical moisture plumes
3) extend jet
4) deep low heavy rain and possible flooding
42
What are reservoir conditions and droughts in California like right now?
Drought:
- Bad drought last year
- basically none right now
Reservoir conditions:
- Most reservoirs in the state are above the historical average
- This is in part due to the snow from last winter
43
What type of clouds is Collision Coalescence most efficient in?
clouds with large distribution of droplet sizes and strong updrafts
44
________ occurs when a mountain range forces air to rise.
orographic lifting
45
What is the most common mechanism of cloud formation?
lowering the air temperature to the dew point by adiabatic cooling of rising air
46
You would most likely expect a rain shadow on which?
- west side of the Andes.
- west side of the Coast Range in California.
- west side of the Sierra Nevada Mountains.
- east side of the Cascade Mountains in the Pacific Northwest.
east side of the Cascade Mountains in the Pacific Northwest.
47
What type of air will keep rising after an initial upward push?
statically unstable air
48
What type of air neither rises on its own following an initial lift nor sinks back to its original level?
statically neutral air
49
When the ELR exceeds both the DALR and the wet adiabatic lapse rate of a parcel of air, that air parcel contains what type of air?
absolutely unstable air.
50
When is the lower atmosphere most likely to have the steepest ELR?
mid-day
51
What is the most important mechanism for stopping the rise of unstable air parcels?
encountering a layer of stable air
52
Clouds that are high and are always composed entirely of ice crystals are called what?
cirrus
53
In what region is Collision-coalescence the predominant cause of precipitation?
the tropics
54
What is the process by which supercooled water droplets freeze onto falling ice crystals called?
riming
55
The collision-coalescence process is dependent upon what?
dependent upon the different downward velocities of different-sized droplets
56
Aggregation is facilitated by what?
a thin coating of water on ice crystals.
57
Which of the following cloud constituents would have the highest terminal velocity?
- raindrops
- hailstones
- ice crystals
- condensation nuclei
hailstones
58
What is the most important principle underlying the Bergeron process?
For a given temperature, the saturation vapor pressure of ice is less than that for supercooled water.
59
Lake-effect snowfall requires what?
requires that the lake be relatively warm.
60
What does rain usually begin as in the midlatitudes?
snow
61
What are 3 processes snow results from?
- deposition.
- riming.
- aggregation.
62
What scale do cyclones, anticyclones, troughs, and ridges, covering hundreds or thousands of square kilometers, occur on?
Synoptic scale
63
Meridional flow is characterized by flow that is ________; while zonal flow is characterized by flow that is ________.
north/south; west/east
64
List the four scales of atmospheric motion from largest to smallest
planetary, synoptic, meso, and micro
65
What does the hadley cell model say happens to air and where does it say it happens?
air rises at the equator and sinks at the poles
66
What area does the hadley cell actually cover?
just areas near the equator
67
What amount of precipitation is there in areas close to the ITCZ?
they receive abundant precipitation
68
What are the southward bulges in the patterns called? What are the northward bulges called?
Troughs (southward) and ridges (northward)
69
The polar front is a region marked by _________.
a sharp change in horizontal temperature
70
Winds in the upper atmosphere are...
- faster in summer than in winter in both the northern and southern hemisphere.
- westerly in both the northern and southern hemisphere.
- westerly only in the southern hemisphere.
- westerly only in the northern hemisphere.
westerly in both the northern and southern hemisphere.
71
Westerly winds in the upper atmosphere at mid-latitudes in the Northern Hemisphere
- are stronger in summer than in winter.
- are the reason most mid-latitude storms move from west to east.
- are moving perpendicular to 500 mb contours.
- are a result of a pressure gradient that moves air from the poles toward the equator.
are the reason most mid-latitude storms move from west to east.
