Geography 140
Introduction to Physical Geography
Lecture: Pressure as an Element of Weather
III. Air pressure is the mediation that gets air moving, so that the two
adiabatic processes can produce changes in air temperature.
A. Pressure is the weight of the atmosphere compressing the air down onto
the surface of the earth.
B. It is measured a variety of ways.
1. Directly as weight:
a. 1 kg/cm2 at sea level
b. 15 lb./sq. in. at sea level
2. As the height of a column of mercury in a mercury barometer:
a. 76 cm at sea level
b. 29.9" at sea level
3. Directly as force exerted
a. The unit of force measurement is the millibar (mb), sometimes
(and increasingly) called the hectoPascal. One mb = 0.01 Pa,
so, 100 Pascals are a hectoPascal, 1 mb = 1 hPa.
b. One mb or hPa equals a force of 1,000 dynes/cm2. Big
help that is!
c. 1 dyne is the force that will accelerate 1 g of mass 1 cm 1
second per second (or 1/100,000 newton)
d. At sea level, air pressure is close to 1,000 mb (it averages
about 1,013.2 mb, actually).
e. 1 cm of mercury, then, is 13.3 mb; 1 in. of mercury is 33.9 mb
f. Just remember that sea level air pressure is 1,013.2 mb.
4. All of these methods can be expressed as a "standard atmosphere" or
"bar": 1 atm is the average air pressure at sea level (therefore,
it's equal to 1,013.2 mb, 101,325 Pa, 1,013.2 hPa, 76 cm of
mercury, and 1 kg/cm2). Got all that?
C. All this attention to measurement expresses the fact that air pressure
varies quite a bit.
1. We've seen that air pressure drops quickly with increases in
altitude.
2. As Torricelli himself noted, air pressure varies at one place at
different times.
3. And, as we'll explore today, air pressure varies from place to
place at one time: This means we can map air pressure variations,
which really excites us geographers!
D. A common cause of air pressure differences is variation in
temperature.
1. Cooled air tends to sink, which increases air pressure.
2. Warmed air tends to rise, which reduces air pressure.
E. These relative variations in air pressure have names:
1. A high pressure area is an "anti-cyclone."
2. A low pressure area is a "cyclone."
a. Possible point of confusion: There are popular usages of the
word, "cyclone," that differ somewhat from scientific usage.
b. In North America, when you hear the word, "cyclone" (especially
if you live in the Midwest or South), you think tornado.
c. In Southern Asia, when you hear the word, "cyclone," you think
hurricane or typhoon.
d. Well, all tornadoes and all hurricanes are cyclones, but not all
cyclones are hurricanes or tornadoes: Our ordinary mid-latitude
winter storms are also cyclones, and, on a really dinky scale,
even a dust-devil is a tiny cyclone.
F. Thus varying from place to place, air pressure can be mapped.
1. Air pressure is mapped with isolines. You remember those, don't
you? An isoline is any line on a map connecting places having the
same value of something or other. We've bumped into them before:
parallels are isolines of latitude, and meridians are isolines of
longitude. Isohyets are isolines connecting all places with the
same precipitation; isotherms connect places with the same
temperature statistics; isagons connect all places having the same
magnetic declination; and contours connect all places having the
same elevation (see Lecture 11 on map symbolization).
2. Isobars are the isolines used to connect all places having the same
corrected air pressure readings (they're normally corrected for
elevation, because increasing elevation decreases air pressure).
3. The conventional isobar interval is 4 mb: This interval gives you
a clear picture of regional highs and lows and the structure of air
pressure variations across space, but it doesn't overwhelm you with
too many lines. On the idealized map below, see how each blue line
has a value 4 mb higher than one neighboring line and 4 mb lower
than the opposite neighbor?
![[ idealized isobar map of generic area ]](http://www.ucompass.com/met1010/lectures/021698/isobar.gif)
4. Here is an isobar map of New Zealand and eastern Australia:
![[ isobar map of New Zealand and eastern Australia ]](https://home.csulb.edu/~rodrigue/geog140/isobarsnewzealand.gif)
Describe the general latitude and longitude of the highest pressure
area on the map. That high has an air pressure in excess of ....
how many millibars (or hectoPascals)? Where is the lowest pressure
area? That low has an air pressure below ... how many mb (hPa)?
You get the idea.
