Chapter 6
Air Pressure and Winds
Atmospheric Pressure
• Air Pressure – is simply the mass of air
above a given level
• Recall as we go up in the atmosphere,
there are fewer molecules above us.
• Pressure always decreases with height!
• Pressure = temperature x density x a
constant
• Standard Atmosphere=1013.25 hPa (mb)
or 29.92 in Hg.
A simple Atmospheric Model
• A model of the
atmosphere where air
density remains constant
with height. The air
pressure at the surface is
related to the number of
molecules above. When
air of the same
temperature is stuffed
into the column, the
surface air pressure rises.
When air is removed from
the column, the surface
pressure falls.
All things equal
• Two columns of air, at
equal temperature
and height.
• p ~T x ρ
p is pressure
T is temperature
r (rho) is density
Pressures equal but Temps different!
• It takes a shorter column
of cold air to exert the
same pressure as a taller
column of warm air...
•
•
•
•
(The difference in height
between the two columns
is greatly exaggerated.)
Pressures are the same
Temperatures are not the
same
p~T  ρ↓
p~T↓ ρ 
Air flows from a High into a Low
• ...because of this fact, aloft,
cold air is associated with low
pressure and warm air with
high pressure. The pressure
differences aloft create a force
that causes the air to move
from a region of higher
pressure toward a region of
lower pressure. The removal of
air from column 2 causes its
surface pressure to drop,
whereas the addition of air into
column 1 causes its surface
pressure to rise.
Measuring Air Pressure
• Air pressure – mass of the atmosphere above any level
or the force exerted by the air molecules over a given
area.
• Barometer – Instrument used to detect and measure
pressure changes
• Millibar (mb) – unit of pressure most commonly found on
surface weather maps. One thousandth of a bar.
• Hectopascal (hPa) – new preferred unit of pressure on
surface maps
• 1 mb = 1 hPa
• Inch of mercury (Hg) – english unit of measure for
pressure. (Used on TV quite a bit)
Atmospheric pressure in inches of mercury and in millibars.
Diurnal Pressure Changes
• Daily sea-level pressure variations in middle and low
latitudes. Middle-latitude pressure changes are primarily
the result of large high- and low-pressure areas that
move toward or away from a region. Diurnal surface
pressure changes in the tropics reflect the influence of
the atmospheric tides.
Barometers
• “Barometric Pressure” – term often applied
to atmospheric pressure because it is
measured using a barometer.
Mercury Barometer
• Mercury Barometer –
Torricelli – 1643 –
long glass tube filled
with mercury
• The height of the
mercury column is a
measure of
atmospheric
pressure.
Aneroid Barometer
• Aneroid barometer –
most common type of
home barometer. No
fluid but rather an aneroid
cell (small flexible metal
box) sealed to outside so
when pressure changes
the box
expands/contracts.
Altimeter and barograph
are two types of aneroid
barometers.
A recording barograph
Pressure Readings
• Station Pressure – barometer reading at a particular
location and elevation
• Altitude corrections – must be made so that a barometer
reading taken at one elevation can be compared with a
reading taken at another site. (Near the surface,
pressure decreases on the average by about 1 mb for
every 10 m increase in elevation.
• Sea level pressure – adjusted barometer reading. Size
of correction depends on how high station is above sea
level.
• Isobar – lines connecting points of equal pressure
•
The top diagram (a) shows four cities (A, B, C, and D) at varying elevations
above sea level, all with different station pressures. The middle diagram (b)
represents sea-level pressures of the four cities plotted on a sea-level chart.
The bottom diagram (c) shows isobars drawn on the chart (gray lines) at
intervals of 4 mb.
Sea-level isobars drawn so that each observation is taken into account. Not
all observations are plotted.
Sea-level isobars after smoothing
Surface and Upper Air Charts
• Sea level pressure chart (surface map) –
chart with sea level pressure values
plotted on it
• Surface weather map (chart) – a map that
shows the distribution of sea level
pressure with isobars and weather
phenomena. “Surface Chart”
Surface map showing areas of high and low pressure. The solid lines are
isobars drawn at 4-mb intervals. The arrows represent wind direction.
Surface Charts
• Winds – arrows on a surface map indicate
the direction from which the wind is
blowing (a frommy!)
• Blue H’s – represent High pressure – or
anticyclones
• Red L’s – represent Low pressure – or mid
latitude cyclones- extratropical cyclones
• Isobars – lines of equal pressure [millibars
(mb)]
Because of the changes in air density, a surface of constant pressure (shaded
gray area) rises in warm, less-dense air and lowers in cold, more-dense air.
Where the horizontal temperature changes most quickly, the constant pressure
surface changes elevation most rapidly.
The 500-mb map for the same day as the surface map. Solid gray lines on the
map are contour lines in meters above sea level. Dashed red lines are
isotherms in °C. Arrows show wind direction.