72
The Ferrel Cell is associated with the ________
mid-latitudes
73
What are jet streams?
bands of high-speed wind found at elevations of 9-15 km
74
In which direction do jet streams generally travel?
west to east
75
Where are the two main hemispheric jet streams located?
between 50-60 degrees latitude and at about 30 degrees latitude
76
What are rossby waves?
major undulations in the path of a jet stream
77
How can the jet stream return to normal zonal flow after Rossby waves build?
through separation of a mass of cold air from the jet stream
78
The Hadley model of atmospheric circulation assumes __________.
a planet covered entirely by water.
79
The Hadley cell...
- creates a high-pressure area at the equator.
- does not account for the formation of trade winds.
- originates with strong solar heating at the equator.
- does not explain upper air movement in the troposphere.
originates with strong solar heating at the equator.
80
The northeast trade winds are the result of what?
air flowing from the subtropical high to the ITCZ
81
Usually, the pressure gradient force would be strongest at what level?
700 mb level
82
Which of the following best defines an air mass?
- A large body of air containing fronts
- A large body of air with similar temperature and moisture characteristics
- A large body of air with uniform temperature and no precipitation
- A large body of air residing over a water mass
A large body of air with similar temperature and moisture characteristics
83
Where do maritime air masses originate?
over water?
84
Which air masses are cold?
polar and arctic
85
Which air masses are warm and humid?
tropical and maritime
86
Which air masses are warm and dry?
continental and tropical
87
Which air masses originate at high latitudes?
polar and arctic
88
Continental is a term referring to _______
dry air masses
89
The following air masses are listed in what order?
- a wintertime cP air mass
- a wintertime mP air mass,
- a wintertime mT air mass
- a summertime cT air mass
coldest to warmest
90
Rainstorms are generally shorter when ________ comes in quickly and collides with ________.
a cold air front, a warm air front
91
When warm air collides with a cold air front, its slow journey up the slope of the cold air front causes what?
longer rains
92
Where are air-mass source regions least likely to be found?
middle latitudes (because the weather is to variable)
93
Most of the air masses in the central part of the United States are either _______ or _______.
continental polar or maritime tropical
94
Which of the following best describes a front?
- A narrow boundary separating different air masses
- A narrow boundary between air masses where cloudiness occurs
- A narrow boundary between air masses where precipitation occurs
- A narrow boundary separating cold air from colder air
A narrow boundary separating different air masses
95
If you observe short, intense, scattered rainfall as a front passes, you are likely to be experiencing what?
A cold front
96
What do we find between a cold front and a warm front?
a warm, moist, and unstable air mass
97
What type of front has a mass of warm air cut off from the surface?
occluded front
98
Why do the sources of air masses occur only in low and high latitudes?
middle-latitude weather is too variable.
99
Northeasters often bring what?
heavy snowfall
100
Which air mass is responsible for bringing virtually all of the moisture that impacts the United States east of the Rocky Mountains?
maritime tropical
101
After passing over a series of mountain ranges, what happens to maritime polar air?
it becomes drier
102
Which of the following best describes midlatitude cyclones that move along the U.S. East Coast and bring moist maritime air towards New England, often producing cold winds and heavy snowfall in winter?
- Southeasters
- Northeasters
- Drylines
- Overrunning
Northeasters
103
Which front does not separate tropical from polar air masses?
occluded front
104
________ fronts usually have showery precipitation while ________ fronts usually have continuous precipitation.
cold, warm
105
What is the first type of cloud an observer will see when a warm front is approaching?
cirrus
106
Which of the following frontal systems often move with greatest forward velocity?
- occluded front
- warm front
- cold front
- stationary front
cold front
107
Drylines in the United States
- can lead to a situation in which warm, moist air overtakes dry, cold air.
- can lead at worst to mild thunderstorms.
- separate saturated air from unsaturated air.
- typically have continental air to the west.
typically have continental air to the west.
108
According to the polar front theory, midlatitude cyclones form along _______.
a boundary separating polar air and warmer air to the south.
109
The areas of greatest absolute vorticity occur along _____.
the trough axis.