5. Remember a very important point: Air pressure is RELATIVE to the
regional context. No specific value is always the breakpoint
between a high and a low. On this map, the high is somewhat
greater than 1,016 hPa (but less than 1,020 hPa, or that would have
been mapped with an isobar), and the low is less than (992 hPa, but
greater than 988 hPa). On another map, it could well be that 992
is the regional HIGH, while, somewhere else, maybe 1,020 is the
regional LOW. You must interpret values within their regional
context.
G. Nature can't stand inequality. In this case, variations in air
pressure are compensated for by transferring air from high pressure
zones to low pressure zones to try to balance out the differences.
This horizontal air flow is "wind" or "advection."
1. Mechanics of air transfer.
![[ highs, lows, winds in cross-section, modified from K.A.
Kepple, USA Today Weather Book ]](https://home.csulb.edu/~rodrigue/geog140/highlowwind.gif)
a. A high is at the bottom of a column of air being forced down
from above. Air at the surface, then, is being squished out
away from the high.
b. A low represents a lifting of the air above a given place. This
creates a partial vacuum, which sucks air to it from all
directions. The greater the lifing, the more effective the
vacuum and its pulling effect.
2. The direction and speed of winds are determined by the relative
location and intensity of highs and lows.
a. This information comes from an isobar map.
b. The intensity of a high or low and the speed of the winds
between them is shown on the map by the spacing of the isobars.
i. If they're close together, then there's a rapid change of
pressure in a short distance, so wind will move across
these isobars at a high speed.
ii. The change in pressure as shown by isobar spacing is called
the "pressure gradient." Wind will flow down the pressure
gradient from high to low, sort of the way water will flow
down an elevation gradient from high country to lowland.
You can visualize the analogy with an elevation gradient by
constructing an X-Y graph showing distance between a high
and a low as the abcissa (X or horizontal axis) and air
pressure on the ordinate (Y or vertical axis). For each
distance reading, you'd look at the map to get (or infer)
the air pressure reading and plot that reading above the
distance. When you were done, you'd have created a cross-
section of the pressure gradient and could instantly
visualize where the air is trying to flow. This is an easy
way to understand the "pressure gradient force," which sets
the direction and speed of the air flow. Here are a couple
of graphics that convey the general idea,
![[ pressure gradients, Australian Government Board of Meteorology ]](http://www.bom.gov.au/nmoc/MSL/wmfig4a.gif)
|
Shallow Pressure Gradient
Steep Pressure Gradient |
3. Coriolis Effect.
a. Now that I've carefully established this analogy between the
behavior of air around a pressure gradient and the behavior of
water around an elevation gradient, I'm going to mess it up a
little.
b. Once wind is set in motion by the pressure gradient force, it
does not flow directly from high to low in a straight line as
might be expected. Its path is knocked off course because of
the earth's rotation.
c. This distortion due to rotation is called the "Coriolis Effect,"
after Gaspard Gustave de Coriolis (1792-1843), a French
mathematician who made the first observation of this distortion
in the early 19th century.
d. The influence of Coriolis Effect is stated in Ferrel's Law:
"Any horizontally moving object in the Northern Hemisphere will
exhibit an apparent right-hand deflection and, in the Southern
Hemisphere, an apparent left-hand deflection."
e. So, any object in the Northern Hemisphere trying to move north
is deflected to its right, winding up east of its destination;
if it tries to move east, it winds up south of its destination;
if it tries to move south, it winds up west of its target; and,
if it tries to move west, it winds up north of the destination.
Exactly the opposite happens in the Southern Hemisphere. On the
globe below, the black arrows are the planned paths and targets,
and the magenta arrows are the actual, deflected paths.
![[ target and deflection due to Coriolis Effect, C.M.
Rodrigue, 2000 ]](https://home.csulb.edu/~rodrigue/geog140/corioliscircle.gif)
f. Isn't that weird? It actually does make sense.
i. Remember, the earth rotates eastward at a constant angular
speed: 15°/hour?
ii. That same angular speed translates into different linear
speeds at different latitudes: roughly 1,660 km/h at the
equator, but only about 830 km/h by the time you get up to
60° N or S (how'd I get that? The cosine of 60° is
0.5 or half; so what do you suppose our actual linear speed
of rotation is here at 34° N?).
