Atmospheric Pressure and Wx Charts 5&6

Atmospheric Pressure and Altimetry


  • Evangelista Toricelli who was a student of Galileo invented the barometer.
  • The Italian discovered in 1643 a way to measure the atmospheric air pressure. He placed a tube fully filled with mercury upside down in an open mercury container. The mercury level in the tube balanced around a height of 30 inches.


  • There are 2 common types of Barometers:
  • Mercurial


  • Aneroid Wafer type.


  • There is a third type
  • The water barometer


  • Atmospheric pressure defined as the force per unit area
  • –For example 14.7 pounds per square inch
  • –Or force per square centimeter (millibar)
  • –Or the force of 1 newton per square meter (pascal)
  • 1 hectopascal is = to 1 millibar


  • 1013.2 Millibars, or Hectopascals
  • 29.92 inches of mercury
  • 14.7 psi
  • 59° F
  • 15° C


  • Station pressure is simply the pressure measured at a particular airport
  • –Higher elevation airports have lower pressure than low elevation airports
  • Pressure drops at an average of 1 inch/ 1000’ as we go up in the atmosphere


  • As altitude goes up pressure goes down, Thus the air pressure on Mt Denali is much lower than that at MWH
  • So there’s got to be a method to make station pressure relevant to other station pressures taken at different elevations
  • First we must understand how temperature effects pressure


  • Causes an airmass to expand or contract
  • –Warm temps cause expansion
  • –Cold temps cause contraction
  • Therefor a given pressure line will be higher when warmer
  • The pressure line will be lower when colder
  • So when it’s colder than standard at your altitude, you are closer to the terrain than your altimeter indicates
  • CFIT


  • As illustrated in the diagram, the atmosphere shrinks due to colder temps, our true altitude becomes less
  • –This can become hazardous in mountainous terrain


  • We can account for non-standard temps by using our E6b
  • We will need:
  • –OAT
  • –Pressure altitude
  • –Indicated altitude
  • PA = 10,000
  • Temp = -40
  • True Alt. = 8689
  • So because the temperature at 10,000 is far below standard, you are 1,311 feet closer to the ground than your altimeter says!


  • The same idea applies to non-standard pressure
  • However the altimeter has the Kollesman window for us to input the correct pressure
  • It does not however have a mechanism for compensating for non-standard temps


  • If station pressure were used on pressure charts what we’d see would be a contour map of the terrain
  • –Lower pressures at higher altitudes and vise versa
  • So there are two methods for leveling the pressures across elevations
  • Reduction to SLP involves using an empirical approach to essentially predict what the temp would be if you dug a hole all the way to sea level and measured the pressure
  • Altimeter Setting uses a different math formula to guess what the pressure would be at that station if it were located at sea level

Altitude Correction

  • At the surface pressure changes 1”/1000 or 10mb/100m
  • If station elevation is 300m then 30mb need to be added to get sea-level pressure
  • Once this correction is applied isobars can be drawn


  • METAR KMWH 142352Z 36013KT 10SM OVC100 M08/M14 A3010 RMK AO2 SLP217 T10781139
  • Station Pressure is the actual pressure reading at the station
  • Altimeter Setting is the value of atmospheric pressure used to adjust the altimeter so that it indicates the height of the aircraft above a known reference surface
  • Sea Level Pressure (SLP) is empirically determined from station pressure accounting for temp and elevation
  • –1021.7 millbars would be the pressure if MWH was at 0’ elevation
  • If we convert 30.10 we get 1019.30 millibars at station elevation of 1189
  • –However SLP is reported at 1021.7
  • To convert inches to millibars multiply by 33.8637526
  • To convert millibars to inches multiply by .0295301


  • Density ρ(rho) is the degree of compactness of a substance, in this case air molecules
  • The book gives us Boyle’s law
  • –M is molar mass (found by adding the atomic mass of the elements that make up air)
  • –P is pressure
  • –R is the universal gas constant 8.3144621(75)JoulesKelvin-1 mol-1
  • –T is temperature in Kelvin
  • But we can look at it a simpler way
  • –P=T x ρ x constant
  • –Reworking the equation for ρ we get
  • If the temp is constant and pressure increases the density will increase
  • If the pressure is constant and temp increases density decreases
  • So if we work a problem:
  • –288K is 273+15°C
  • –1.226 kg/m3 sea level density
  • –2.87 is the constant for air
  • Using the formula P=T(rho)(C)
  • P=(288)(1.226)(2.87)
  • P=1013mb


  • Density is also affected by humidity
  • Dry air has a molecular weight of about 29 g/mol
  • Water vapor has a molecular weight of about 18 g/mol
  • Adding water vapor then decreases density in a given air volume
  • A 500’ error in density altitude is possible for temps above 90 and RH above 80%


