METEOROLOGY
GEL-1370
Chapter Six
Air Pressure and Wind
Goal for this Chapter
We are going to learn answers to the following questions:
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How and Why Atmospheric Pressure Varies?
What forces influence atmospheric motions at aloft and surface?
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
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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 decrease
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.
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IT TAKES A SHORTER COLUMN OF COLD, MORE DENSE AIR TO EXERT THE SAME SURFACE PRESSURE
AS A TALLER COLUMN OF WARM, LESS DENSE AIR
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
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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 changes
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
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Relationship between temperature, pressure, and density is:
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For constant temperature, P density
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Pressure = constant X temperature X 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 1/density
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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
Barometers
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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
pressure
Barometer – contd.
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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
barometers
Pressure Readings
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Problems associated with reading Hg column (in obtaining air pressure) are:
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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’
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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
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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 tropics
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.
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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
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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.
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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
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Newton’s Laws of Motion:
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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 down)
Forces that affect the horizontal movement of air are:
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Pressure Gradient Force
Coriolis Force
Centripetal Force
Frictional Force
Pressure at the bottom of each tank is  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
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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 winds
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Pressure gradient force (PGF) is directed from higher toward lower pressure at right angles to the isobars
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Magnitude of this force is directly related to the pressure gradient
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
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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
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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
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It is fictitious force resulting from the rotation of the earth
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 Hemisphere.
The amount of deflection depends upon
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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
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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 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.
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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:
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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 2
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.
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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 influence
Centripetal acceleration: Force directed towards the center
Winds on Upper-level charts: On the upper-level 500-mb 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
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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
SFO
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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.
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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
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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
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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.
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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
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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
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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
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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.
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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
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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????
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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