![[ same angular speed = different linear speed at
different latitudes, D. McConnell, 1999 ]](https://home.csulb.edu/~rodrigue/geog140/angularlinearanim.gif)
iii. So, any horizontally moving object heading north or south
away from the equator does so, carrying the eastbound
motion it had on the equator (1,660 km/h), which becomes
faster and faster relative to the motion of the latitudes
through which it passes, so it winds up east of its target.
iv. Any horizontally-moving object moving toward the equator
starts out with the relatively slower linear speed of its
origin, which means that it is moving eastward more and
more slowly relative to the lower latitudes through which
it passes: It winds up west of its target.
v. It gets a little gnarlier explaining why objects moving
east or west ALSO deflect. It has to do with centripetal
acceleration. If the object is going too fast with respect
to the rotation of the globe, it tries to move to a higher
"orbit," if you will, to a region moving faster. If it's
going too slow with respect to the spinning globe, it tries
to "fall" to a region with a slower, more compatible speed.
a. By moving east, a horizontally-moving object is now
going faster than the rotation of the earth, so it tries
to move towards the equator, which is moving faster.
b. By moving west, the object is now going more slowly than
the earth is, so it tries to move towards the pole,
where speeds are lower and lower (and 0 km/h at the
pole).
vi. The Coriolis Effect is zero for objects moving east and
west along the equator: There's no place farther on the
earth with a higher speed to go for the eastbound object,
and the equator divides the Southern Hemisphere with its
left hand deflection from the Northern Hemisphere with its
right hand deflection, so there's no basis for a westbound
moving object to make a "decision" about where to "fall."
vii. If the explanation mystifies you, just memorize that
horizontally-moving objects (e.g., winds) in the Northern
Hemisphere are deflected off course to their right and
those in the Southern Hemisphere are deflected off course
to their left.
viii. Coriolis Effect is zero at the equator and becomes
progressively stronger as you approach the poles.
ix. It is stronger the faster the motion involved and the
greater the distance of travel.
g. You can perform an experiment in the privacy of your dwelling
unit to become a true geonerd (and demonstrate Coriolis Effect
in a very graphic way). You'll need a friend, a pizza, a stick,
and a nail. After sharing the pizza with your pal and lab
assistant (Igor), save the circular cardboard platter the pizza
came in. You may need to clean it off a bit to make it
serviceable.... Make a hole right in the middle a bit wider
than the stick. Have Igor stick the stick in it and press it
firmly against your table top. You grab the nail and have Igor
spin the pizza platter (while s/he is holding the central axis,
of course) and keep it spinning. Now, compose yourself and use
the nail to scratch a line on the spinning platter, drawing the
nail toward you in as straight a line as possible. You saw it,
and Igor saw it: You drew a straight line. When Igor stops
spinning the platter, though, what will that line look like?
It'll spiral, exhibiting the apparent deflection of Coriolis
Effect. Cool, eh? So, the actual, absolute path is straight,
but it describes a spiraling path on the rotating object.
That's the essence of Coriolis Effect! Each hemisphere of the
earth is kind of like a 3-d pizza platter for the purposes of
Coriolis Effect.
4. So what does all this have to do with winds? Well, once a wind
begins to move, it is affected by four things:
a. The pressure gradient force (the pressure difference divided by
the distance between the high and low) determines its speed of
flow from the high to the low and the direction for which it
aims.
b. The Coriolis Effect, which distorts its path and actually
balances and cancels out the net effect of the pressure gradient
force at high enough speeds.
c. Frictional resistance to its flow.
d. Centrifugal force due to bends in the isobars around the highs
and lows (isobars tend to form concentric circles defining peaks
and pits of air pressure).
e. These act in contradictory manners:
i. The pressure gradient force moves at right angles to the
isobars.
ii. Coriolis Effect acts at right angles to the direction of
movement (parallel to the isobars here): to the right in
the Northern Hemisphere and to the left in the Southern
Hemisphere.
iii. Friction opposes motion, slows it down, so it acts in the
direction opposite the motion; it significantly slows winds
(and, therefore, weakens Coriolis Effect) within about a
kilometer of the surface.
iv. Centrifugal force acts against the pressure gradient force
but it is in dynamic balance with it: if it weakens, the
pressure gradient force strengths and vice-versa.
f. Let's start with the simplest situation and understand the
balance of forces there, then gradually bring in complications.
Let's start with the balance of forces on air aloft, more than a
kilometer up above a flat countryside like much of the American
Midwest. Up here, there's no frictional resistance from the
ground to slow down the wind, so it can really boogie up there.
Let's say, further, that the isobars in the region happen to
form straight lines between the regional high and the regional
low on that day.
i. The wind will start to flow in response to the pressure
gradient force from high to low, crossing the isobars at
right angles to do so.
ii. As they move faster and faster, though, Coriolis Effect
kicks in and directly opposes the pressure gradient force.
