AVIATION 120

1. AVIATION 120 Meteorology

2. Today�s Agenda Atmospheric Pressure and Air Circulation Study and Reference Material Atmospheric Pressure What it is Relationship with temperature � The Gas Law How it varies with place and time How it is measured Sea Level Pressure vs. Station Pressure Surface weather charts � isobars Upper Level weather charts � contours Pressure Features - Ridges, Troughs, Highs and Lows

3. Atmospheric Pressure Atmospheric pressure is also known as: Air Pressure Barometric Pressure Atmospheric pressure is the mass of air above sea level, ground level, or any other level Units of pressure can be expressed as: Millibars (mb) Hectopascals (hPa) Pascals (Pa) Inches of Mercury (in. Hg) Millimeters of Mercury (mm Hg) Pounds per Square Inch (PSI or lb/in.2) Refer to Meteorology Today, Appendix A for conversions: 1 mb = 1 hPa = 100 Pa = 0.02953 in. Hg = 0.75 mm Hg = 0.01450 PSI

5. Air Pressure, Density and Temperature The relationship between atmospheric pressure, air density and temperature is described by the gas law or equation of state (see the FOCUS section on pg. 196 in M.T.): The gas law states that the pressure of a gas is equal to its temperature times its density times a constant In other words, p = T � ? � Constant, where p is pressure in mb, T is temperature in �K and ? (rho) is density in kg/m3 For our purposes, we can simplify the gas law to the following: p ~ T � ? where �~� means �proportional to� So, if we keep temperature constant and pressure increases, density must also increase If we keep pressure constant and temperature increases, density must decrease It is evident that small changes in temperature or density will bring about a change in atmospheric pressure

8. Measuring Atmospheric Pressure Atmospheric Pressure is measured with a device called a barometer (hence the term barometric pressure) Barometers can be one of two varieties: Mercury barometer Aneroid barometer Aircraft altimeters Barographs Barometers are subject to the following errors: Temperature (mercury barometer) Gravity Instrument Error

10. Station Pressure vs. Sea Level Pressure Station Pressure The pressure at a particular location after barometer errors are corrected This pressure is NOT corrected for the elevation above sea level of the station This pressure is not very useful due to the difficulty comparing it to other places which may not be at the same altitude Sea Level Pressure Sea level pressure is the station pressure adjusted to an altitude of zero which is mean sea level The correction for altitude is about 10 mb per 100 m but is not constant, it varies with temperature and density Since sea level pressure of a station is adjusted to an altitude of zero, this is the pressure used to compare atmospheric pressures between different locations The sea level pressure is the pressure depicted on surface weather maps with isobars Altimeter settings are MSL pressure corrected using the standard atmosphere.

11. Station Pressure vs. Sea Level Pressure

12. Surface Weather Charts As you have already learned, surface weather charts depict atmospheric pressure as mean sea level pressure This pressure is depicted with isobars and are smoothed by computers to eliminate mean sea level pressure conversion errors Other information depicted on surface charts may include: Temperature Wind Sky condition and clouds Pressure trend Visibility Dew point temperature Surface charts are constant height charts � the constant height is mean sea level

13. Upper Level Weather Charts Upper Level charts are known as constant pressure charts or isobaric charts These charts depict how a particular pressure level changes height from place to place These charts use height contours as opposed to the isobars used on surface charts Examples of constant pressure or isobaric charts: 250 hPa 500 hPa 700 hPa 850 hPa

14. Upper Level Weather Charts

15. Upper Level Weather Charts

16. Upper Level Weather Charts

18. Constant Pressure Surfaces Not only are upper air weather charts depicting a constant pressure surface, but the altimeter in your aircraft is as well Beware flying from high to low� your aircraft will be closer to the ground than you think if you don�t change the altimeter setting �When flying from high to low, look out below�

19. Pressure Features - Highs High pressure regions are areas where mean sea level pressure is relatively high when compared to surrounding sea level pressures High pressure regions normally correspond to warm temperatures aloft, divergence, descending air and fair weather Surface winds in the northern hemisphere circulate clockwise and outward from surface highs, known as anticyclonic flow Highs are known as anticyclones

20. Pressure Features - Lows Low pressure regions are areas where mean sea level pressure is relatively low when compared to surrounding sea level pressures Low pressure regions normally correspond to cold temperatures aloft, convergence, rising air and poor weather Surface winds in the northern hemisphere circulate counterclockwise and into surface lows, known as cyclonic flow Mid-latitude storms, cyclones, typhoons, hurricanes and tornados are all low pressure phenomena

21. Highs and Lows - Circulation

22. Pressure Features � Ridges and Troughs A ridge is an elongated area of high pressure Weather features and circulation associated with a ridge are similar to those associated with a high A trough is an elongated area of low pressure Weather features and circulation associated with a trough are similar to those associated with a low Ridges, troughs, highs and lows can occur both at the surface of the earth as well as at upper levels in the atmosphere

23. Ridges and Troughs - Circulation