Archer Study Guide Flashcards
What is a geostrophic wind?
An upper-level wind that flows parallel to the isobars. At Geostrophic winds, the Coriolis force is equal to the pressure gradient force.
True or False: Friction is absent in the upper atmosphere.
True
For surface winds, what are the four types of forcing acting on wind?
Pressure Gradient force, Coriolis force, centripetal acceleration, and friction.
How does friction affect the direction of wind?
Friction slows down wind, causing the flow to cut across the isobars, rather than parallel.
Why are Westerly winds aloft?
Since cold air is denser than warm air, pressure decreases more rapidly with height in a cold column of air. Thus, at high latitudes we find lower pressure
aloft and high pressure aloft at lower latitudes. Pressure gradient force pulls to the pole and Coriolis pulls toward the equator causing westerly wind.
What are the two main jet streams?
Polar front jet stream and Sub-Tropical jet stream.
How big are jet streams?
Thousands of miles long (nearly circumpolar), hundreds of miles
wide, and only a few miles thick.
What are basic characteristics of the Jet Streams?
Located between 9000m and 15000m (roughly 30-50 thousand feet)
- Average wind velocity of 75mph, can exceed 250mph (60knots is threshold for
classification
- Jet streams form in zones of maximum thermal and pressure gradients at the top of the troposphere.
Polar Front Jet Stream:
Strongest in the winter, weakest in the summer
o Shifts equatorward in the winter, poleward in the summer.
o It is variable in character.
o Strength varies daily, weekly, and seasonally.
o Position varies daily.
o It controls the surface weather (creates divergence and subsidence aloft, so cyclogenesis
and anticyclogenesis at the surface)
o Controls movement of surface pressure cells (low pressure, high pressure)
o Related to thermal contrast at surface through upper atmosphere.
o Strong seasonally
o Geographically and temporally unstable
o Meteorologically important
Sub-Tropical Jet Stream:
o Not seasonal in nature
o Geographically stable
o Meteorologically less important, but climatologically important (linked to position of Sub-Tropical High)
What happens when these two jet streams merge?
El Nino Winter
Sub-Tropical jet stream becomes very active, and it cuts across the southern U.S. and occasionally merges with the polar front jet stream causing a lot of moisture being delivered into the Eastern U.S. along with lifting creating significant weather systems.
What is an Air Mass?
An extensive, homogeneous body of air
Air mass size
Size: subcontinental (~1000km diameter)
o Size vertically: from surface through troposphere
o Homogenous: thermal, moisture
What are source regions?
The region in which an air mass forms over/obtains thermal/moisture characteristics.
What are the names/characteristics of/alphabetic code for the main air mass types in North America?
Latitude: P= Polar, T= Tropical
o Underlying Surface: m=maritime, c= continental
o cP- Continental Polar
o mP- Maritime Polar
o mT- Maritime Tropical
o cT- Continental Tropical
Relative temperature:
w = warmer than surface moving over
k = colder than surface moving over
What air mass types are associated with steam fog, lake-effect snow, dust devils, etc.?
cPk air mass: Net negative radiation balance (low sun angle, short days) low moisture
cPk:
air moving into the southern U.S. from Canada during winter.
Would cPk promote stability or instability?
Unstable: warm surface and cooler air aloft.
Low level instability can cause formation of cumulus clouds. Bumpy, turbulent air for planes during takeoff/landing.
Steam Fog:
cold air (at or below dew point temperature) over warm water can
result in steam fog.
Lake effect snow:
cP air out of Canada crosses the Great Lakes, which are warmer than the air mass, so it is cPk. Upward flux of Q_h and Q_e. Clouds form, Speed convergence
-Many lake effect snow days are under high pressure conditions.
mP air mass:
Cool, high relative humidity (specific humidity not that great- cool air)
Winter fog:
mP air mass moving inland will often be warmer than the surface mPw). Warm, moist air above a cold surface can lead advection fog (if moving) or radiation fog. Common in California, Oregon, Washington
Summer fog:
Advection fog- mP air moving toward California is warm (~80F),
moist. Air mass crosses California current (60F water), so becomes mPw.