110
The region of lowest absolute vorticity occurs near ________.
the ridge axis.
111
Which of the following are associated with the formation and intensification of surface mid latitude cyclones?
- upper-level divergence and lower-level divergence
- upper-level divergence and lower-level convergence
- upper-level convergence and lower-level convergence
- upper-level convergence and lower-level divergence
upper-level divergence and lower-level convergence
112
At which level does the wind direction most closely approximate the path that a midlatitude cyclone tends to take?
The 500 mb level
113
In what conveyor belt model, does the belt enter the storm flowing westward toward the surface cyclone?
cold conveyor belt
114
According to the polar front theory, cyclogenesis begins when what?
cold air begins to advance southward and warm air begins to advance northward.
115
In which of the following areas is cyclogenesis most likely to occur?
- In the tropics
- In regions of strong temperature contrasts
- Beneath an upper-air ridge
- In river valleys
in regions of strong temperature contrasts
116
Mid-latitude cyclones in the Northern Hemisphere typically travel primarily in what direction?
east
117
The warm sector between the warm and cold fronts is generally characterized by what?
clear conditions
118
When does a mid-latitude cyclone reach its most intense stage?
the storm system undergoes occlusion
119
Earth vorticity...
- is a function of latitude.
- does not affect absolute vorticity.
- is greatest at the Equator.
- is vorticity relative to the earth's surface.
is a function of latitude.
120
Rossby waves can generate what?
upper-level convergence and upper-level divergence
121
As a cold front moves into a warm air mass, what happens?
there is a substantial horizontal pressure gradient at heights of a kilometer or more above the surface of the two air masses
122
What do zonal patterns show?
little north-south displacement between contour lines on a 500 mb map.
123
________ motions within anticyclones generally bring ________ skies
Sinking; clear
124
What does thunder result from?
the explosive expansion of air.
125
What are some characteristics of severe thunderstorms?
- hailstones one inch or larger in diameter
- wind speeds in excess of 58 mph
- tornadoes
126
What is required for the formation of a mesocyclone?
vertical wind shear.
127
Waterspouts tend to form in areas with ________ water and ________ atmospheric conditions
warm; unstable
128
A typical cloud-to-ground lightning event consists of what?
several distinct steps that look to the human eye like a single lightning strike.
129
Which portion of the lightning process is the most visible?
return stroke
130
Supercell storms...
- are larger than mesoscale convective complexes.
- account for a majority of tornadoes.
- are not readily visible on Doppler radar.
- do not exhibit any rotational aspects.
account for a majority of tornadoes.
131
An outflow boundary is which?
- the leading edge of cold air from a thunderstorm downdraft.
- can be clearly seen on radar images.
- a favorable place for future severe storm development, especially if two intersect.
- all of the above
all of the above
132
A common Doppler radar signature associated with a supercell tornado is the
hook echo
133
air mass thunderstorms extinguish themselves through what?
through the formation of extensive downdrafts
134
Tornadoes are associated with what clouds?
cumulonimbus clouds
135
Which is NOT a significant factor in explaining why the United States experiences so many tornadoes?
- spans wide range of latitudes.
- relatively flat in eastern portion.
- bordered by a cold ocean current on west coast.
- lack of major east-west mountain range in east.
bordered by a cold ocean current on west coast.
136
Tropical storms in the East Pacific
- eventually pass over Hawaii.
- miss Mexico.
- generally move westward.
- are the typhoons that hit Asia.
generally move westward.
137
Hurricanes
- are smaller than Mid-latitude cyclones.
- never spawn tornadoes.
- generally have very little thunder and lightning.
- are larger than Mid-latitude cyclones.
are smaller than Mid-latitude cyclones.
138
How many fronts do hurricanes have?
none
139
Why is a hurricane almost symmetric (round) in shape?
because a hurricane has only warm air.
140
A hurricane typically...
- is a cold-core cyclone.