At this altitude and at high speeds, the two forces cancel
out their net effects (toward the low and away from the
low).
iii. This means that the wind, caught between these two opposing
forces, flows ALONG the isobars, parallel to them!
iv. Such winds aloft flowing parallel to straight isobars,
balanced exactly between the pressure gradient force and
the Coriolis Effect, are called "geostrophic winds."
g. Now, let's muddy the picture a bit: Let's permit the isobars to
bend around, the way they usually do, to enclose the high and
the low in concentric circles. This introduces centrifugal
force.
i. The geostrophic wind "wants" to run in a straight line,
along the isobars, but now the isobars are themselves bent.
ii. So, the winds are rotating around the high and the low.
iii. This means that the wind tends to drive outward away from
the low and the high: centrifugal force.
iv. Winds rotating around the low respond to centrifugal force,
which in this case means in the same direction as the
Coriolis Effect. To preserve the balance, Coriolis Effect
slackens, which allows the pressure gradient force to
strengthen, which pushes the air back on a path paralleling
the isobars!
v. Winds rotating around the high also respond to centrifugal
force, moving away from the high and thus in the same
direction as the pressure gradient force. The PGF slackens
now, which strengthens Coriolis Effect, which redirects the
wind back onto a path parallel to the isobars!
vi. So the wind is not really geostrophic anymore, not
technically, but the air is still flowing parallel to the
isobars: Such a wind is called a "gradient wind."
vii. If my explanation leaves you sputtering, have a look at a
great site with really effective animations to illustrate
all this by clicking here
h. Now, let's come closer to Earth and consider the situation
underneath these balanced geostrophic and gradient winds.
Closer to the surface of the earth, the air flow is more and
more affected by surface roughness and the resulting frictional
resistance.
i. Frictional resistance slows the winds.
ii. This weakens Coriolis Effect.
iii. The balance of power between the two opposing forces now
shifts towards the pressure gradient force.
iv. The wind now is able to cross the isobars and actually
spiral in towards the low and out of the high. Because
Coriolis Effect is still present, though weakened, the wind
cannot cross the isobars at right angles the way the
pressure gradient force would "like." It crosses them,
deflected some 15-30 degrees (depending on latitude and
wind speed).
i. The resulting spirals are opposite one another in the northern
and the southern hemispheres:
i. In the Northern Hemisphere, winds spiral CLOCKWISE out of a
high and COUNTERCLOCKWISE into a low.
ii. In the Southern Hemisphere, winds spiral COUNTERCLOCKWISE
out of a high and CLOCKWISE into a low.
iii. It is very important to remember these: They're confusing.
5. Winds are named for the horizontal direction FROM which they blow,
not the direction TOWARDS which they blow:
a. A westbound wind, then, is an east wind.
b. A "Nor'easter" blows from the northeast to the southwest.
c. The Prevailing Westerlies blow from the west to the east.
d. The Polar Easterlies blow from the east to the west.
e. A sea breeze blows onshore from the sea.
f. A land breeze blows offshore from the land.
g. Just to make your lives miserable, however, this applies to
horizontal airflow, not vertical airflow: An upslope breeze is
moving up and a downslope breeze is moving down.
6. The Buys-Ballot Law is kind of neat. It takes into account
Coriolis Effect, the pressure gradient force, and friction to
predict where the wind is coming from. If you're in the Northern
Hemisphere and you put your back to the wind, the low from which
the wind is coming is to your left and the high towards which it's
blowing is on your right. In the Southern Hemisphere, it's exactly
the opposite: I've heard it summarized as 3-L there: Look into
the wind, the Low from which the wind is coming is on your Left.
The 3-L approach, of course, is only valid in the Southern
Hemisphere. Oh, I don't expect you to memorize this: It's just
kind of cool and might help you break the ice at a party!
H. The global-scale pressure and wind pattern does tend to show some
predictable regularities. In order to make sense of these, I'll again
follow a style of exposition, in which I'll present an analysis of the
patterns in as simple a circumstance as possible and then gradually
add in "grubby reality" factors at a pace we can handle.
1. This style of thinking is characteristic of the scientific method,
by the way: Scientists tend to proceed by analyzing the things
they're interested in by isolating them from complications and
forming laws that explain their behavior in isolation. Then, once
we think we have it down, we then gradually synthesize more and
more real world factors into the simple model to make it more
comprehensive while still preserving the essence of what we learned
during the analytic phase. We tend to admire best those models,
which cover the widest array of observations with the fewest and
simplest assumptions: This is what we call "elegance," "economy,"
or "parsimony" in explanation.
