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CHAPTER 3 ATMOSPHERIC PRESSURE

CHAPTER 3 ATMOSPHERIC PRESSURE. KMWH 121552Z 16004KT 4SM -RA BR FEW043 OVC055 01/01 A2974 RMK AO2 RAE04B28UPB04E28 SLP081 P0005 T00060006 KMWH 121507Z 1215/1312 VRB03KT 6SM -SHRA BR BKN035 FM121800 VRB03KT P6SM VCSH BKN015 OVC100 FM130100 32005KT 5SM -SHRA BR SCT006 OVC010

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CHAPTER 3 ATMOSPHERIC PRESSURE

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  1. CHAPTER 3 ATMOSPHERIC PRESSURE

  2. KMWH 121552Z 16004KT 4SM -RA BR FEW043 OVC055 01/01 A2974 RMK AO2 RAE04B28UPB04E28 SLP081 P0005 T00060006 • KMWH 121507Z 1215/1312 VRB03KT 6SM -SHRA BR BKN035 FM121800 VRB03KT P6SM VCSH BKN015 OVC100 FM130100 32005KT 5SM -SHRA BR SCT006 OVC010 • KEPH 121553Z AUTO 35004KT 10SM -RA BKN055 OVC070 03/02 A2972 RMK AO2 SLP075 P0000 T00280022 • http://www.avcanada.ca/forums2/viewtopic.php?f=54&t=70372

  3. Weight of Earth’s Atmosphere • Given: Weight per square inch = 14.7 lbs • Given: Earth’s surface area = 196,000,000 sq miles (statute) • Weight per square foot = 2116.8 lbs or 1.0584 tons • Weight per square mile = 29,506,498 tons • Total weight of atmosphere = 5,783,000,000,000,000 tons

  4. 14.7 lbs. per square inch

  5. Volume of Earth’s Atmosphere • Given: 99% is contained within 31 miles of the surface • Total volume = 6,076,000,000 cubic miles

  6. ATMOSPHERIC PRESSURE • Evangelista Toricelli who was a student of Galileo invented the barometer in 1643. • There are 2 common types of Barometers: Mercurial and Aneroid (without liquid) Wafer type. • Baro =Greek for weight • Aneroid = not wet

  7. BAROMETERS • Any instrument that measures pressure is called a barometer • Aneroid Barometers work similar to Altimeter

  8. Aneroid barometer

  9. Mercury Barometer

  10. BAROMETERS • Mercury Barometers need to be corrected before any of the pressure readings can be used for maps. • Elevation must be corrected (set for sea level) • Temperature (corrected to 0 degrees C) • Acceleration of gravity (45 degree latitude) http://www.engineeringtoolbox.com/acceleration-gravity-latitude-d_1554.html

  11. BAROMETERS ELEVATION

  12. BAROMETERS ELEVATION

  13. MERCURY BAROMETERS • Atmospheric pressure forces mercury from the open dish upward into the evacuated glass tube. The height of the mercury column is a measure of atmospheric pressure.

  14. MERCURY BAROMETERS • Standard sea level pressure = 29.92 inches of mercury or 1013.25 hectopascals (=millibars) • pressure = force per unit area

  15. AIR HAS WEIGHT • Air is heavy. It is a lot heavier than most people realize. One cubic yard of air at sea level pressure and at a temperature of 70 F weighs almost exactly 2 pounds. The air in a room 12 ft x 14 ft with an 8-foot ceiling weighs almost exactly 100 pounds. • To come to the point quickly, the average air pressure at sea level is about 14.7 psi. It varies from day to day, but this is the average pressure in pounds per square inch, in round numbers.

  16. Water barometer • Water can be used as the liquid to make a pretty good barometer, but there is a practical difficulty in that the arms of the U tube need to be about 34 feet long. That is, atmospheric pressure is able to push a column of water upward a distance of 34 feet above the level at which the pressure is applied. • The first barometer was developed by Evangelista Torricelli in 1643, in the era of Issac Newton. One of his earlier models used water as the liquid. Story has it that he mounted the thing (34 feet tall) in the village square so the townspeople could see it. But they made him take it down because every time the liquid dropped down, the weather turned bad. And they didn't need any more bad weather. • After Mr. Torricelli had developed a workable and portable mercury barometer, he and his friends read the thing very carefully while down in the valley, and then carried it up to the top of a nearby mountain. Ahh ha! The barometer indicated a lower pressure at the top of the mountain than it did down in the valley. This gave the first indication that the atmosphere of the earth had a finite depth rather than extending upward forever. • Atmospheric pressure = 14.7 psi = 30 in/Hg = 34 feet of water • All these numbers are approximate, of course.

  17. PRESSURE VARIATION • Pressure Varies with • Altitude - Pressure drops at an average of 1 inch/ 1000’ as we go up in the atmosphere • Also with: Temperature

  18. Approximately 1 inch per 1000ft • Here is a table that gives the atmospheric pressure at various altitudes. The altitude is given in feet and the pressure is in inches of mercury. • AltitudePressureAltitudePressure •     0,000                 29.92              20,000               13.75    1,000                 28.86              25,000               11.10    2,000                 27.82              30,000               8.886 •     3,000                 26.82              35.000               7.041    4,000                 25.84              40,000               5.538    5,000                 24.89              45,000               4.355 •   10,000                 20.58              50,000               3.425  15,000                 16.88              60,000               2.118  18,000 *               14.94            100,000               0.329 • * This is almost exactly one-half the sea-level value.      To convert in/Hg to psi, multiply by 0.491. • It is interesting to note that the pressure drops to one-half its sea-level value at about 18,000 feet. The implication is that one-half of all the mass of the atmosphere lies below this altitude. Further, almost (but not quite) a third of the total lies below 10,000 feet.  • http://www.challengers101.com/Pressure.html

  19. Average decrease in pressure/1000ft • Average decreaseAltitude Rangeper 1,000 feetFeet per in/Hg •   Sea level to 5,000 ft         1.006 in/Hg                     994        5,000 to 10,000                  0.862                       1,160     10,000 to 15,000                  0.740                       1,350Sea Level to 10,000 ft             0.934                       1,070 • At atlitudes below 5,000 feet, you won't be in error too much is you simply say 1 in/Hg per 1,000 feet, or, 1,000 feet for each change in pressure of 1 in/Hg. Or even, 10 feet for each 0.01 change in pressure.