- contains bands of precipitating clouds that spiral clockwise in the - Northern Hemisphere.
- forms over relatively cool ocean waters.
- has temperatures near its center that are much higher than surrounding temperatures.
has temperatures near its center that are much higher than surrounding temperatures.
141
Where do the strongest winds within a hurricane occur?
in the eyewall.
142
Hurricanes are
- cold-core systems with high pressure aloft.
- warm-core systems with low pressure aloft.
- warm-core systems with high pressure aloft.
- cold-core systems with low pressure is a.
warm-core systems with high pressure aloft.
143
Horizontal air flow in a hurricane goes in what direction?
counter-clockwise at the surface, clockwise aloft.
144
Where does the strongest rainfall within a hurricane occur?
in the eyewall.
145
Air ________ inside the eye of a hurricane.
sinks and warms adiabatically
146
A shrinking hurricane eye generally means
- decreasing levels of heavy rain.
- high winds in the eye.
- an intensifying hurricane.
- a storm that is evolving into a tropical storm.
an intensifying hurricane.
147
What do the shapes of ice crystals depend on?
temperature and degree of supersaturation (for ice)
148
What happens to form freezing rain?
1) Snow melts
2) hits cold air
3) freezes on ground and objects
149
What happens to form sleet?
1) snow smelts
2) refreezes into sleet as it travels into cold air
150
What changes do fronts bring about?
changes in temperature and humidity as one air mass is replaced by another
151
What are the 4 general types of fronts associated with midlatitude cyclones
- cold front
- warm front
- stationary front
- occluded front
152
If a tank with half warm water and half cold water had a wall separating them and the wall was then taken away, what would happen?
- cold water would slide under the warm water
- b/c the cold is more dense and moves the warm water upward as it advances
153
What is a cold front?
mass of cold air advancing toward warm air
(usually continental polar advancing toward maritime tropical)
154
What are cold fronts typically associated with?
- heavy precipitation (rain/snow)
- rapid temperature drops
155
What do doppler radars do?
use microwave radiation to calculate precipitation intensity and type (rain, snow, hail, etc.)
156
What are warm fronts?
mass of warm air advancing toward cold air
(usually maritime tropical advancing toward maritime polar)
157
What are warm fronts typically associated with?
- slow predictable changes
- light precipitation for days
- frontal fog
158
Which moves faster: cold front or warm front?
- Cold front moves faster over a shorter distance
- Warm front moves slower over a longer distance
159
Is warm or cold more dense?
Cold: more dense
Warm: less dense
160
What are stationary fronts?
- fronts that do not move or move very slowly
- 2 unlike air masses side by side
161
Describe occluded fronts
- warm front slower than cold front
- both fronts meet and the warm air mass moves up
- at the ground, 2 cold air masses (1 rlly cold, 1 cool)
162
Compare the 2 types of occlusion
Cold-type occlusion:
- occurs in eastern half of continent
- a cold front associated w/ continental polar air meets a warm front with maritime polar air ahead
Warm-type occlusion:
- occurs in western edges of continents
- cold front associated with maritime polar air migrates to an area occupied by continental polar air
163
To observers at the ground, the weather for a cold-type occlusion looks like a _____ front when it
approaches, and a _____ front after it passes.
warm, cold
164
What are drylines?
boundaries between humid air and dry air, without large temperature differences
165
What are drylines typically associated with?
- thunderstorm development
- tornado outbreaks
166
True or false: At the same temp. and pressure, dry air is heavier than moist air.
True, moist air goes up because its lighter/less dense
167
Which is more dense, dry air or moist air?
