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Transcript CHAPTER – 3

Chapter Six
Air Pressure and Wind
Goal for this Chapter
We are going to learn answers to the following
• How and Why Atmospheric Pressure Varies?
• What forces influence atmospheric motions at aloft and
• How the Wind should flow in a particular region?
• What is pressure gradient?
• What is Coriolis Force?
• What is Geostrophic and Gradient Wind?
• Why in Northern Hemisphere winds flow clockwise
around regions of high pressure?
• Various methods & instruments to measure wind speed
and direction?
Atmospheric Pressure
• Air pressure: Measure of mass of air above a given level
• Atmospheric Pressure decreases with altitude, as there
are fewer molecules above us
• Temperature, pressure and density of air are related
to each other
• For a given volume of air, adding more to the column
will increase the surface air pressure (temp constant) & if
we remove air from the column, the pressure will
• If we have two identical column, one undergoes cooling
and other warming, the one that cools becomes more
dense; one that warms, becomes less dense
Air density remains constant with height; when
more air is stuffed (at same temp), in to the
column, pressure increases;
Shorter column of cold air & taller column of
warm air exert same pressure – aloft cold air
is associated with low pressure
Atmospheric Pressure – contd.
• Atmospheric pressure decreases more rapidly with
elevation in the cold column of air; in the warmer,
less denser air (associated with high atm pressure),
this pressure does not decrease as rapidly with height,
as there are fewer molecules
• Horizontal difference in temperature creates a
horizontal difference in pressure – pressure gradient;
pressure difference creates Pressure Gradient Force
that causes air to move from high to low pressure
Atmospheric pressure and measurement
• Heating/Cooling of air leads to horizontal variations in
pressure that cause the air to move; net accumulation of
air above the surface causes the surface air pressure to
rise, whereas a decrease in the amount of air --- pressure
to fall
• Measuring Air Pressure
• Air Pressure = Force exerted by the air molecules/area
• Barometers: Instruments that detect & measure pressure
• Units: Bar --- millibars (mb) --- Hectopascal (hPa)
• 1013.25 mb = 1013.25 hPa = 29.92 in. Hg
Atmospheric pressure in inches of Hg & mb
Tinkering with Gas Law
• Relationship between temperature, pressure, and density
is: Pressure = constant X temperature X density
• For constant temperature, P a density
– For nearly the same temperature and elevation, air above a
region of surface high pressure is more dense than air
above a region of low pressure
For constant pressure, T a 1/density
– For a given atmospheric pressure, air that is cold is
more dense than air that is warm
Hg barometer – height of Hg is a measure of
atmospheric pressure
• Aneroid barometer: No fluid; an aneroid cell (small,
flexible metal piece), air is partially removed --- small
changes in external air pressure cause the cell to expand
or contract; size of the cell is calibrated to measure
Barometer – contd.
• Higher the reading --- more likely clear weather; lower
the reading --- inclement weather
• Surface high pressure areas are associated with sinking
air and normally fair weather; surface low-pressure
areas are associated with rising air and usually cloudy,
wet weather
• A steady rise in atmospheric pressure usually indicates
clearing weather or fair weather; steady drop in
atmospheric pressure often signals the approach of a
storm with inclement weather
• Altimeter (calibrated to indicate altitude) and barograph
(recording aneroid barometer) are two types of aneroid
Pressure Readings
• Problems associated with reading Hg column (in
obtaining air pressure) are:
– Temperature (expands when heated & contracts when cooled);
Corrections are made as if they were read at the same temp.
