Download Lecture 4

Air Pressure Experiments
Lessons from Paper Cup experiment:
1. Air pressure is present everywhere
2. Air “tries to” move from an area of higher
pressure to an area of low pressure
Lesson from Pop Bottle experiment:
3. Warm air occupies more space than an
equal number of molecules of cold air
Wind Power Generation
in Southern Alberta
“Don’t try this at home”
The speed and direction of wind
is determined by three forces:
1. Pressure Gradient Force
2. Inertial Coriolis Force
3. Friction Force
Pressure Gradient Force
Definition:
The difference in
atmospheric
pressure per unit
distance
PGF acts at right
angles to isobars
of equal pressure
H
L
102.2
99.8
101.4
100.6
600 km
Pressure Gradient Force = 2.4 kPa / 600 km
= 0.4 kPa / 100 km
Where is the PGF forecast to be
strongest today ?
Regina or
Lethbridge?
Solution:
Check the
spacing of the
isobars of
equal surface
pressure
Source: http://weatheroffice.ec.gc.ca/data/model_forecast/592_100.gif
The Inertial Coriolis Force
Objects moving in an “absolute” straight line
between two points on the Earth’s surface are
deflected:
To the RIGHT in the N hemisphere
To the LEFT in the S hemisphere
Why ?
The Earth rotates more quickly at the equator.
Visualizing the Coriolis Force
Source: NASA
The Friction Force
Surface roughness decreases wind speed
Reduces impact of Inertial Coriolis Force
Winds cross isobars, spiralling out of
ANTICYCLONES (H), and into CYCLONES (L)
H
L
Can you infer wind direction and relative
speed from this map ?
weather.unisys.com
Sea level
pressure:
Altitude
Correction
Source: Ahrens (1994)
Weather
symbols and
wind barbs
Classic Low Pressure System
In Temperate Latitudes
0600h GMT
APRIL 5
2003
NORTHEAST
WINDS
SHARP
COLD
FRONT
WARM,
MOIST
SOUTHERLY
FLOW
Cold Front
Arctic high pressure drives cold arctic air behind low
Warm Front
•Not as steep a division as in a cold front
•It takes longer to scour out surface air
(warm air rises)
The weather pattern right now
COOL
NW WIND
WARM FRONT
WARM,
SOUTH
COLD FRONT WIND
HURRICANE ISABEL
Main Low and High Pressure Zones
1. Equatorial Low Pressure Trough
2. Subtropical High Pressure Cells
3. Subpolar Low Pressure Cells
4. Weak Polar High Pressure Cells
Atmospheric Circulation Overview
POLAR
CELL
FERREL
CELL
HADLEY
CELL
Equatorial low pressure trough (warm, wet)
High solar angle
Consistent daylength
Convergence
Heating
ITCZ shifts with season
Hadley Cells
1. Warm, moist air rises in equatorial low
Cools, condenses, and causes heavy rain
2. Outward flow to subtropical high at high
altitude
3. Air descends in subtropical high
Heats, compresses and becomes very dry
4. The subtropical high provides the gradient
for trade winds and westerlies
eg. Bermuda/Azores and Pacific/Hawaii highs
Strahler and Strahler (2002)
Ferrel Cells
Between subtropical highs and subpolar lows
Poleward transport of excess heat through
eddies and migration of lows toward polar front
Strong low pressure develops in a belt around
Antarctica, near the Aleutians and near Iceland
Lows strongest in winter (shift and diminish
periodically, especially in the summer)
Why ?
Water much warmer than land in winter
leading to lower pressure over oceans
H
L
Air tends to be unstable in low pressure (tendency to rise)
Air tends to be stable in high pressure (tendency to fall)
(more on stability in next class)
WINTER
SUMMER
Generalized Overview of Seasonal Surface Pressure
Average Global
Surface Pressure
in January and
July
Can you explain
the monsoon season
of the Indian subcontinent with this
chart ?
Polar High Pressure Cells
Tendency for higher pressure near poles
than at the polar front
Anticyclonic flow develops
Weak and variable polar easterlies result
(stronger in southern hemisphere)
In northern hemisphere winter, the polar
front usually lies over Canada and Russia,
(further south than in the summer)
Geostrophic Winds
500 mbar
height map
Lower heights
where air is cold
Airflow parallel
to isobars in
upper troposphere
Why ?
Combination
of PGF and
Coriolis force
Source: http://weatheroffice.ec.gc.ca/data/model_forecast/134_100.gif
Effect of Air temperature on 500 mb heights
Source: Ahrens (1994)
Upper Atmospheric Circulation
Jet Streams
A band of wind in the upper troposphere
150 – 500 km wide
0.9-2.2 km thick
Speeds may exceed 300 km/h
Polar Jet Stream:
Between Polar and Ferrel cells
Subtropical Jet Stream:
Between Hadley and Ferrel Cells
Source: http://apollo.lsc.vsc.edu
Source: http://apollo.lsc.vsc.edu
Jet Stream Cross Section
“Rivers” of strong wind
where cold and warm meet
18 000 m
12 000 m
Tropopause
height
6 000 m
Discontinuity or
step in tropopause
height
See: www.avsim.com/avwx/avsim_wxus_jetstream.html
Polar Jet Stream
Meanders from 30-70° N or S
Moves more poleward in summer
Influences (and is influenced by) storm paths
Subtropical Jet Stream
Meanders from 20-50° N or S
May occur simultaneously with Polar Jet in NA
Rossby Waves
The polar jet stream follows the Rossby Waves
Rossby Waves are undulations in the upper-air
westerlies extending from the middle to upper
troposphere
Form along the polar front
Mechanism of poleward heat transport
(Strahler and Strahler, 2002)
Smaller Scale Winds
1.
The Land-Sea Breeze
Land heats up
in morning sun,
causing air
molecules to rise
Air pressure
drops over land
as molecules
removed
http://cimss.ssec.wisc.edu/wxwise
High pressure
develops over
the ocean
In the evening,
the land cools
faster than the sea
and the process is
reversed
2.
Mountain Valley Breezes
Daytime
The sun heats the
hillslope, causing
air to move up the
slope - a convection
loop forms
Night
Night radiation cools
the slopes
Cooler, denser air
moves downslope
Source: http://apollo.lsc.vsc.edu
3.
Katabatic Winds
Air cools on a plateau or sloping terrain, becomes
more dense and descends
Winds get faster and faster downslope
Relatively warm water at base can further increase
winds, which can be very strong as a result
Can occur on large scale (eg. Greenland, Antarctica)
Also referred to as gravity drainage winds
•Water piles up around equator due to trade winds
•Along western edge of oceans, water spills N and S
along shorelines of continents (also downwelling)
•Upwelling occurs near east edge of oceans (west coasts)
Upwelling of cool waters
The Thermohaline Circulation
(1) Intensive cooling at the ocean surface in North Atlantic
(2) Northward transport of salty surface water from lower
latitudes (both increase the density).
Result: The water column
becomes unstable and
mixes vertically in the
north. This newly formed
water is carried southward
at great depths - North
Atlantic Deep Water
(NADW)