2. Anyhow, for this presentation, I'll create an ideal world by making
two simplifying assumptions (to get rid of complications we'll deal
with later):
a. First, let's simplify away the continents (yow!): Instant
"Waterworld." Why on Earth would we do something so ridiculous?
Think back a bit to the last lecture, about the differential
response of land and sea to heat energy inputs. Land and sea
have different specific heats, heating and cooling at different
rates. You can see, from earlier in this lecture, that this
would screw up pressure patterns by creating different
temperature zones at the same latitudes. So, let's not go there
just yet.
b. Second, let's throw out the tilt in the earth's axis of
rotation. Whatever for? So we can avoid dealing with
seasonality and the migration of the direct ray of the sun into
one hemisphere and then the other. We have enough on our plates
as it is for now.
3. On such a "perfect," simple, ideal world, we would see the
following patterns of alternating high and low pressure bands and
winds in between them:
![[ ideal pressure and wind belts on oceanic world with no
axial tilt, from New Media Studio]](http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Easterly_Waves/Trade_Winds/Trade_Winds_fig01.jpg)
a. Let's deal with the pressure bands first.
i. Along the equator, you'll notice an area called the
"Intertropical Convergence Zone" (or ITCZ to its friends).
This area is also called the "Equatorial Low. It is an
area of chronic low pressure, created by the uplift of air
by the concentrated heating below the direct ray of the
sun. It's called the ITCZ, too, because the tropical Trade
Winds converge in this area to be uplifted by concentrated
equatorial heating (more on the Trades in a bit). Because
the airflow here is vertical (convection, assisted by
convergence), there is no wind here (no horizontal
advection). This meant that in the early days of
colonialism, sailing ships were often becalmed in these
waters, a very depressing situation, so this area is also
called the "Doldrums."
ii. Up around 30° N or S, you'll see an area labelled the
"Subtropical High." This is an area of subsidence, the
downward movement that balances upward equatorial
convection. Remember in the previous lecture how I
stressed that descending air never precipitates and that it
warms adiabatically, which lowers its relative humidity?
Well, the Subtropical High is what accounts for the Sahara
and similar deserts (which you'll notice tend to be around
30° N and S on the west coasts of landmasses). Okay.
It's bone dry here. And again the air movement is
vertical. No wind. This helps us understand yet another
name for the region, which also dates from early
imperialism and sailing ships: The "Horse Latitudes."
Ships were becalmed here, too, but this was more than
depressing, this was very, very life-threatening, because
the crews could exhaust their fresh water supplies while
waiting for a breeze. Well, one of the weapons the
Europeans used to conquer and rule the New World was ...
horses. Horses drink a LOT of water. Especially when it's
as hot and dry as the Sahara on board ship. So, the
sailors would often be forced into making the soldiers'
horses "walk the gangplank." It was such a common sight
then to see bloated horse corpses floating in these waters
that this pressure belt came to be called the Horse
Latitudes. Will this tidbit make you popular at parties!
iii. Looking still higher, you'll notice another pressure band
around 60° N or S: The Subpolar Low. This is an area
where two more wind systems converge (the Prevailing
Westerlies and the Polar Easterlies). When air converges,
it has nowhere else to go but up. So, again, you get an
area of vertical airflow, rising here, just as we saw along
the equator. This is an area of powerful uplift and
condensation/freezing and storminess (we'll see later that
most of our winter storms are spawned in this pressure
belt. No cutesy names from the old days of empire and
sailing ships for you, though.
iv. Last of all, above the north and south poles, at 90° N
or S, we have the Polar High. Air is so badly chilled here
that it subsides, again creating an area of vertical
airflow, downward again, like the Horse Latitudes. This,
too, is extremely dry air, dry because of the downward
movement and also dry because of the extreme cold
(remember, cold air cannot contain much water vapor).
Greenland and Antarctica are polar deserts. They don't get
much precipitation but what falls doesn't melt: It's
accumulated to 2 and 3 km in places over thousands of
years!
b. Now, about those wind belts.
i. First, between the Subtropical High and the Equatorial Low
lie the Trade Winds. These blow toward the ITCZ and are
slightly deflected by the Coriolis Effect at the surface.
