welcome to chapter 5 atmospheric
pressure and wind we are going to talk
about some of the processes in the
atmosphere that move the air across the
world but first we need to talk about
air pressure and causes of wind so one
of the most basic examples or probably
the most common examples of the link
between air pressure and wind is inside
a hurricane now in the eye of a
hurricane or I should say really around
the eye wall we have extreme differences
in pressure which also as you may know
create very very strong winds but inside
a hurricane's eye we can get some of the
lowest atmospheric pressure readings on
the surface of the earth so in this
chapter we're going to look at a couple
of terms we're going to get into
something called an isobar and then your
three forces behind wind pressure
gradient force Coriolis and friction
talk about a couple of terms in it we
deal with winds going around a low and a
high pressure various kinds of high and
low pressure systems something called
the ITCZ Intertropical Convergence zone
and so the other terms here that we have
are linked to the global flow or the
global circulation of the atmosphere one
of those things that you may have heard
of before is the jet stream and we'll
get into that toward the end
so a question I have for you right now
is have you ever had your ears pop when
you go up or down in an airplane or even
over a hill if that is the case then you
have experienced differences in air
pressure your ears are trying to
calibrate to different air pressure
readings so exactly what is their
pressure well we got to think about air
as a fluid it's full of molecules in the
gas form and they're continuously in
motion and the force that's exerted by
all those gas molecules create a certain
kind of pressure on you and so it's not
just pressure that surrounds you also
but it's pressure that's above you and
so the deeper you are in the atmosphere
basically closer to the surface of the
earth you're going to have more air
pressure
a couple of links between temperature
density and air pressure density
basically means the particles are closer
together and as you can see in this we'll
say a vertical profile of the of air
pressure higher density means higher
pressure so as you get farther up into
the atmosphere we have a lot less air
pressure because one of the reasons is
because the molecules are very thin
spread out in the upper part of our
atmosphere temperature and pressure are
linked if the volume remains the same so
if we have let's say a container or even
let's say a bottle of air and you warm
that air up then you would have
increasing pressure but in the case for
this chapter we're going to really just
focus on air pressure and density so
here's another way of looking at it
pressure is force divided by area and
that force is really gravity that's
pulling most of these air molecules down
close to the surface of the earth in
fact if you're at the sea level then
it's about 15 pounds per square inch
that's a lot of weight now that unit is
not commonly used in in geography or
meteorology so what we have is something
called a millibar so that's 1,013
millibars at sea level so keep that in
mind that one number 1,013 millibars
so this is just another look at the
vertical part of the atmosphere when it
comes to pressure and the point really
here is that the air pressure decreases
very dramatically within the troposphere
so in the area around here where you
might see airplanes fly the air pressure
is much lower than it is at the surface
right here but then that decrease in air
pressure kind of flattens out as you get
farther up into the upper parts of our
atmosphere
so back down to the surface what kind of
air pressure readings do we usually have
well if we have a extreme low pressure
system we might see values down to 980
if we have a high pressure a strong high
pressure system we might see values up
to 1050 this here is a normal range now
in California we barely see these this
range where work much closer to let's
say between a thousand and ten thirty
around the world though hurricanes are
known to have the lowest air pressure
readings the record low pressure is 870
that's at the surface of the earth
that's quite quite low the highest air
pressure reading on the surface with
1084 and that was in Siberia due to
extremely cold weather in this case the
cold weather actually created such an
area with such a high density that it
created also a very high or very strong
high pressure
so what does that have to do with wind
what causes wind you can see some
dramatic examples of wind and wind all
over the world has done quite a bit of
damage to certain structures to
communities whether we're talking about
hurricanes tornadoes or even in
California with Santa Ana winds so maybe
you don't really know what causes the
wind one of the most important force
that is behind all wind is the pressure
gradient force and mathematically is the
pressure gradient force well we're going
to skip this part for now but if you
want to know what the mathematical
treatment is for pressure gradient force
it's really just a change in pressure
keep it simple it's a change in pressure
over a horizontal distance it's that
simple we can look at it this way so we
have air molecules on the left hand side
and there's a lot of air molecules on
the left versus what's on the right so
nature doesn't like imbalance so what's
going to happen is is you're going to
have these molecules moving from left to
right and the movement of air molecules
is wind on a map we draw lines of equal
air pressure and those lines are called
isobars so over here on the left it's
showing a pressure reading of 10 20 but
on the right 10 18 so air wants to move
directly from that high to the low and
by the way with this force air is moving
perpendicular to the isobars on a real
world map you may not see air moving
directly
across isobars because there are other
forces but generally speaking air wants
to leave high-pressure systems and move
into low-pressure systems
another way of looking at it is looking
at from the side air down here leaving
the high and going toward the low and