  • Indicated altitude
  • –read off a correctly set altimeter
  • –Can be found by using your eyeballs
  • Pressure altitude
  • –altitude of the 29.92” line or read off altimeter when set to 29.92
  • –Can be found using your calculator
  • Density altitude
  • –pressure altitude corrected for nonstandard temp.
  • –Can be found using your E6B
  • –Rule Of Thumb: 120’ for each degree above or below standard
  • Absolute altitude
  • –the height above the surface (AGL)
  • –Can be found by dropping your mother in law out the door and timing the drop
  • True altitude
  • –actual altitude above sea level
  • –Can be found using your E6B
  • –Could be inaccurate if the temp doesn’t follow a standard lapse rate


  • High Density altitude refers to height not density
  • –Reduced power
  • –Reduced thrust
  • –Reduced lift
  • Causal factors behind high density altitude
  • –High temp
  • –High altitude
  • –High humidity
  • –Low pressure
  • Use the same indicated airspeeds but TAS is higher ground speed is higher in no wind conditions
  • Effects of high density altitude
  • –Longer takeoff roll
  • –Longer landing roll
  • –Lower climb rate
  • –Lower service ceiling


  • When flying from High to Low “Look out below”
  • When flying from Low to High “High in the sky”
  • Above 18,000 feet the altimeter is set to 29.92 and only pressure altitudes are flown


  • Cyclonic
  • Rotates counterclockwise
  • Area of rising air
  • Usually clouds present
  • Bad weather
  • No Kid’n Serious Low Pressure


  • Anticyclonic
  • Rotates clockwise
  • Area of descending air
  • Usually no clouds
  • Good weather

Chapter 9

  • Chapter 9
    Global Circulations and Jet Streams

    Weather books love to show the single-cell model
    This is an over simplification of the heat engine at work
    Heat rises at the equator, cools at the poles, sets up an advective wind whereby it flows back to the equator heats and rises then repeats the process

    But we have an earth that spins, land masses, a tilting axis and ocean currents so that results in a three cell model
    The three cells have names
    The Hadley cell
    The Ferrel cell
    The Polar cell

    This 3 cell model sets up some semi-permanent highs and lows
    At the equator we have a semi-permanent low
    Horizontal pressure gradients are weak here so winds are light
    This is referred to as the Doldrums
    Convective “Hot Towers” form here often
    Where the northeast trade winds converge with the southwest trade winds is known as the Intertropical Convergence Zone or ITCZ
    At 30°we have a semi-permanent high
    This belt is referred to as the Subtropical High
    Sometimes referred to as the Horse Latitudes
    Winds are light here so sailing ships were stuck here for days or weeks forcing the sailors to either eat their horses or toss them overboard
    This is where we find large deserts like the Sahara
    This is also where the trade winds originate or Westerlies
    At 60°we again have a semi-permanent low
    This is where warmer air meets the Polar Front producing the Subpolar Low
    North of the polar front we have the Polar Easterlies
    Remember from our discussion on lows and highs
    Descending air is characteristic of a high pressure area
    Rising air is a characteristic of a low pressure area

    Remember Coriolis and Pressure Gradient Force result in the curved flow of the
    Polar Easterlies
    NE trade winds
    SE trade winds
    10 years of weather in 9 minutes

    As World War II was approaching its conclusion, the United States introduced the first high-altitude bomber plane called the B-29. It could fly at altitudes well above 20,000 feet.
    When the B-29s were being put into service from a Pacific island base, two air force meteorologists were assigned the task of producing a wind forecast for aircraft operations at such altitudes.

    To make their prediction, the meteorologists used primarily surface observations and what is known in meteorology as the “thermal wind” relationship.
    In plain language, this relationship implies “that if you stand with your back to the wind, and the air is colder to the left and warmer to the right, the wind will get stronger on your back as you ascend in the atmosphere.”
    Using this relationship, the meteorologists then predicted a 168-knot wind from the west. Their commanding officer could not believe the estimate. However, on the next day, the B-29 pilots reported wind speeds of 170 knots from the west!
    The jet stream was discovered.

    Narrow river of wind
    50-150 knots
    100-400 miles wide
    1 mile thick
    Subtropical jet 33,000 to 43,000
    Polar Front jet 25,000 to 35,000

    Stronger in winter than summer
    Farther south in winter than summer
    Occurs at breaks in the tropopause
    Has to be 50 kts or greater to be classified as a Jet Stream

    The subtropical jet will form between breaks between the Hadley and Ferrel cells
    The polar jet will form between the Ferrel and Polar cells

    The jet blows west to east and occasionally shifting north and south
    Jet streams follow the boundaries between hot and cold air
    Since the temp gradient is stronger in the winter, the jet is also stronger
    Great contrast of temperatures in adjacent air frontal zones
    Rapid change in temp causes rapid change in pressure
    Steep pressure gradient intensifies wind speed and causes the Jet
    Temp difference is greatest in winter between polar and tropical air masses
    This causes the jet to be more intense in winter than summer