So, they have a slight easterly bias. So the ones in the
Northern Hemisphere are called the Northeast Trades, and
the ones in the Southern Hemisphere are called the
Southeast Trades. Talk about euphemism! "Trade" -- that's
what they called the conquest of the New World, plundering
of its empires' riches and their use to finance the
takeover of much of the rest of the Old World. Orwellian
language goes back long before George Orwell!
ii. Second, you'll notice the Westerlies or the Prevailing
Westerlies blowing from the Subtropical High to the
Subpolar Low in each hemisphere. Being at a higher
latitude, they are more strongly affected by Coriolis
Effect and have a marked westerlies bias. "Prevailing"
means that, in this area, the prevailing or most common
direction of the wind is from the west.
iii. Third, blowing from the Polar High to the Subpolar Low
(where they converge into the Prevailing Westerlies) are
the Polar Easterlies. These guys are REALLY strongly
biased from the east, and they are generally very fast-
moving winds.
4. The mechanism behind this pattern of alternating pressure bands and
wind systems in between them is essentially a convection engine,
distorted by Coriolis Effect. George Hadley is the fellow who
figured this out, back in 1735!
a. Heat is absorbed in greatest concentration along the equator,
which produces convectional uplift, which accounts for the
Equatorial Low.
b. The air in the upper troposphere chills as it climbs, but it
cannot sink over the air rising below it, so it spreads out and
drifts towards the poles. The air flow, distorted by the
Coriolis Effect, flows from west to east, accumulating around
30° N or S. This constant accumulation of air in the upper
troposphere becomes a great river of air, the subtropical jet
stream, some of which "leaks" downward, subsiding to the surface
to form the Subtropical High.
c. At the surface, some of the air subsiding over 30° N and S,
flows towards the equator as the Trade Winds (with their slight
Coriolis defletion). Some of it goes the other way, toward the
poles, becoming the Prevailing Westerlies (with their stronger
Coriolis deflection).
d. Meanwhile, back up at the north and south poles, the intensely
chilled air contracts (remember the gas laws from the last
lecture), becomes more dense, and subsides to form the Polar
High.
e. Air is pushed out from under that high towards the equator, very
strongly affected by Coriolis Effect: These are the Polar
Easterlies.
f. The Polar Easterlies collide with the Prevailing Westerlies
around 60° N and S, which creates convergent uplift and the
Subpolar Low. Above the Subpolar Low is the "polar jet stream,"
the first jet stream to be discovered. It is a current of very
fast flowing air about 10 km above the Subpolar Low, supplied by
the uplifted air from that low.
i. This current flows from west to east somewhere between 100
to 200 km/h.
ii. This current affects air traffic ("jet" stream): It takes
less time to fly from west to east than from east to west.
Going east, the jet stream is a tailwind and speeds you on
your way; going back west, it's a headwind and it delays
you about an hour from the East Coast to the West Coast.
iii. This jet stream is also important because it affects the
track of our winter storms, more about which later.
5. Let's bring some "grubby reality" back into this model, folks.
Let's allow the planet to resume its tilt of 23½° from
the vertical of the plane of ecliptic. This means the declination
of the direct ray of the sun can now move from 23½° N in
June to 23½° S in December. Let the seasons begin!
a. As the declination of the sun migrates into the Northern
Hemisphere, the whole world pressure and wind system shifts
north, following its power source. Each band shifts north.
Those in the Northern Hemisphere are compressed latitudinally,
while those in the Southern Hemisphere expand latitudinally.
b. The exact opposite happens when the system follows its power
source into the Southern Hemisphere.
c. An apt physical analogy would be holding a glass pot filled with
water (and some glitter or pepper to make this visible) over a
candle until a convection cell develops, rising over the candle
and sinking along the sides. Move the candle around a bit, and
you'll see the convectional uplift move with it.
d. There are many two-season climates in the world strongly
affected by the movement of the world pressure and wind belts
north and south each year.
i. West Africa and other tropical wet-and-dry climates are
covered by the Subtropical High when it shifts south in the
Northern Hemisphere winter, giving it a hot, dry winter.
The movement of the ITCZ north in summer brings it a rainy,
warm summer.
ii. Southern California and other Mediterranean climates are
covered by the Subtropical High in the summer when it moves
poleward of its customary 30° N (and S) position,
giving us our hot, dry summer so reminiscent of West
Africa's winter. Comes winter, and we are affected by the
Westerlies and the Subpolar Low and its storminess.