– Changes in Gravity: Earth mass distribution leads to
differences; must be corrected
– Instrument Error: Mainly due to the surface tension of Hg
against the glass tube
The corrected pressure is called ‘Station Pressure’
– Pressure changes vertically; Monitoring changes in
horizontal pressures that we normalize with respect to
altitude (sea level pressure)
– Atmos. pressure decreases ~10 mb/100 m (0.1mb/m)
A Recording Barograph
Cities A,B,C,D at 4 elevations with different station pressures;
b) sea level pressures of 4 cities on a sea level chart; c) isobars
drawn on the chart at 4 mb intervals
Surface Map
• Isobars do not pass through each point, but with the
values interpolated from the data given on the chart
• With isobars plotted, the chart is called ‘sea level
pressure chart’ or simply ‘Surface Map’
• When weather data are plotted are in this map, it
becomes ‘Surface Weather Map’
Surface and Upper-Air Charts:
H’s: Centers of high pressure (also called anticyclones)
L’s: Centers of low pressure (also known as depressions
or mid-latitude depressions or extra-tropical cyclones)
– they form in the middle latitudes, outside of the
Surface Map showing areas of high & low pressure; solid lines
are isobars at 4 mb intervals; arrows wind direction; winds
blow across the isobars
Surface & Upper-Air charts contd.
• The upper-air map is a constant pressure chart --constructed to show height variations along a constant
pressure (isobaric surface) – Isobaric maps
• Contour lines connect points of equal elevation above
sea level
• Contour lines of low height represent regions of lower
pressure & lines of high height represent region of
higher pressure;
• Contour lines decrease from south to north; isotherms
(dotted line) shows north is colder than south --- cold air
aloft is associated with low pressure
• Contour lines bend $ turn indicating elongated highs
(ridges, warmer air) & depressions (troughs, colder air)
Upper-level 500 mb map for the same day; solid lines: contour
lines in meters above sea level; dashed lines:isotherms (°C);
wind directions are parallel to the contour lines
Upper-air charts contd.
• The winds on the 500-mb chart tend to flow parallel to
the contour lines on a wavy west-to-east direction
• Surface maps describe where the centers of high & low
pressures are found and winds and weather associated
with these systems
• Upper-air charts are important for weather forecast;
upper-level winds determine the movement of surface air
pressure systems, as well as whether these surface
systems will intensify or weaken
Less dense air in the south; cold air in the north; Height of the
pressure surface varies; Changes in elevation of a constant
pressure surface shown as a contour lines on a isobaric map
Forces that influences the wind
• Newton’s Laws of Motion:
– First Law: An object will continue to rest or its uniform
motion unless it is compelled by an external force
– Second Law: F = ma (Acceleration of an object is caused by
all the forces acting on it); Force acting on it is proportional to
acceleration (Acceleration is the speeding-up, the slowing
Forces that affect the horizontal movement of air are:
– Pressure Gradient Force
– Coriolis Force
– Centripetal Force
– Frictional Force
Pressure at the bottom of each tank is a weight of
water above; pressure at the bottom of A > pressure at
the bottom of B; greater the difference higher the force
Pressure Gradient Force
• Pressure Gradient = Pressure Difference/distance
• Pressure Gradient Force is the force that causes the wind
to blow; closely spaced isobars on a weather chart
indicate steep pressure gradients, strong forces, and high
• Pressure gradient force (PGF) is directed from higher
toward lower pressure at right angles to the isobars
• Magnitude of this force is directly related to the pressure
PGF between 1 & 2 is 4 mb/100km; PGA: Net force
directed from higher toward lower pressure
Closer isobars--- greater pressure gradient--stronger PGF--- greater the wind speed– length of
arrows indicate magnitude of PGF
Surface Weather Map
Dark Grey lines: Isobars in mb
A deep low with a central pressure of 972 mb
Distance along X-X’ is 500 km
Difference in pressure between X & X’ is 32 mb
Pressure gradient = 0.064 mb/km
Tightly packed isobars along the green line associated
with northwesterly winds of 40 knots
• Wind speeds are indicated by barbs and flags;  would
be a wind from the northwest at 10 knots
• Solid blue line is a cold front; solid red line is a warm
front; heavy dashed line is a trough
Surface weather map
Coriolis Force
• It is fictitious force resulting from the rotation of the
• To an observer on the earth, objects moving in any
direction (north, south, west, east) are deflected to the
right of their intended path in the Northern hemisphere
and to the left of their intended path in the Southern
• The amount of deflection depends upon
– Rotation of the earth
– Latitude (0 at equator and maximum at the poles)
– Objects’s speed
If we watch from above, the ball moves on a straight
path; for anyone in platform B, the ball appears to
deflect to the right of its intended path
All freely moving objects (ocean currents, aircraft,
artillery projectiles, air molecules) seem to deflect
from a straight-line path; it is greater at the poles and 0
at the equator
Geostrophic (Earth turning) Wind
• Why winds aloft more or less parallel to the isobars or
contour lines?
