Weather Chapter: 1

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  2. Troposhere
    a transition zone between the troposphere and the stratosphere. The temperature in this layer is constant with altitude.
  3. Flying conditions associated with the Troposphere
    The strongest winds, those of the jet stream, occur just below the tropopause. Moderate to severe turbulence is sometimes associated with the wind shear caused by the jet stream. Contrails frequently form and persist near the tropopause since it is normally the coldest area within the lower atmosphere. While clouds and weather are generally confined to the troposphere, severe thunderstorm tops may penetrate the tropopause into the stratosphere.
  4. Characteristics of Troposphere
    You can sometimes identify the tropopause while in-flight by the following characteristics: the average height of the tropopause over the US is 36,000 feet MSL, anvil tops of thunderstorms will spread out at the base of the tropopause, and a haze layer with a definite top frequently exists at the tropopause.
  5. Stratosphere
    characterized by increasing temperature with increasing altitude. This increase in temperature is due to the gas ozone, which plays a major part in heating the air at this altitude.
  6. Flying Conditions associate with Stratosphere
    Flying in stratosphere is generally smooth with excellent visibility. The air is thing and offers very little resistance to the aircraft. The general lack of weather in this layer makes for outstanding flying.
  7. Average Lapse Rate
    The average or standard lapse rate is 2° Celsius (3.5° Fahrenheit) per 1000 feet.
  8. Atmospheric (barometric) Pressure
    pressure exerted on a surface by the atmosphere due to the weight of the column of air directly above that surface. Pressure, unlike temperature, always decreases with altitude.
  9. Standard Units of Pressure Measurement
    • Two units of measurement are used to measure and report atmospheric pressure:
    • Inches of Mercury (in-Hg) measure of height of a column of mercury that can be supported by atmo pressure
    • Milibars: (mb) direct representation of pressure, which is defined as a force per unit area. 
    • Normal sea level pressures in the atmo vary as low as 28 in-Hg (about 950 mb) to as high as 31 in-Hg (about 1060 mb)
  10. Standard Atmosphere
    In aviation, everything related to standard day conditions at sea level (29.92 in-Hg/1013.2mb) and 15°C/59°F. In the lower atmosphere, and thus for most aviation applications, a 1000 foot increase in altitude will result in a pressure decrease of approximately 1 in-Hg (34 mb) and a temperature decrease of 2° C (3.5° F). These values are the standard day pressure and temperature lapse rates.
  11. Sea Level Pressure
    pressure that would be measured from the existing weather if the station were at mean sea level (MSL). This can be measured directly at sea level, or calculated if the station is not at sea level using the standard pressure lapse rate.
  12. Station Pressure
    Atmospheric pressure measured directly at an airfield or other weather station.
  13. Density Altitude (DA)
    Density altitude is pressure altitude corrected for nonstandard temperature deviations. Density altitude is not a height reference; rather, it is an index to aircraft performance. 

    On a hot day, air molecules are farther apart, decreasing the air density and increasing the density altitude. The opposite is true on a colder day: Increased air density causes a decreased density altitude and a DA lower than the published field elevation and the pressure altitude.
  14. Indicated Altitude
    • Altitude read directly from the altimeter. 
    • For an indicated altitude to be useful, however, the altimeter needs to have the correct reference for the situation by dialing either the local altimeter setting or 29.92 in to the Kollsman window. This way, the indicated altitude will be equal to either the MSL or the pressure altitude.
  15. Pressure Altitude
    Pressure altitude is the height above the standard datum plane.
  16. True Altitude
    Mean Sea Level (MSL) or True altitude is the actual height above mean sea level (MSL). It is found by correcting calibrated altitude for temperature deviations from the standard atmosphere. On a standard day, MSL/true altitude is equal to calibrated altitude.
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Weather Chapter: 1
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