    Why does the jet blow west to east?
    The earth’s rotation is responsible for developing the 3 primary cells
    Recall the difference in rotational velocity along different points of latitude causes this
    As air moves north it keeps it eastward flow
    This is know as conservation of angular momentum
    The Earth below the air, however, moves slower as that air travels toward the poles.
    The result is that the air moves faster and faster in an easterly direction (relative to the Earth’s surface below) the farther it moves from the equator.
    The farther the air moves northward the slower the earth’s rotation
    This causes a shift in the speed of eastward movement of the air causing faster velocity
    Because of this the jet moves from west to east in both the northern and southern hemispheres

    As we move northward air moves closer to axis of rotation of the earth
    Air must move faster east which looks like a wind out of the west to an observer on the ground
    The best example is an ice skater

    At 30 and 60 degrees latitude, temp changes are the greatest
    As the temp difference increases so does the wind speed

    The actual appearance of the jet is a complex interaction between many variables:
    Location of high an low pressure systems
    Warm and cold air masses
    Seasonal changes
    They meander around the earth dipping and rising in altitude and latitude
    They can split, form eddies, disappear and then reappear elsewhere
    The jet will tend to be located where the strongest surface temp contrast exists

    Jet streams form along the upper air boundaries of large masses of warm and cold air.
    Often, the jet stream contains “jet streaks” of wind speeds faster than the surrounding regions.
    These jet streaks can play a very important role in precipitation and storm formation.
    Depending on the strength and position of a jet streak, a weak area of low pressure can quickly become a raging monster.
    Highest velocities in jet streaks have hit 260 kts

    The curvature in the jet stream and upper-air flow can have significant implications for surface weather.
    Curvature’s effects are the strongest in a highly amplified, meridional flow pattern with large troughs and ridges.
    Meridional flow is north – south
    Zonal flow is east – west
    As the graphic shows, the anticyclonic curvature in the western USA ridge induces a clockwise circulation while the cyclonic curvature in the east trough tends to favor a counter-clockwise circulation.

    Meteorologists use the jet position to forecast weather
    The polar front jet is a critical piece when generating a forecast
    Changes in it indicate changes in weather
    Typically north of the polar jet is relatively cold air exists, warmer to the south
    The polar jet can promote development of storms
    Storms are most likely to develop under a jet streak
    The polar jet generally steers storms in the direction of its travel
    There are usually 2 jet streaks located somewhere over the US at any given time

    Rising air on the equatorial side can cause cirriform clouds
    Descending air on the polar side causes them to dissipate

    The more serious turbulence is found on the polar side of the Jet
    Rate of decrease of wind speed is greater on polar side
    Usually associated with an upper level trough
    The bend caused by the trough is where pressure gradients change over the smallest distance increasing the chance for turbulence (CAT)
    Cold air colliding with warm air causes shear and presto CAT
    The worst turbulence is found
    In winter when temp differential is greatest
    Where greatest wind velocity is found
    On the polar side where there is combo of strong shear, curvature in the flow and cold air advection
    Make no mistake, it is found on the equatorial side as well
    Located most of the time in an upper trough on the polar side of the jet
    Cat is found in sharply curved contours of strong lows, troughs and ridges
    Light 10-15% of the time
    Moderate 2-3% of the time
    Severe to Extreme 2/10ths of 1% of the time or 1 in 500 chance

    Usually climbing or descending will get you out of it
    Use FSS and/or Flight Watch on 122.00 find out pireps
    Remember stress is cumulative (Turb + Manev = total stress)
    Maintain constant attitude, slow to Va, accept variations

    Apr 4, 1981 DC 10 at 37,000 over central Illinois
    Dropped 2,000 feet
    21 out of 154 injured including
    Fractured hip
    Jabbed nose with a fork then landed in the seat in front of him on it

  • CAT AND THE B-52
    January 10 1964, USAF B-52H (S/N 61-023) took-off from Wichita, Kansas on a structural flight test mission.  The all-Boeing air crew consisted of instructor pilot Charles Fisher, pilot Richard Curry, co-pilot Leo Coors, and navigator James Pittman.  The aircraft was equipped with accelerometers and other sensors to record in-flight loads and stresses.
    At approximately 345 KCAS, the Stratofortress and its crew experienced an extreme turbulence event that lasted roughly 9 seconds.
    Amazingly, after flying 6 hours sans a vertical tail, the Stratofortress and her crew landed safely.
    Safe recovery of crew and aircraft brought additional benefits.  It was found that at least one gust in the severe CAT encounter registered at nearly 100 mph.
    B-52H (61-023) was repaired and returned to the USAF inventory.  The aircraft spent the latter part of its flying career as a member of the 2nd Bomb Wing at Barksdale AFB, Louisiana.  
    The venerable bird was retired from active service in July of 2008.

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