Upper Air Charts
• Upper air charts are charts of equal pressure,
therefore the heights of these equal pressures
vary.
• Isoheights – contour lines – lines that connect
points of equal elevation above sea level
• Blue H’s – are high heights
• Red L’s – are low heights
• Ridges – elongated highs- warm ridges
• Troughs – (trofs) – elongated lows – cold trofs
Why the wind blows
• Newton’s First Law – an object at rest will
remain at rest and an object in motion will
remain in motion (travel at a constant
velocity along a straight line) as long as no
force is exerted on the object (i.e. thrown
baseball)
• Newton’s Second Law – the force exerted
on an object equals its mass times the
acceleration produced; F=ma
Why the wind blows (cont)
• F=ma
• When mass of an object is constant, the force on
the object is directly related to the acceleration
that is produced. (Acceleration is the speeding
up, the slowing down, or the changing of
direction of an object) – Change in velocity
over time.
• More than one force acts on an object, and
Newton’s second law refers to the net (total)
force that results.
• Objects accelerate in the direction of the total
force acting on it
Forces that influence the wind
• The higher water level
creates higher fluid
pressure at the bottom of
tank A and a net force
directed toward the lower
fluid pressure at the
bottom of tank B. This net
force causes water to
move from higher
pressure toward lower
pressure.
Since it is easier to visualize a tank of water than a tank of air…
Forces that affect the horizontal
movement of air
•
•
•
•
Pressure Gradient Force (PGF)
Coriolis Force (CF)
Centripetal Force
Friction
Pressure Gradient and PGF
• The pressure gradient
between point 1 and
point 2 is 4 mb per
100 km. The net force
directed from higher
toward lower pressure
is the pressure
gradient force.
Winds always blow from Higher toward Lower Pressure!!
Pressure Gradient and PGF
• Pressure gradient = difference in pressure/distance
• Pressure Gradient Force – the force due
to differences in pressure within the
atmosphere that causes air to move and,
hence, the wind to blow. Directionally
proportional to pressure gradient
• PGF is directed from higher toward lower
pressure at right angles to the isobars.
• PGF causes the wind to blow!!!!
PGF
• The closer the spacing of
the isobars, the greater
the pressure gradient.
The greater the pressure
gradient, the stronger the
pressure gradient force
(PGF). The stronger the
PGF, the greater the wind
speed. The red arrows
represent the relative
magnitude of the force,
which is always directed
from higher toward lower
pressure.
Pressure Gradient Force
• Surface weather map for 6
A.M. (CST), Tuesday,
November 10, 1998. Dark gray
lines are isobars with units in
millibars. The interval between
isobars is 4 mb. The distance
along the green line X-X' is
500 km. Wind directions are
given by lines that parallel the
wind. Wind speeds are
indicated by barbs and flags.
The solid blue line is a cold
front, the solid red line a warm
front, and the solid purple line
an occluded front. The heavy
dashed line is a trough.
Coriolis Force
• If PGF was the only force acting upon air, we would
always find winds blowing directly from higher pressure
to lower pressure.
• However, the moment air starts to move from higher
pressure to lower pressure it is deflected (to the right in
the N. Hemisphere and to the left in the S. Hemisphere)
by the Coriolis force.
• An apparent force that is due to the rotation of the earth.
• As speed of wind increases, deflection due to the
Coriolis force increases.
• Deflection is zero at the equator and a maximum at the
poles.
Coriolis Force
Summary
• Objects moving any direction (N,S,E,W)
are deflected to the right of their intended
path in the NH and to the left in the SH
• The amount of deflection depends on:
– The rotation of the earth
– The latitude
– The object’s speed
Coriolis Force
• On nonrotating platform A, the thrown ball moves in a
straight line. On platform B, which rotates
counterclockwise, the ball continues to move in a straight
line. However, platform B is rotating while the ball is in
flight; thus, to anyone on platform B, the ball appears to
deflect to the right of its intended path
Coriolis Force
• Except at the equator, a
free-moving object
heading either east or
west (or any other
direction) will appear from
the earth to deviate from
its path as the earth
rotates beneath it. The
deviation (Coriolis force)
is greatest at the poles
and decreases to zero at
the equator.
Coriolis Force
• Except at the equator, a
free-moving object
heading either east or
west (or any other
direction) will appear from
the earth to deviate from
its path as the earth
rotates beneath it. The
deviation (Coriolis force)
is greatest at the poles
and decreases to zero at
the equator.
Straight line flow aloft
• Above the level of friction,
air initially at rest will
accelerate until it flows
parallel to the isobars at a
steady speed with the
pressure gradient force
(PGF) balanced by the
Coriolis force (CF). Wind
blowing under these
conditions is called
geostrophic.