  20. Stations then take the local pressure and plot it on maps to follow the pressure patterns.

  21. Lines of equal pressure are then connected called isobars.

  22. LOW PRESSURE • Low = center of pressure surrounded on all sides by higher pressure also called a cyclone. Cyclonic • rotates counterclockwise • area of rising air • usually clouds present • bad weather

  23. HIGH PRESSURE • High = a center of pressure surrounded on all sides by lower pressure also called an Anticyclone. Anticyclonic • rotates clockwise • area of descending air • usually no clouds • good weather

  24. Other PRESSURE Definitions • Trough - an elongated area of low pressure with the lowest pressure along a line marking maximum cyclonic curvature. • Ridge - an elongated area of high pressure with the highest pressure along a line marking maximum anticyclonic curvature.

  25. Other PRESSURE Definitions • Col = the neutral are between two highs and two lows (like a mountain pass on a map

  26. Surface/Upper Air Maps • We will discuss more in detail latter on. • You can find many different kinds of weather maps for different pressure analysis. • 250, 500, 700 etc… • These charts can be very useful in determining the weather at specific altitudes • Example 700mb chart is approximately 10,000 ft MSL

  27. ALTIMETRY • The Altimeter is basically an aneroid barometer (measures height) • Indicated altitude - read off a correctly set altimeter • Pressure altitude - altitude of the 29.92” line or read off altimeter when set to 29.92 • Density altitude - pressure altitude corrected for nonstandard temp.

  28. ALTIMETRY • Absolute altitude - the height above the surface (AGL) • True altitude - actual altitude above sea level

  29. TEMPERATURE • Causes an airmass to expand or contract • This however does not necessarily effect pressure with a given volume of air • therefore the pressure line will be higher when warmer • the pressure line will be lower when colder

  30. INDICATED ALTITUDE • Temperature affects indicated altitude • Cold temperature correction charts

  31. DENSITY ALTITUDE • High Density altitude refers to height not density. Gives: • reduced power • reduced thrust • reduced lift • Use the same airspeeds but ground speed is higher

  32. DENSITY ALTITUDE

  33. ICAO cold temperature error table • http://www2.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html

  34. PRESSURE CHANGES IN FLIGHT (read Pages 18-19) • 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

  35. Chapter #3What causes a L or H pressure? 1 • Temperature • In a closed container more temp = more pressure • You might think that the higher the temp the higher the pressure • But No!

  36. What causes a L or H pressure? 1 • Usually the highest pressures are found in cold regions • Why? • Because of Density • Usually the higher density offsets the lack of movement of the molecules

  37. What causes a L or H pressure? 2 • Convergence • movement of air aloft is not always at the same speed • where it slows down it piles up into a High pressure • the piling up of air is called convergence

  38. What causes a L or H pressure? 3 • Divergence • opposite of convergence • the upper level wind speeds up and stretches the air out creating a Low pressure • usually good wx under an upper level divergence

  39. What causes a L or H pressure? 4 • Thermal tides • At an average altitude of 60 mi (thermosphere) changes of over 500ºC • the rapid warming and cooling of upper air causes great density oscillations • shows up as small pressure changes at the surface because of the high altitude

  40. CHAPTER 4 WIND

  41. WIND • Differences in temperature create differences in pressure. These pressure differences drive a complex system of winds in a never ending attempt to reach equilibrium. Wind also is a transportation device for water vapor and cloud condensation nuclei.

  42. CONVECTION • Warm air rises • Cold air sinks • With convection, warm air rises cools then sinks. Uneven surface heating. • The wind sets up an advection process whereby the cool air is blown along the ground until it is warmed then it rises again and repeats the process.

  43. CONVECTION (24)

  44. PRESSURE GRADIENT • Pressure gradient = difference in pressure / distance • Sets up a flow from high to low • The closer the isobars, the stronger the pressure gradient force and the stronger the wind

  45. PRESSURE GRADIENT • Think of a Topographical map. If you’re a ball on the top of a steep mountain (high pressure system) and you roll off into the low lying are below (low pressure system) the steeper the gradient the faster the wind.

  46. CORIOLIS FORCE • This force describes the apparent force due to the rotation of the earth • All free moving objects such as ocean currents, artillery projectiles, air molecules and aircraft seem to deflect from a straight line path because the earth rotates under them.

  47. On this non-rotating platform the ball travels in a straight line from one guy to another On this counter-clockwise rotating platform the ball seems to veer to the right from the perspective of the persons on the platform

  48. CORIOLIS FORCE • Flow would normally be 90º to isobars except for Coriolis Force • Causes a deflection of winds to the right in the Northern Hemisphere • To the left in the Southern Hemisphere • The deflection turns the winds parallel to the isobars at altitude • Near the ground, the deflection depends on surface friction

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