Warm air gets chilled by cold ocean to dew point temperature – advection fog
forms.
mT air masses:
low latitude, over water, unstable side of STH, very high specific humidity.
mTk air:
In the southeastern U.S., summertime mT air is colder than the surface it moves over. Cooler, humid air over a hot surface is a recipe for convective storm activity (steep lapse rate + moisture).
Coastal fog:
mTw air, common on the Gulf Coast in winter. mT air is often warmer than the land in winter- moves inland and gets chilled to dew point.
cT air mass:
Hot, dry, forms on stable side of STH – lots of subsidence.
Dust devils:
cTk air. Lower levels of air = extreme lapse rates. 125-degree soil and 90-degree air 1m up (lapse rate of 35 degrees per meter). Superheated airparcels break free from ground and rise rapidly, creating vacuum, air rushes in to fill the void.
What is a front?
any front is just a boundary between dissimilar air masses
Polar Front:
continental
General boundary between Polar and Tropical air masses
The position of the polar front and jet stream are the same.
Synoptic scale:
cold, warm, stationary, and occluded fronts
Cold front:
cold air is the aggressor (cold air moves into area occupied by warm air)
Warm front:
Warm air is the aggressor.
Stationary front:
Neither air mass is doing much moving.
Occluded front:
when a cold front catches up to and overtakes a warm front
What is a mid-latitude wave cyclone?
An atmospheric disturbance involving the interaction of cold and warm air masses along sharply defined fronts.
o Most common weather disturbance – mid to high latitude
Why do mid latitude wave cyclones develop?
Radiation imbalance in the system.
Increasing Coriolis force with latitude
Results in horizontal air mass change
Dynamic lifting is caused by upper-level flow.
Stages of mid-latitude cyclone development:
Stationary:
Developing
Mature:
Occluded
Stationary:
polar air next to tropical air (baroclinic zone)
Mature:
The pressure surrounding the low drops.
Winds strengthen.
Frontal weather drops
Advancing Cold front:
rapid lifting creates cumulus/cumulonimbus clouds.
Advancing Warm front:
gradual lifting creates (from farthest to closest) cirrus, cirrostratus, altostratus, and nimbostratus.
Development of dynamically induced low-pressure cell:
Key element: curvature vorticity
o Vorticity: spin or rotation in the atmosphere
o Curvature vorticity = vorticity (spin) related to Earth’s rotation. Earth rotates eastward:
N. pole perspective = counterclockwise. S. pole perspective = clockwise
Curvature vorticity:
Absolute vorticity is conserved by balancing changes in ⌠k and horizontal area
(A) aloft.
⌠a = « ⌠k + « A
Divergence aloft
positive change in Area aloft – leads to surface convergence, uplift, rising air, and low pressure at the surface.
Convergence aloft
negative change in area, leads to subsidence (sinking), surface high pressure.
From ridge to trough
there is positive change in curvature vorticity. This is balanced by a negative change
From trough to ridge
there is a negative change in curvature vorticity. This is balanced
by a positive change in the area aloft. Thus, we get upper divergence, rising air, low pressure. This creates cyclogenesis.
Vertical motions are maximized between the trough and ridge
Thus, expect the surface low to form to the North and East of the base of the upper-level trough.
Troughs:
cyclogenesis
Ridges:
Anticyclogenesis
Ridges/Troughs and surface weather
In general, the greater the amplitude of the trough/ridge, the more extreme the vertical air motions (1993 superstorm).
Wind velocity is also a factor: shear vorticity.
Meridional (up and down) flow: strong highs and lows
Zonal (westerly) flow: weak highs and lows.
Continental Polar air:
sitting for a few days (less cold and dry).
Easterly/Southeasterly winds.
Maritime Tropical air:
Warm, moist air. Southernly/Southwesterly winds