Why is this wind blowing from the west?
Pt. 1 PGF wind moves toward low pressure
Coriolis deflects the air to right. As parcel
increases speed, deflection increases.
Pt. 5 the wind speed increases to a point
Where the CF balances the PGF (net force
zero) and the wind blows parallel to isobars.
The isobars and contours on an upper-level chart are like the banks along a
flowing stream. When they are widely spaced, the flow is weak; when they
are narrowly spaced, the flow is stronger. The increase in winds on the chart
results in a stronger Coriolis force (CF), which balances a larger pressure
gradient force (PGF).
Wind flow around low pressure
• Winds and related
forces around an area
of low pressure above
the friction level in the
Northern Hemisphere.
Notice that the
pressure gradient
force (PGF) is in red,
while the Coriolis
force (CF) is in blue
Cyclonic flow around a low!!!
Wind flow around high pressure
• Winds and related
forces around an area
of high pressure
above the friction
level in the Northern
Hemisphere.
• Clockwise flow
around a high
Anticyclonic flow around a High!!
Wind flow around Highs and Lows
• Gradient wind – wind that blows at a
constant speed parallel to curved isobars
above the level of frictional influence
• Centripetal Force (Centri: center; petal: to
push toward) – the radial force required to
keep an object moving in a circular path.
Directed toward the center of that curved
path.
Winds on Upper level charts
• Winds tend to parallel the contour lines
• Widely spaced contour lines = light winds
• Closely spaced contour lines = strong
winds
• Meridional flow – a type of atmospheric
circulation pattern in which the north-south
component of the wind is dominant
• Zonal flow – a pattern in which the eastwest component of the wind is dominant
An upper-level 500-mb map showing wind direction, as indicated by lines that
parallel the wind. Wind speeds are indicated by barbs and flags. (See the green
insert.) Solid gray lines are contours in meters above sea level. Dashed red
lines are isotherms in °C.
Point out zonal and meridional flow!
Surface Winds
• The effect of surface
friction is to slow
down the wind so
that, near the ground,
the wind crosses the
isobars and blows
toward lower
pressure.
Note: Surface winds do not blow exactly parallel to the isobars!
Friction
• This phenomenon at
the surface produces
an inflow of air around
a low and an outflow
around a high. Aloft,
the winds blow
parallel to the lines.
Southern Hemisphere
• Winds around areas of
low and high pressure in
the Southern
Hemisphere. Notice that
at the surface winds blow
clockwise and inward
around the low (L);
counterclockwise and
outward around the high
(H).
Southern Hemisphere
• Surface weather map
showing isobars and
winds on a day in
December in South
America.
Buys-Ballot Law
• In the Northern
Hemisphere, if you
stand with the wind
aloft at your back,
lower pressure aloft
will be to your left and
higher pressure to
your right.
Buys-Ballot at the Surface
• At the surface, the
same relationship
holds if, with your
back to the surface
wind, you turn
clockwise about 30°.
Winds and Vertical Air Motions
• Generally speaking vertical motions are only
about several centimeters per second or about 1
mile per day
• Air rises over a surface low and sinks over a
surface high.
• Why doesn’t air over a low rush off into space?
• Hydrostatic equilibrium – the state of the
atmosphere when there is a balance between
the vertical pressure gradient force and the
downward pull of gravity.
Winds and air motions associated with surface highs and lows in the
Northern Hemisphere.
Measuring and Determining Wind
• Wind is characterized by direction, speed, and
gustiness.
• Wind direction is the direction from which the
wind is blowing
• Prevailing Wind – name given to the wind
direction most often observed during a given
time of day. Prevailing wind in winter in US is
NW, and SW in summer.
(Houses should have windows facing SW to
provide summertime ventilation and few
windows facing NW (where cold winds come
from)
An onshore wind blows from water to land, whereas an offshore wind
blows from land to water.
Wind direction
• Wind direction can be
expressed in degrees
about a circle or as
compass points.
Flag Trees
• In the high country,
trees standing
unprotected from the
wind are often
sculpted into "flag"
trees.
Wind Instruments
• Wind vane – old, reliable weather instrument for
determining wind direction (long arrow with tail)
• Anemometer – measures wind speed
• Aerovane (skyvane) – instrument used to
indicate both wind speed and direction.
• Wind profiler – a doppler radar capable of
measuring the turbulent eddies that move with
the wind. Provides a vertical picture of wind
speed and wind direction.
The aerovane (skyvane).
Wind Rose
• This wind rose represents
the percent of time the
wind blew from different
directions at a given site
during the month of
January for the past ten
years. The prevailing
wind is NW and the wind
direction of least
frequency is NE.
Automated Surface Observing
System (ASOS)
• A wind vane and a
cup anemometer.
These instruments
are part of the ASOS
system.