• Consider air at 1-km above the earth’s surface; the PGE
acts on the air accelerating it northward toward lower
pressure--- when the air begins to move, CF deflects
the air toward its right, curving its path ---as the speed
of air increases (2,3,4) CF increases bending the wind
more; CF increases with latitude; at point 5, net force =
0--- wind flows in a straight path, parallel to the isobars
at a constant speed – This flow of air is called
Geostrophic Wind
• Coriolis acceleration = 2 w x v = 2 wv cos q (q:
latitude; w: angular velocity of rotation of earth; v:
vertical velocity of air mass)
At 1-km above earth’s surface, the isobaric lines are
evenly spaced (constant PGF); parcel of air left at 1;
two forces act-PGF and CF; CF increases with lati.
Isobars and contours on a upper-level chart; when
widely spread, flow is weak; when narrowly
spaced, flow is stronger; increase in winds results in
a stronger CF which balances larger PGF
Geostrophic wind contd.
• When the flow is purely geostrophic, the isobars (or
contour lines) are straight and evenly spaced and wind
speed is constant; the speed of geostrophic wind is
directly related to the pressure gradient
• Curved Winds Around Lows & Highs Aloft:
– The counter clockwise flow of air around Lows (known as
cyclones) is anticyclonic flow
– Clockwise flow of air around a high or anticyclone is called
anticyclonic flow
– In Figure a) at point 1, PGF accelerates the air inward toward
the center of the low and the CF deflects the moving air to its
right, until the air is moving parallel to the isobars at position
Winds and related forces around areas of low and
high pressure above the friction level in the
Northern Hemisphere
Curved winds around lows and highs aloft – contd.
• If the wind were geostrophic, at position 3 the air would
move northward parallel to straight-line isobars at a
constant speed
• Gradient Wind: Wind that blows at a constant speed
parallel to curved isobars above the level of frictional
• Centripetal acceleration: Force directed towards the
• Winds on Upper-level charts: On the upper-level 500mb chart, the winds tend to parallel the contour lines;
wind is geostrophic where it blows in a straight path
parallel to evenly spaced lines; it is gradient where it
blows parallel to curved contour lines
Winds on upper-level charts
When the lines are closer together, winds are stronger
Where the lines are farther apart, the winds are weaker
Meridional Flow: When wind blows in a north-south trajectory
Zonal Flow: Winds blowing in a west-to-east direction
Winds aloft in middle and high latitudes generally flow from
west to east –shorter Time of Flight from SFO to NY than NY to
• Surface Winds:
• Winds on a surface weather map do not blow exactly parallel to the
isobars; instead, they cross the isobars, moving from higher to lower
pressure; the angle at which the wind crosses the isobars varies, but
averages about 30°. The reason for this behavior is friction
Upper-level 500-mb showing wind direction; solid gray
lines are contours in meters AMSL; dashed red lines are
isotherms in degree C
Surface winds – contd.
