METEOROLOGY

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METEOROLOGY

• GEL-1370

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Chapter Six

• Air Pressure and Wind

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Goal for this Chapter

• We are going to learn answers to the following
  questions
• 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?

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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 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

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Air density remains constant with height when
more air is stuffed (at same temp), in to the
column, pressure increases
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Shorter column of cold air taller column of
warm air exert same pressure aloft cold air is
associated with low pressure
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Atmospheric Pressure contd.

• 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

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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 changes
• Units Bar --- millibars (mb) --- Hectopascal
  (hPa)
• 1013.25 mb 1013.25 hPa 29.92 in. Hg

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Atmospheric pressure in inches of Hg mb
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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

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Hg barometer height of Hg is a measure of
atmospheric pressure
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Barometers

• 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

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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 barometers

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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)

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A Recording Barograph
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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
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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
• Hs Centers of high pressure (also called
  anticyclones)
• Ls 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

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Surface Map showing areas of high low pressure
solid lines are isobars at 4 mb intervals arrows
wind direction winds blow across the isobars
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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)

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Upper-level 500 mb map for the same day solid
lines contour lines in meters above sea level
dashed linesisotherms (C) wind directions are
parallel to the contour lines
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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

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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
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Forces that influences the wind

• Newtons 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
  down)
• Forces that affect the horizontal movement of air
  are
• Pressure Gradient Force
• Coriolis Force
• Centripetal Force
• Frictional Force

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Pressure at the bottom of each tank is a weight
of water above pressure at the bottom of A gt
pressure at the bottom of B greater the
difference higher the force
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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 winds
• 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 gradient

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PGF between 1 2 is 4 mb/100km PGA Net force
directed from higher toward lower pressure
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Closer isobars--- greater pressure gradient---
stronger PGF--- greater the wind speed length of
arrows indicate magnitude of PGF
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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

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Surface weather map
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Coriolis Force

• 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
• Rotation of the earth
• Latitude (0 at equator and maximum at the poles)
• Objectss speed

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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
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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
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Geostrophic (Earth turning) Wind

• Why winds aloft more or less parallel to the
  isobars or contour lines?
• Consider air at 1-km above the earths 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)

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At 1-km above earths 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.
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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
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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
  2

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Winds and related forces around areas of low and
high pressure above the friction level in the
Northern Hemisphere
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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 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

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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 SFO
• 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

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Upper-level 500-mb showing wind direction solid
gray lines are contours in meters AMSL dashed
red lines are isotherms in degree C
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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

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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
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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
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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)

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Winds and air motions associated with surface
highs and lows in the Northern Hemisphere
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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

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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

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Onshore and offshore wind
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Wind direction expressed in degrees
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Unprotected (from wind) trees are sculpted into
flag trees
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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

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A wind vane a cup anemometer
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Wind rose represents of time the wind blew from
different direction during January (10 yrs)
prevailing wind is NW least frequency is NE
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The aerovane (skyvane)
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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

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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????
• YES

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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, Newtons 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