Atmospheric Pressure The : An instrument to measure altitude based on an aneroid barometer. It can be adjusted for changes in atmospheric pressure 1 2 Altimeter Setting Is not SLP, but close to it. If set to 0 ft Alt, reads close to station pressure. Sources of Error Changes in Atmospheric pressure due to change in position or time. Changes in Temperature Strong Vertical gusts 3 4 1
Sources of Error: Change in Pressure Flying into a high pressure area you will be flying high Flying into a low pressure area you will be flying low High to Low Lookout Below 5 6 Sources of Error: Change in Temperature Flying into Higher temperature: You will fly higher than indicated Flying into Lower temperature: You will be flying lower than indicated Hot to Cold your in the Snow 7 8 2
The METAR Sources of Error: Vertical Gusts Can create areas of anomalous pressure METeorological Aerodrome Report Two Kinds: 1. Regular: METAR 2. Special: SPECI (Can be pilot requested) 9 10 1. Type of Report: Speci or Metar 2. Location(usually Airport Designation) eg KSJC 3. Modifier: Auto = Automated (ASOS) 4. Wind NNNSSSGgg(Kt) a. NNN is true degrees wind is from b. SSS is Speed, usually Kt, but also KMH, MPS c. G d. gg gust speed e. Kt units of Speed 11 5. Visibility in meters eg 3500 is 3500m a. RVR visibility on a RWY 1500 = 1500m 0050 = 50m b. VV = Vertical Visibility 6. Present Weather eg DZRA drizzle and rain 7. Sky: Oktas 0= clear, 1 to 2 =Few 3 to 4 =SCT Scattered 5 to 7 =BKN Broken 8 = OVC Overcast 12 3
7. SKYAAA: may appear several times for different layers of clouds. a. AAA = Altitude in hundreds of feet b. SKY = Sky conditions CLR,FEW,SCT,BKN Sky: Oktas 0= CLR, 1 to 2 =FEW 3 to 4 =SCT Scattered 5 to 7 =BKN Broken 8 = OVC Overcast 13 8. Temperature/Dewpoint a. In degrees Celsius b. M indicates minus - c. Whole degrees 9. Altimeter Setting a. Preceded by A if in inches of Hg b. Preceded by Q if in hpa +Supplementary, Trend, +Remarks: RMK (slp) 14 10. RMK: Remarks a. SLP: Sea Level Pressure SLPABC A, B, C are digits if A is 0,1,2,3 then ABC is preceded by 10 If A is 6,7,8,9 then ABC is preceded by 9. 15 10. RMK: Remarks b. TemperatureDewpoint: TSNNDSPPD T stands for temperature if S is 1 than it is a minus sign SNN.D is the temperature SPP.D is the dewpoint 16 4
1. Density of atmosphere is not the same for same indicated pressures 2. At Sea Level the standard is: T=15 o C, P=1013.25 hpa 3. At higher Temperatures P may higher but density may be lower 17 18 3. At higher Temperatures P may higher but density may be lower 4. Due to PV = mrt or Density= P/RT 5. Density goes down as Temperature goes up(at constant pressure). Water Vapor also increases Dens. Alt. 19 3. At higher Temperatures P may higher but density may be lower 4. Due to PV = mrt or Density= P/RT 5. Density goes down as Temperature goes up(at constant pressure). Water Vapor also increases Dens. Alt. 20 5
5. Density goes down as Temperature goes up(at constant pressure). 6. Plane flight depends on Density: a. Engine performance decreases with decreasing density b. Propeller force decreases with decreasing density c. Lift decreases with decreasing density LEAP off the RWY: Lift, Engine, & Prop 21 7. Definition: Altitude above MSL at which a given atmospheric density occurs in a standard atmosphere 8. Or the altitude you would have to be at in the standard atmosphere to have the air density of where you are standing. 22 7. Definition: Altitude above MSL at which a given atmospheric density occurs in a standard atmosphere 8. Or the altitude you would be at in the standard atmosphere to have the air density of where you are standing. 23 9. Airplane performance is dependent on density altitude not just altitude. 10.The most important factor affected is takeoff roll. It gets longer with increasing altitude. 11.Takeoff roll must be shorter than the runway.(you d also like to clear trees). 24 6
Finding 9. Airplane performance is dependent on density altitude not just altitude. 10.The most important factor affected is takeoff roll. It gets longer with increasing altitude. 11.Takeoff roll must be shorter than the runway.(you d also like to clear trees). 25 1. From a chart (Not very accurate usually do not factor in water vapor: Dew Point) 2. From an E6B slide rule 3. From an electronic calculator 4. From a Web Calculator: Most accurate uses Altimeter Setting, T, and Dew Point 26 Finding 27 9. Airplane performance is dependent on density altitude not just altitude. 10.The most important factor affected is takeoff roll. It gets longer with increasing altitude. 11.Takeoff roll must be shorter than the runway.(you d also like to clear trees). 28 7
Finding 4. From a Web Calculator: Most accurate uses Altimeter Setting, T, and Dew Point Good Web Calc at: http://wahiduddin.net/calc/calc_da_em.htm Use elevation in feet, temperatures in degrees Celsius, Altitude setting in inches of mercury. Pressure Altitude can be approximated by: PA = (29.92 Altimeter_Setting)*1000ft + station_elevation 29 Finding LAB Good Web Calc at: http://wahiduddin.net/calc/calc_da_em.htm Use elevation in feet, temperatures in degrees Celsius, Altimeter setting in inches of mercury. For Row Labels: One Station from Florida, Colo., & New Mexico on Aug 1, 2015 2200Z Use Plymouth Metar Archive to lookup Metars w SLP Columns Labels: Find elevation, Temp, Dewpoint, Altimeter Setting, Pressure Alt., Density Alt. from chart, Den. Alt. from Web page, take off roll at airport Elevation at field elevation, take off roll at ( from Web Calc) Use Plymouth metar archive for 8/1/2015 at 2200 zulu Contrast each Station Elev. & its & Why Contrast T.O. rolls at F.E. and the current 30 8