• Friction Layer: The atmospheric layer that is influenced
by friction (planetary boundary layer) usually extends
upward to an altitude near 1000m above the surface
• In Fig a) wind aloft is blowing at a level above the
frictional influence of the ground; the wind is ~
geostrophic and blows parallel to the isobars with the
PGF on its left is balanced by the CF on its right
• Near the surface, friction reduces the wind speed, which
in turn reduces the coriolis force; weaker CF no longer
balances the PGF and the wind blows across the isobars
toward lower pressure
Effect of surface friction is to slow down the wind; near the
ground, the wind crosses the isobars & blows toward lower
pressure; this produces an outflow of air around a high and
an inflow around a low
Surface weather map showing isobars and winds in
December in South America; b) boxed area shows
the idealized flow around surface-pressure systems in
the Southern Hemisphere
Surface winds & Vertical air motions
• In the Northern hemisphere, surface winds blowing
counterclockwise and into a low; they flow clockwise
and out of a high
• In the Southern hemisphere, winds blow clockwise and
inward around surface lows, counterclockwise and
outward around surface highs
• Vertical Air Motions: As air moves inward toward the
center of a low-pressure area, it must go somewhere.
Since the converging air cannot go into the ground, it
slowly rises and begins to spread apart
• The vertical motions ~ several cm/s (1.5 km/day)
Winds and air motions associated with surface
highs and lows in the Northern Hemisphere
Vertical motion – contd.
• Hydrostatic Equilibrium: When the upward-directed
pressure gradient force is exactly balanced by the
downward force of gravity, equilibrium exists
• Hydrostatic equilibrium does not exist in violent
thunderstorms and tornadoes where the air shows
vertical acceleration
Measurement of Wind Speeds
Onshore Wind: Wind blowing from the water onto land
Offshore Wind: Wind blowing from land to water
Upslope Wind: Air moving uphill
Downslope Wind: Air moving downhill
Wind Direction: calm is zero; 360° is North
Prevailing Wind: Wind direction most often observed
during a given time period; prevailing winds greatly
affect the climate of a region
• In the Northeastern half of the US, the prevailing wind in
winter is northwest & in summer, it is southwest
• Wind Rose: Indication of the percentage of time the
wind blows from different direction
Onshore and offshore wind
Wind direction expressed in degrees
Unprotected (from wind) trees are sculpted into
‘flag’ trees
Wind Instruments
• Wind Vane: Old, yet reliable, weather instrument to
determine wind direction; arrow always points into the
wind direction
• Anemometer: An instrument to measure wind speed
• Aerovane: An instrument used to indicate both wind
speed and direction
• Wind information can be obtained during a radiosonde
observation (temp, pressure & humidity); instrument in
the ground constantly tracks the balloon, measuring its
vertical and horizontal angles as well as height above
the ground (up to 30 km or so)
• Wind speed & direction can be obtained from satellites
A wind vane & a cup anemometer
Wind rose represents % of time the wind blew from different
direction during January (10 yrs); prevailing wind is NW &
least frequency is NE
The aerovane (skyvane)
Wind measurements – contd.
• Satellite track the movement of clouds --- direction of
cloud movement indicates wind direction & horizontal
distance the cloud has moved for a given time is a
measure of wind speed
• Doppler Radar is used to obtain a vertical profile of
wind speed & direction upto an altitude of 16 km or so;
such a profile is called wind sounding and the radar is
called wind profiler
• The amount of force exerted by the wind over an area
increases as the square of the wind velocity; when the
wind velocity doubles, the force it exerts on an object
goes up by a factor of four
Wind Power
• Wind Turbines produce energy in winds as low as 5
knots and as high as 45 knots
• In 1997, over 15,000 wind machines were generating
over 3.5 billion KWH (~1% of the total need)
• Can we go completely Wind Power-based, rather than
solar, nuclear, fossil-fuel based????
chapter –6- Summary
Variation of air pressure with altitude
Barometer, anemometer,
Relationship between atmospheric pressure & gravity
Station pressure
Surface weather map, 500-mb contour map
Wind direction in upper-level chart; ridge, trough
Acceleration, Newton’s laws, friction, gravitational force
Pressure gradient, Pressure gradient force
Coriolis force, variation with latitude
Zonal & Meridional flow
Geostrophic wind, Gradient wind, Offshore & onshore winds
Hydrostatic equilibrium
Wind rose, Wind profiler