this is just showing you this diagram
right here is showing you another way of
looking at it through isobars again with
the pressure gradient force air goes
from the high to the low and there's a
right angle here between the isobar and
the force which is also in this case the
wind
now when we get wind that moves across
the earth long distances because the
earth is rotating we get something
called the Coriolis force Coriolis force
is kind of hard to explain
that's why I've posted some videos on
Canvas to help you understand the
Coriolis force a little bit better
essentially the Coriolis force is there
because the Earth rotates let's let's
look at an example here we have an
airplane and it's moving from the North
Pole and it wants to go to South America
Quito South America now pilots are aware
of the Coriolis effect if they are not
that they will never get to their
destination especially if it's a long
distance so if the pilot did not know that
the Coriolis force existed then that
pilot would end up somewhere in the
middle of the Pacific Ocean because as
this airplane is flying from the North
Pole toward South America the whole
earth is rotating counterclockwise and
so it makes it offset a little bit so the
bottom line air if you if you're in the
direction that the airplane is flying or
if you're in the wind if you are the
wind you want to veer off to the right
in the northern hemisphere but it's to
the left in the southern hemisphere
now what that does is is if we're
looking at the northern hemisphere we
have air that wants to go from the high
to the low but then eventually it gets
curled to the right at the moment that
this air is parallel to the isobars then
we have then the wind actually stops
changing direction we have something
called a geostrophic wind this really
only happens in our upper atmosphere
because in our upper atmosphere we
have the pressure gradient force and the
Coriolis force
so around centers so we're talking about
circular isobars around centers of highs
and lows we have have we've set up a
certain kind of flow air always moves
around a low in the Northern Hemisphere
counterclockwise and it moves clockwise
around a high in the southern hemisphere
air moves clockwise around the low but
counterclockwise around a high you can
kind of think of the Coriolis force as a
twisting force so it makes air want to
you know move around and the curl and it
offsets from the original direction that
the pressure gradient force wanted it to
go
so that's in the upper atmosphere what
about at the surface well at the surface
we have something called friction and
friction slows the wind down we need
friction to actually walk a great
example if we're trying to walk over ice
but what it does with the air at the
surface creates a little turbulence and
it slows the wind down so winds higher
up in the upper atmosphere as shown as
it are much faster and stronger so what
that does in terms of isobars is that it
the Coriolis force by the way is not as
strong if the wind is slower so faster
the wind the greater the effect of
Coriolis what that does here is instead
of the wind instead of the wind going
parallel to isobars
so that doesn't happen wind actually
moves toward toward the low but at an
angle and so the wind actually crosses
isobars but at about a 45 degree angle
if you're dealing with rough surfaces
like mountains and maybe around 30
degrees if you're dealing with flat
terrain and around centers of highs and
lows give air that still moves
counterclockwise but it spirals into a
low air spirals out of a high clockwise
and then in the southern hemisphere air
spirals into the low clockwise and out
of a high counterclockwise so this is an
example where we add pressure gradient
force Coriolis and friction all together
to create surface winds
so as a recap we started out with
pressure gradient force alone air moves
directly from high to low at a
perpendicular angle with the upper
atmosphere
technically the upper troposphere we
have wind parallel to isobars we call
those geostrophic winds so for example
the jet stream if you're looking at jet
stream in relation to isobars then the
jet stream would move parallel to isobars but once we get back down to the
surface of the earth then we have air
that spirals and cross
the isobars at about it let's say
anywhere between a thirty to a 45 degree
angle now keep in mind that the Coriolis
force won't really be in a factor with
short distance winds or really slow
winds so when we deal with the Coriolis force
we have to deal with winds that move
over let's say more than a thousand
miles or so large scale winds
okay so what we want to do now is since
we went ahead and talked about the three
main forces let's talk a little bit
about patterns of air pressure across
the earth and then we're going to get
into how the earth has set up this
global circulation so in January
it is winter in the northern hemisphere
and summer in the southern hemisphere
and what we notice is some of the
highest pressure readings are actually
over some of the coldest areas so
temperature is one way that we can set
up strong high pressures and strong low
pressures it's not the only way but is
one way and in this case I just want to
point out the Siberian high which is
quite cold now in the in the winter time
up north here we have dynamic closed air
air that sets up and becomes
low-pressure because there is movement
of air not because it's hot or cold and
these dynamic loads are responsible for
some of the rain systems that we have
here in California especially the
Aleutian low up here near Alaska so
we're going to stick with the northern
hemisphere now look go to July now in
July we have cooler temperatures over
the water than we have over the land
this low-pressure here over land is
responsible for the Indian monsoon this
high pressure center here west of us
west of California helps inhibit any
kind of rainfall that we could we could
possibly get in Southern California at
least large-scale rain systems where we
can get rain systems in the in the
winter time because this high-pressure
system isn't really there
now that high-pressure system is there