6. Now, let's relax the second simplifying assumption and let there be
land. Pretty impressive, huh? This means we have to deal with the
different specific heats of land and water, land heating up and
cooling down a lot faster than oceans adjacent. This sets up
intense pressure gradients from land to sea, which introduces a
longitudinal factor cutting across the latitudinal world pressure
and wind belts.
a. This particularly affects the Northern Hemisphere. Anyone want
to hazard a guess why? Hint:
![[ Mollweide equal area map of the world ]](http://www.colorado.edu/geography/gcraft/notes/mapproj/gif/mollweid.gif)
b. So, I'll focus on the Northern Hemisphere here.
c. The Northern Hemisphere summer produces heating and convectional
uplift over the North American and Eurasian landmasses, which
produces low pressure areas right where you'd expect the
Subtropical High to be at that time of year.
i. This breaks up the Subtropical High into two oceanic cells
or concentrated peaks of high pressure, with steep pressure
gradients on all sides of the central peaks (not just north
and south).
a. The one in the Pacific is called the "Hawai'ian High."
b. The one in the Atlantic is called either the "Azores
High" or the "Bermudas High."
ii. This breakup and consequent concentration of high pressure
strengthens its impact on weather: There's a steep
longitudinal pressure gradient between land and sea. This
accelerates air flow out of these oceanic highs.
iii. This produces very different summer weather patterns on the
west coasts and the east coasts of continents, however,
because of ocean currents. Hunh? Ocean currents? I
thought we were talking about weather!
a. Yep. Ocean currents. On p. 58 of your textbook, you'll
see a map with warm currents shown with black lines and
cold currents with white lines. Look at the east coast
of North America -- warm current (Gulf Stream). Look at
the east coast of Asia -- warm current (Kuroshio or
Japan Current). Now, have a gander at the west coast of
North America -- COLD current (California Current) --
which never fails to surprise visitors from Back East
who want to jump into the Pacific! How about the west
coast of Europe and North Africa -- yep, another cold
current (Canaries Current).
b. So, when the Bermudas High builds up into a concentrated
peak of high pressure, winds spiral (clockwise) out of
it ... and right across the warm Gulf Stream. Passing
over the warm ocean water, this wind evaporates a lot of
water vapor into itself and schleps it right onto the
American South and East. That's why summer there is so
sticky and hot and why it rains constantly!
c. When the Hawai'ian High puffs up into a concentrated
peak of high pressure, it, too, sends winds spiralling
clockwise out of it in all directions. But the air that
comes to California passes over that cold California
Current bringing water down from the North Pacific.
Little water will evaporate into it, so it's drier. In
fact, the air may be chilled by conduction and radiation
below its dew point out at sea, forming those huge fog
banks you see building out there in the summer (and our
marine layer along the coast in June). The air is
drier, and that partially explains our long hot dry
summer in California (and other Mediterranean climates
around the world). Sometimes, too, the subsidence of
the Hawai'ian High moves over us, bringing warm air down
from above, and that's when we really bake here. That
subsidence also creates a huge regional inversion layer,
too, and that's the summer inversion that gives us that
awful summer smog season here.
d. Meanwhile, during the Northern Hemisphere summer up at the
Subpolar Low, the land at that latitude is also warming up (yes,
it does get warm in Canada, believe it or not). This means that
there is uplift and low pressure over land. This is not too
remarkable, since there normally is uplift and low pressure at
this latitude. In other words, there isn't all that much
contrast between land and sea in air pressure at this latitude,
the way there is farther south.
i. So the Subpolar Low remains connected over land in summer,
with relatively little land and sea differential.
ii. The low pressure remains smeared out in a band with a
pressure gradient only going north and south.
iii. This means the influence of the Subpolar Low declines in
summer, and it, therefore, generates fewer storms.
e. The Northern Hemisphere winter produces opposite results. Now,
the North American and Eurasian landmasses cool and up at
60° N, it's awfully cold. Cold air subsides, producing a
high pressure area over Canada, Northern Europe, and Siberia.
Now, it's the Subpolar Low's turn to be broken up into two
oceanic cells.
i. The North Pacific one is called the "Aleutian Low" (you've
heard the weathercasters mention that, I bet).
ii. The one in the North Atlantic is called the "Icelandic
Low."
iii. Being broken into two oceanic cells means there is now a
strong, steep pressure gradient between land and sea at
this latitude, which forms two concentrated, deep pits of
pressure out at sea. Each one really sucks air into it
fast, which creates convergent uplift and triggers storms,
which then track eastbound across the northern landmasses,
pulled along by the Polar Jet Stream. So, the influence of
the Subpolar Low on weather is profoundly strengthened by
being broken into two concentrated pressure pits out at
sea.
f. Further south, meanwhile, the cooling of the land mass in winter
allows more subsidence over land, which permits the Subtropical
High to re-extend itself over the land. This smears the high
pressure back into a latitudinal, non-focussed band, as opposed
to a sharp peak out at sea. This reduction in the land and sea
contrast at this latitude during the winter reduces the
significance of the Subtropical High on the winter weather. We
in Southern California experience the southern movement and
weakening of the high pressure as the loss of our protection
from Aleutian storms, which give us our rainy winter.