- dry air is more dense
- moist air is less dense
168
What are the steps of the polar front theory? (formation/dissipation of midlatitude cyclones)
1) Stationary polar front
- continental polar air meets maritime polar
- air flows parallel to front in opp. directions
2) Frontal wave (cyclogenesis)
- frontal wave forms along front, giving rise to a cold & warm front
- low pressure center begins to form at junction btwn 2 fronts
3) Open wave (cyclogenesis)
- open wave forms around low pressure center
- precipitation along both frontal boundaries
- winder circles around and towards low presure center (friction)
4) Mature Cyclone
- pressure decreases at low pressure center (deepens)
- entire system moves toward east-northeat
- cold front moves faster than warm front
5) Occlusion
- marks peak in cyclone intensity/wind speeds
- then fronts occlude and cyclone intensity starts to decrease
6) Cut-off cyclone
- original front gradually disappears
- new staitonary front forms, leaving a cut-off weakened low pressure center
169
Describe precipitation in the mature phase of a midlatitude cyclone at different points
- heavy near low pressure center
- light near warm front
- heavy near cold front
170
What is vorticity?
- the spinning motion of air parcels on its own axis
- useful to describe the amount of ROTATION in the wind
171
What is planetary vorticity? What has it?
- vorticity due to the rotation of the planet
- any object/air parcel sitting on top of planet has vertical vorticity unless it is at the equator
172
What is relative vorticity?
- vorticity associated with wind motions
- rotation where wind causes air parcels to rotate
173
In the northern hemisphere, which center/direction is positive relative vorticity and which is negative? Are they associated with troughs or ridges?
Positive:
- counter-clockwise around low pressure center
- trough
- divergence
- cyclone
Negative:
- clockwise around high pressure center
- thumb points in
- convergence
- anti cyclone
174
What is the explanation for why cyclonic wind rotates counterclockwise and anticyclonic wind rotates clockwise?
Because the earth's rotation is counter-clockwise and cyclonic motion is the same direction as earth's rotation
175
If divergence aloft is stronger than the convergence at
surface, will the low pressure at the surface deepen, intensifying the storm, or will it decay?
the low will deepen
176
For convergence, which center and direction is it? For divergence?
Convergence:
- clockwise around high pressure center
Divergence:
- counter-clockwise around low pressure center
177
Are winds in the troughs supergeostrophic or subgeostrophic?
Subgeostrophic
178
Cumuliform clouds...
- are typically much wider than they are tall.
- form in absolutely stable air conditions.
- typically have higher water content than stratiform clouds.
- have very weak vertical velocities within them.
typically have higher water content than stratiform clouds.
179
Which air mass type generally has the highest dew point?
maritime tropical
180
What does the stepped leader do?
creates the pathway for the flow of electrons.
181
List in order of size from biggest to smallest:
- hurricane
- mid-latitude cyclone
- thunderstorm
- tornado
mid-latitude cyclone, hurricane, thunderstorm, tornado
182
Is an easterly wave favorable or unfavorable for hurricane development?
favorable
183
When do hurricanes lose strength?
When they move over cooler water or move over land.
184
What do hurricanes gain most of their energy from?
latent heat released by condensation.
185
Where is there an absence of CF? How does this affect hurricane formation?
At the equator, prohibits hurricane formation there
186
Why do almost all hurricanes form in the equatorial regions of the earth?
Because it has the largest source of warm water.
187
What does a hurricane's greatest threat to life and property along coasts comes from?
storm surge
188
After the trough, there is what?
upper level divergence
189
What do the predominantly zonal height patterns do?
* Prevent development of intense cyclones
* Usually mild atmospheric conditions at the surface
190
What do the predominantly meridional height patterns do?
Support cyclone development as vorticity changes between troughs and ridges
191
Surface systems move in ________ direction as the 500mb flow, at about ______ the speed (steering level)
the same direction, 1/2 the speed
192
Are upper-level winds stronger in winter or summer? How much stronger?
about twice as strong in winter
than summer
193
Describe the conveyor belt model of cyclones.
* Connects upper-level circulation with the surface cyclone system
* A good three-dimensional representation of mature cyclone
194
Anticyclones are associated with what weather?
clear skies and calm conditions
195
If anticyclones linger over a region for long periods of time, this can lead to what?
droughts and heat waves
196
Anticyclones are responsible for which specific wind conditions?