for a couple of reasons the water here
is is cooler the land nearby that's one
way and because of dynamic factors that
we'll talk about in a second here
alright so in order to understand the
global circulation of the atmosphere
we're going to start really small here
we're going to assume okay that the
earth does not rotate the earth is
there's no land on the earth and we
don't have seasons so there's very
strong heating at the equator very
little heating at the poles so what air
wants to do is it wants to rise up and
diverge away from the equator aloft that
sets up a low-pressure system there's
less air above the equator now on the
flip side air wants to converge at the
poles and sink that sets up by high
pressure so then air at the
surface returns goes from high to low
that's very basic way of looking at the
global circulation we have one vertical
circulation in each hemisphere and
that's basically what I've listed here
in the slides we have a thermal low a
thermal high then we have ascending air
over the equator descending air over the
poles and then the surface winds blow
from the equator to I'm sorry
winds blow equatorward at the surface
poleward aloft
now at this point what we're going to do
is we're going to make it a little bit
more complex we're going to add the
Coriolis effect and we're adding land we
still have air that wants to rise over
the equator and diverge but instead of
making it all the way to the pole it's
the air tends to sink around 30 degrees
north and 30 degrees south what's
interesting is that the sinking air does
not allow rain to form because we need
rising air for rain to form so in or
around 30 degrees we have much of our
deserts around the world on the flip
side around the equator we have air that
converges and rises up and we have a lot
of our rain forests right around the
equator in fact that itcz Intertropical
Convergence zone is a belt of
thunderstorms that hugs the equator now
those are vertical circulations now what
happens is we have we set up high
pressure systems along 30 degrees like
the subtropical high and air wants to
move from a high to a low so air goes
from subtropical high down to the
equator we call those the north easterly
trade winds and we have the south
easterly trade winds in the southern
hemisphere and then after that we have
the westerlies north of the subtropical
high the westerlies
this steers most of our weather that we
have in the northern hemisphere now
those are just some of the pieces but
one of the most important vertical
circulations that we have here is called
the Hadley cell starts at the equator
and it goes down to 30 degrees north and
30 degrees south another way of looking
at this kind of like this diagram as
well because it shows the vertical
circulation so we have the westerlies
that spiral clockwise out of the
subtropical highs and steers most of our
weather that we have here in North
America from west to east we have the
Northeast trade winds which blow from
the high to the equator or the south
easterly trade winds still from
from the high in the southern hemisphere
to the equator we have that ITCZ
Intertropical Convergence zone and we
have our Hadley cells now there's a
video that I've posted on canvas that
actually as a professor drawing each
piece of this and I strongly suggest
watching that video to get a even better
example or a better understanding of the
global circulation I do want to point
out there are other vertical
circulations we have the Ferrell cell
that goes from the high around 30
degrees to another set of lows around 60
degrees and then we have the polar cell
which hugs the North Pole and the South
Pole
that's not drawn very well in this
diagram but it is drawn very well on
that video I've posted
okay so to take a step back jet streams
not well represented on the global
circulation but they're very important
because they are fast moving quickly
moving rivers of air at the very top of
our troposphere and they steer that
helps steer much of our weather just
like the westerlies they help steer much
of our weather in North America from
west to east and we have two kinds we
have the subtropical jet stream which is
focused in the southern part of our
country and it's not as strong but the
polar jet stream is much stronger and
this is especially in the wintertime it
drives much of our rain from the Pacific
Ocean into California and then out
of California across toward the East
Coast
now jet streams can be as you know they
can be extremely fast we're talking
maybe up to 200 miles per hour these jet
streams are responsible for much of our
weather systems that get very strong
across east of the Rocky Mountains we
should say and also they help steer
aircraft so if you've ever noticed that
airplanes if you fly let's say from LA
to New York in the winter time it takes
a little bit less time to get to New
York than the other way around from New
York to LA the jet stream helps move
airplanes even faster and by the way
this is one of the best examples of a
geostrophic wind
now down at the surface another kind of
wind that I like to talk about is the
sea breeze and we're going to ignore
Coriolis because this sea breeze is very
small it's not big enough to be affected
noticeably by the Coriolis effect but
we're going to look at pressure gradient
force and specific heat so during the
day this happens a lot it's summer time
we have high pressure I should say up
let's start with temperature we have
warmer temperatures over the land than
we do over the water so air will want to
rise up and that helps create a thermal
low but over the water air sinks and
that helps create a thermal high so we
have high over the water low over the
land pressure gradient force says air
must go from high to low so therefore if
you're standing up on the beach you will
have a sea breeze air that goes from the
sea toward the land sometimes we can get
a land breeze at night because the land
will cool off more than the water so we
have a return flow a land breeze so down
here we have a high over the land and a
low over the water in California we
don't get that very often but every once
in a while we do