I. Not all wind and pressure operate at the global scale. There are
regional and local scale wind and pressure systems, too.
1. Continental wind systems are called "monsoons."
a. These are seasonally-reversing winds.
b. They are related to the different specific heats of land and
sea, much like the material just discussed, so I'll try to be
brief.
c. Nice geotrivia first: The differential heating of land and sea
was first proposed as an explanation for monsoonal circulation
by Edmund Halley (of Halley's Comet fame) back in 1686. Oh, he
preferred to be called "HAH-lee" as opposed to "HAY-lee" as we
usually hear it pronounced.
d. In the summer, the land heats up and develops a low pressure
area. The surrounding sea lags in heating, so it remains cooler
and develops a relative high pressure area. The result is an
onshore monsoon, which brings rainy summers. Where the land
mass is especially huge (Asia), the monsoon can be REALLY rainy.
e. In the winter, the land cools down quickly, developing a
regional high, while the sea remains relatively warm, developing
a regional low. The result is an offshore flow, a land wind,
which brings drier conditions.
i. In a manner of speaking, our Santa Ana winds are triggered
by this process, so we get to participate in part of the
monsoonal circulation on our continent.
ii. The winter monsoon is really well developed in the gigantic
Eurasian landmass, but it is present in North America as
those polar Canadian winds that rush down the Mississippi
plains right into the Gulf of Mexico and the Caribbean.
2. Then, there are local breeze systems. They are really local,
affecting no more than a couple kilometers at best, and they are
diurnally-reversing, which means they change direction from night
to day.
a. Land and sea breezes.
i. These are mechanically similar to monsoons, but they're
very small scale and diurnally-reversing, where the monsoon
is contintental scale and seasonally-reversing.
ii. In the day, the land heats up, creating a very local low.
The sea is cooler and has relatively high pressure
offshore. This draws a sea breeze onshore, best developed
in the mid to late afternoon (the only reason Miami and
Chicago are humanly habitable in the summer!).
iii. At night, the land quickly loses its heat, which results in
a local high. The water, however, loses its heat more
slowly, which results in uplift and a local low pressure
area. This means that the breeze reverses itself, and you
now have a landbreeze blowing offshore in the wee hours.
DAY <-\----- NIGHT ----\--->
/ /
land \ sea land \ sea
| |
heats | cool cools | warm
\ \
L <-|--- H H ---|--> L
/ /
b. Upslope/downslope breezes.
i. These are associated with the cold air drainage that
produces inversion layers. On a slope, the formation and
sloughing off of cold air creates a downslope breeze.
ii. During the day, the land heats rapidly, but the peaks are
likely to respond to the sun first, because the cold air
layer is thinnest there. This creates a small low pressure
area on top of the hills. Pretty soon convection begins to
mix up the inversion layer, and some of that uplift will be
drawn to those peaks, creating an upslope breeze.
iii. This kind of micro-breeze system is important for
firefighters to understand, as fire will tend to move
downslope a little faster at night and upslope a little
faster in the day time.
Well, that's another lecture in the hamper, folks. Come away familiar with
the different ways air pressure is expressed (units of mercury, weight, force,
and that millibars and hectoPascals are the same thing). Be very clear on
isobar maps and how to interpret them. Understand the basic mechanics of air
transfer and what Coriolis Effect is and how wind maintains a balancing act
among the pressure gradient force, Coriolis Effect, friction, and centrifugal
force. Know what a geostrophic and a gradient wind are and why we don't
experience them at the surface. Know that winds are named for the direction
from which they blow (in general). Memorize the global pressure and wind
pattern and its explanation, as well as how seasonality and the differential
specific heats of land and ocean affect it. Be able to explain the mechanisms
among seasonally-reversing continental-scale winds (monsoons) and diurnally-
reversing local-scale breezes.
The next lecture will examine moisture in more detail as an element of weather
in the troposphere.
Document and © maintained by Dr.
Rodrigue
First placed on web: 10/15/00
Last revised: 06/19/07