Santa Ana wind conditions
197
What are the 4 main types of lightning?
- cloud-to-ground
- cloud-to-air
- intra-cloud
- spider lightning
198
Describe the 4 steps of lightning formation.
1. Charge separation:
* Separation of positive and negative charges into different regions of cloud
* positive charges at top of cloud, negative charges in lower part of cloud
* Likely produced by interaction between ice crystals and hail
2. Stepped leader:
* rapid and staggered advance of a shaft of negatively charged air
* Advances about 50 m at a time in about a millionth of a second (1 microsecond)
* In between pauses of about 50 microseconds for electrons to pile up at the tip
* usually not visible
3. Ground spark:
* As the leader approaches the ground a spark is created
4. Return stroke:
* When the leader and the spark connect a flow of electrons illuminates the cloud by strokes, or return strokes
* Stroke propagates upward
* Current heat the air around to about 30,000K (5X the temperature of the surface of the sun)
* Dart leader follows, producing other strokes
* lightning flash is the combination of the strokes
199
Why do we have much less lightning in the northern part of the United States?
Because lightning formation requires warm moist air near the surface
200
Describe thunder formation.
* pressure & temperature increase suddenly
* causes surrounding air to expand violently REALLY fast
* extends outward for 1st 30 ft , after which it becomes an ordinary sound wave (thunder)
* lag in lightning strike and thunder b/c sound travels slower than light
201
Which are stronger: positive or negative lightning strokes?
- positive strokes stronger
- negative strokes less strong
202
Describe positive lightning strokes.
- Positive charges at the top of the cloud can result in lightning strikes that shoot positive charges to the surface, ahead of the storm
- (stronger than negative strokes)
203
What are Transient Luminous Events?
Rarely observed and poorly understood electrical phenomena occurring above large thunderstorms in the high atmosphere (above troposphere)
204
What are the key processes in development and evolution of thunderstorms?
1. Source of moisture
2. Unstable atmosphere
3. Lifting mechanism to initiate updraft
4. Vertical shear in wind
205
What does the US National Weather Service classify as a "severe" thunderstorm?
- has potential to threaten lives and property: must have at least1 of....
- hail with diameter of at least 1 in
- Winds in excess of 93km/h (68mph)
- tornadoes
206
Describe single cell thunderstorms. When and where do they form?
* most common and least destructive
* Relatively small, localized, and short-lived (~ 1hour)
* Not classified as “severe”
* Form in the absence of wind shear, with weak winds aloft
* Form away from frontal systems, triggered by surface heating or orographic lifting
207
What is the lifetime of a single cell thunderstorm?
about 1 hour (very short)
208
Describe Mesoscale Convective Systems
* clusters of thunderstorms (2 types)
* Mesoscale convective complexes (MCCs): circular clusters
* Squall lines: linear bands of thunderstorms
* (These can be severe thunderstorms with lifespan of 12+ hours)
209
What is a key factor of the structure of mesoscale convective systems (MCSs)? Describe.
- Wind shear...
- strong winds aloft push updraft ahead preventing the downdraft from suppressing updraft
- moisture source is not cut off!
210
What are derechos?
* large scale horizontal winds produced from strong downdrafts associated with MCCs
* long-lived (several hours)
* straight-line winds generally exceed hurricane-force
211
Where do Frontal Squall Lines form?
- Usually form in the warm section of midlatitude cyclones, just ahead of the cold front
- Sometimes form in front of drylines
212
Describe the development of an MCS Squall Line
1. Disorganized cluster of thunderstorms
2. Storms organize into an arc-shaped line (squall line)
3. One segment of the squall bows outward producing a bow echo
4. Widespread less-intense precipitation develops in the stratiform region behind the squall line
5. Stratiform region grows and a gust front detaches from storm
6. Thunderstorm decays leaving wide stratiform region and new thunderstorms develop
213
The lifting associated with the gust front in frontal squall lines produces what?
shelf clouds and roll clouds
214
Describe supercell thunderstorms.
