REMINDERS: Midterm 2: Friday, February 28
- lecture material covering chapters 6, 7, and 15 (since first midterm and through Wed lecture) - same Format as first midterm
UPCOMING REVIEW SESSIONS:
- Thursday, Feb 27, 6:30-8:00pm in HSS 1330
Because of land masses, subtropical highs and polar lows
are not con7nuous bands
Simulated Water Planet Reality with Land Masses
par7cularly true in N. Hem where there is more land mass
Semi-permanent Subtropical Highs and Polar Lows - Pacific High is an important factor for California's climate
Bermuda High Pacific High
Aleutian Low Icelandic Low
In Northern Hemisphere:
Highs Lows
Pacific High Aleu7an Low Bermuda High Icelandic Low
weaken and strengthen and shiG south shiG south strengthen and weaken and
shiG north shiG north SUMMER
WINTER
34°N 34°N
U.S. West vs. East Coast Weather/Precipitation
West US: Prevailing wind is from the Northwest bringing cool and moist air onto land. Because air is cool, still not much actual water vapor in air, so over land RH is low. => inhibits thunderstorm development
East US: Prevailing wind is from the Southeast bringing warm and moist air onto land. Because air is warm there is large amount of water vapor in air, so over land RH is high. => good for thunderstorm development
Also important: in summer ocean water off West coast (50-70°F) much colder than water off East coast (70-85°F)
Polar Front
~60°N N.Pole ~30°N
COLD AIR WARM AIR
300 mb
300 mb PGF
Polar Front
~60°N N.Pole ~30°N
COLD AIR WARM AIR
300 mb
300 mb PGF
Polar Front
JT
Coriolis Deflects air to the right => Polar Jet (into the screen = westerly) => Maximum speed near top of troposphere JET
Clicker Question Set Frequency to "BB"
When do you think the Polar Jet Stream would be strongest? (A) Equal at all times of the year (B) Winter: When temperature contrast across front is largest (C) Summer: When temperature contrast across front is smallest
Clicker Question Set Frequency to "BB"
When do you think the Polar Jet Stream would be strongest? (A) Equal at all times of the year (B) Winter: When temperature contrast across front is largest (C) Summer: When temperature contrast across front is smallest
~60°N N.Pole ~30°N
COLD AIR WARM AIR
300 mb
300 mb PGF
Polar Front
Stronger Temperature Difference: ==> Stronger PGF ==> Stronger Coriolis needed to balance ==> Stronger Winds
Polar Jet (aka Midlatitude Jet)
Subtropical Jet
Polar Front
Polar Easterlies
Mid-Latitude Westerlies
Horizontal Shear Zone - wind changes direction and/or speed - will induce air to rotate
Now, back to the Surface Winds.....
Polar Front
Polar Easterlies
Mid-Latitude Westerlies
Horizontal Shear Zone
Clicker Question Set Frequency to "AB"
The horizontal shear along the polar front will induce the air near the surface to rotate. Which direction will it rotate? (A) Cyclonically (CCW in N. Hem) (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth
Polar Front
Polar Easterlies
Mid-Latitude Westerlies
Horizontal Shear Zone
Clicker Question Set Frequency to "AB"
The horizontal shear along the polar front will induce the air near the surface to rotate. Which direction will it rotate? (A) Cyclonically (CCW in N. Hem) (B) Anti-Cyclonically (CW in N. Hem) (C) Will vary back and forth
Anti-Cyclonic Flow clockwise in N. Hem
Cyclonic Flow counter clockwise in N. Hem
Polar Front
Polar Easterlies
Mid-Latitude Westerlies
Horizontal Shear Zone - wind changes direction and/or speed - will induce air to rotate => Cyclonic Flow is induced here => Mid-latitude storms (low pressure) may form if conditions are right
Now, back to the Surface Winds.....
Divergence aloft can lead to low pressure forming/intensifying at surface and possible storm formation. When does divergence aloft occur?? 1) Waves in Jet Stream 2) Jet Streaks
Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere
isobars V1
V2
V3
V4 HIGH
LOW
Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere
isobars V1
V2
V3
V4 HIGH
CW Flow
CF = PGF + V2/R
CF = PGF - V2/R CCW Flow
LOW
Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant).
Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere
isobars V1
V2
V3
V4 HIGH
CW Flow
CF = PGF + V2/R
CF = PGF - V2/R CCW Flow
LOW
Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant). V2 > V3 and air in region A is converging (more entering than leaving)
==> Causes surface pressure to increase
A
Waves in Jet Stream high above surface (roughly 300 mb height) assume equal isobar spacing and Northern Hemisphere
isobars V1
V2
V3
V4 HIGH
CW Flow
CF = PGF + V2/R
CF = PGF - V2/R CCW Flow
LOW
Since Coriolis Force (CF) is proportional to velocity, flow around HIGH is faster than flow around LOW for the same isobar spacing (PGF=constant). V2 > V3 and air in region A is converging (more entering than leaving)
==> Causes surface pressure to increase
V3 < V4 and air in region B is diverging (more leaving than entering) ==> Causes surface pressure to decrease => surface low forms
A B
JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM
Jet Streaks
isobars
JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM
Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds
isobars
LOW
HIGH
isobars
JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM
Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds
isobars
A
At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left)
LOW
HIGH
isobars
JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM
Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds
isobars
A
At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left)
At B: winds decrease quickly => Coriolis takes time to adjust
=> Forces temporarily out of balance (too much Coriolis and wind veers to right)
B
LOW
HIGH
isobars
JET STREAKS = REGIONS OF FASTEST WINDS IN JET STREAM
Largest ∆T => Largest PGF => Smallest Isobar Spacing => Fastest Winds
isobars
A
At A: winds increase quickly => Coriolis takes time to adjust => Forces temporarily out of balance (too little Coriolis and wind veers to left)
At B: winds decrease quickly => Coriolis takes time to adjust
=> Forces temporarily out of balance (too much Coriolis and wind veers to right)
B
Strong Convergence
Region
Weak Convergence
Region
Strong Divergence
Region
Weak Divergence
Region
LOW
HIGH
Divergence aloft – creates/enhances low pressure at surface – leads to rising air
Forecast of 300 mb winds (roughly Jet Stream level) for Friday a=ernoon
Forecast of 300 mb winds (roughly Jet Stream level) for Friday a=ernoon
base of trough jet streak