* Contain a single updraft zone (unlike MCSs) and are more severe and powerful
* Characterized by the presence of a deep, persistently rotating updraft called mesocyclone
215
Where are supercell thunderstorms found?
* in the warm sector of a low pressure system
* in a north easterly direction in line with the cold front of low pressure system
- (often isolated from other thunderstorms)
216
How long do supercell thunderstorms last?
Can last for a few hours and spin up large tornadoes
217
We know that supercell thunderstorms form in environments characterized by strong shear in the mean wind. What can we say about the vorticity that exists in this environment?
Environment has mostly horizontal vorticity
218
What does the rotation in the mesocyclone come from? Describe.
- Comes from the vorticity that exists in the environment (caused by the wind shear)
- wind shear sets air spinning
- updraft bends the spinning air upwards
- the updraft rotates with the spinning column of air
219
What is a vortex?
a region in a fluid in which the wind (or water flow) rotates around an axis line, which may be straight or curved
220
What happens to a vortex when it gets stretched by the wind? Why?
It becomes more intense, because its vorticity increases to conserve angular momentum
221
What is a tornado? What must it be in contact with?
a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud
222
How long does a tornado last?
Usually last only a few minutes
223
Tornadoes are visible in the form of a ____________ with a cloud of rotating debris and dust beneath it
condensation funnel
224
What is the tornado intensity scale?
- Enhanced Fujita Scale
- Classification based on structural damage
- EF-0 to EF-5
- only 1% of tornadoes are 4s and 5s but they account for 70% of the deaths
225
What is the pressure at the center of a tornado?
- characterized by low pressure at center
226
What country has the most tornadoes? Where in that country do they happen?
- the US
- to the East of the Rockies
227
What is a funnel cloud?
- narrow rapid rotating vortex
- oftern start of torando
228
How is a wall cloud formed?
formed by stretching of mesocyclone
229
What is tornadogenesis?
the process by which a tornado forms
230
What is a tornado outbreak?
Multiple tornadoes spawned by the
same synoptic scale weather system
231
Describe tornado forecasting.
* Numerical models can forecast likelihood of development of supercell thunderstorms
* Mostly based on identification ofan existing mesocyclone (i.e. storm rotation) as seen on Doppler radars
232
What are waterspouts?
intense vortices that occur over warm-water bodies (weaker than supercell tornadoes)
233
How are dust devils formed?
formed from the bottom up by convection
234
What are similarities between midlatitude (extratropical) cyclones and tropical cyclones (hurricanes)?
* Both are low pressure centers
* Both have counterclockwiserotation (they are cyclones!)
* Both strongly influenced by CF
* Both cause heavy precipitation
235
What are differences between midlatitude (extratropical) cyclones and tropical cyclones (hurricanes)?
Midlatitude Cyclones:
* Cold and warm air masses (fronts)
* Driven by horizontal temperature gradients
* bigger
* lasts 6 days
* higher surface pressure
* Maximum winds near tropopause
Hurricanes:
* Warm air masses (no fronts)
* Driven by warm oceans
* smaller
* lasts 4 days (storm ~ 2 weeks)
* slightly smaller surface pressure
* Maximum winds near surface
236
What are the 2 main factors that determine regions of formation of tropical cyclones?
- sea surface temperature (ocean too cold at high and low latitudes)
- coriolis (no CF at equator)
237
Where is the most intense storm activity in a hurricane?
- eyewall
- fast winds, heavy precipitation, and big clouds
238
What does a shrinking eye of a hurricane indicate?
storm intensification
239
What is a hurricane eye?
an area of descending air, light winds, and clear skies
240
In the eye of a hurricane, vertical wind is blowing from low pressure aloft to high pressure below. How is this possible?
Gravitational force is larger than PGF, producing the downward winds
241
Describe the lifecycle of a hurricane.
1. Tropical disturbances:
-disorganized clusters of thunderstorms with weak pressure gradients & no rotation
2. Tropical depressions:
- organized with cyclonic rotation and at least one closed isobar
- sustained wind speeds below 39 mph (62 km/h)
3. Tropical storms:
- intensify to sustain winds of at least 39 mph
4. Hurricanes status
- further intensification to 74 mph (119 km/h)
242
What are the 6 conditions necessary for hurricane formation?
1. Warm ocean waters (at least 27 deg. C/ 81 def. F needed for high evaporation rates)
2. High humidity in lower/mid troposphere
3. Unstable conditions (to promote convection and condensation)
4. Coriolis effect, to produce cyclonic circulation
5. Low wind shear, to preserve vertical structure
6. Preexisting disturbance, such as those produced by the easterly waves
243
Tropical disturbances and depressions are guided by _________ and move _________.
guided by the trade winds and move west
244
Tropical storms and hurricanes are guided by _________ and move _________.
guided by upper-level winds and tend to move poleward
245
What are the 3 main processes of hurricane dissipation?
1. Hurricane moves over land
* Loses source of moisture and latent heat
* Friction deflects flow towards low
pressure at the center, “filling” the low pressure
2. Hurricane moves further pole-ward
* Cooler sea surface temperatures (most hurricanes weaken as they track northwards along the east coast of the U.S.)
3. Hurricane reaches a location where flow aloft is unfavorable (counteracting hurricane movement)
246
What are some main impacts of hurricanes that can cause destruction?
- Hurricane winds exceed 120km/hr (74mph)
- Hurricanes produce intense rainfall of 25cm/day (10in/day)
- Many have clusters of tornadoes
- Cause rises in water level called storm surge (water piles due to heavy winds and low atmospheric pressure)
247
Which cell is the primary driver of all 3 cells in the general circulation model?
- the hadley cell
248
the upper branch of the hadley cell has a PGF pointing from _______ to the ______ due to _______
- from the equator to the poles
- due to heating difference
249
What part of what cell in the general circulation model is fake>
upper branch of ferrel cell
250
the subtropical jet stream is the result of what? (in relation to general circulation model)
result of upper branch of hadley cell reaching geostrophic balance & blowing from west to east
251
the polar jet stream is a result of what? (in relation to general circulation model)
- result of warm air from bottom of ferrel cell meeting cold air from polar cell, creating a very large PGF
- PGF points north so creates a jet stream that blows west to east
252
what drives the polar jet stream?
strong temperature gradients at the polar front
253
what feature of the general circulation model is associated with the subtropical jet stream?
the top branch of the hadley cell
254
The polar jet stream blows from _______. The subtropical jet stream blows from _______.
west to east (for both)
255
Which jet stream is stronger? Why?
- polar jet stream stronger than subtropical jet stream
- due to polar front's strength
256
Precipitation events tend to happen when we get a bunch of ________ air ______.
warm, moist ar rising
257
Moving fronts often leads to _________.
precipitation events
258
During charge separation when lightning is forming, the top of the cloud becomes _________ and the bottom becomes __________.
- top: positively charge
- bottom: negatively charged
259
Single cell thunderstorms occur when only the _______ condition and _______ condition are met.
source of moisture and unstable atmosphere
260
The cyclonic updraft in supercell thunderstorms is produced when the wind shear creates _________ which is then bent upward by _________.
horizontal vorticity which is then bent upward by the convective updraft
261
In tornado formation, a wall cloud occurs at _______ when __________.
occurs at the interface of the updraft and downdraft when cold air from the down draft is pulled back into the updraft
262
What are rain bands? (in a hurricane)
curved bands of thunderstorms that spiral out from the center of the storm
263
In hurricanes, rain and winds get progressively less intense as _______.
you move away from the center of the storm
264
How does getting closer to Earth's axis of rotation affect zonal velocity?
It increases zonal velocity
