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DESCRIBE the characteristics of the troposphere

  • adjacent to the earth's surface.
  • It varies in height depending on location and time of year.
  • Height Varies from 28,000 to 55,000 ft. Average height over the US is 36,000 feet.
  • The temperature normally decreases with increasing altitude.
  • Large amounts of moisture is found in the troposphere.
  • Winds are generally light near the surface and increase with altitude.


DESCRIBE the characteristics of the tropopause.

  • transition zone between the troposphere and starosphere.
  • Temperature is constant with altitude.
  • The strongest winds (jet stream) is just below the tropopause.
  • Contrails form and persist.
  • Average height of over the US is 36,000 ft.
  • Anvil tops of thunderstorms will spread out at the base of the tropopause.
  • A haze layer with a definite top frequently exists at the tropopause.


DESCRIBE the characteristics of the stratosphere.

  • Increasing temperature with increasing altitude
  • Gas ozone plays major part in heating the air at this altitude


DESCRIBE the flight conditions associated with the troposphere

  • Hazardous weather sometimes exists
  • Wind is light near the surface but increases with altitude
  • Winds of up to 200 knots may occur near the top of the troposphere


DESCRIBE the flight conditions associated with the tropopause

  • The strongest winds (those of the jet stream) occur just below the tropopause
  • Moderate to severe turbulence may be associated with wind shear caused by the jet stream
  • Contrails form and persist
  • Severe thunderstorms may benetrate the tropopause


DESCRIBE the flight conditions associated with the stratosphere

  • Smooth flying with excellent visibility
  • The air is thin and offers little resistance to aircraft
  • The general lack of weather makes for outstanding flying


DEFINE a lapse rate

The decrease in atmospheric temperature with increasing altitude is called the temperature lapse rate. There is also a pressure lapse rate.


STATE the average lapse rate in degrees Celsius

2ºC per 1000 ft (3.5ºF)


DEFINE atmospheric pressure

The force exerted by the weight of the atmosphere from the level of measure to its outer limits.


STATE the standard units of pressure measurement

Inches of Mercury (in. Hg.) and millibars (mb)


DEFINE the standard atmosphere

A hypothetical vertical distribution of the atmospheric temperature, pressure, and density, which by international agreement is considered to be representative of the atmosphere for pressure-altimeter calibrations and other purposes (29.92 in. Hg. or 1013.2 mb)


DIFFERENTIATE between sea level pressure and station pressure

  • Station Pressure is the atmospheric pressure measured directly at an airfield or other weather station
  • Sea Level Pressure is the pressure that would be measured from the existing weather if the station were at mean sea level (MSL). This could be measured at sea level or calculated using the standard pressure lapse rate.


DEFINE the types of altitudes

Indicated Altitude is the altitude read directly from the altimeter

Calibrated Altitude is indicated altitude corrected for instrument error.

Mean Sea Level (MSL) or True Altitude is the actual height above mean sea level. It is found by correcting calibrated altitude for temperature deviations from the standard atmosphere.

Above Ground Level (AGL) or Absoulte Altitude is the aircraft's height above the terrain directly beneath the aircraft and is measured in feet above ground level.

Pressure Altitude is the height above the standard datum plane. The standard datum plane is the actual elevation above or below the earth's surface at which the barometric pressure is 29.92 in. Hg.

Density Altitude is not a height reference, rather it is an index to aircraft performance.





DEFINE indicated altitude

The altitude read directly from the altimeter


DESCRIBE the effects of pressure changes on aircraft altimeters

  • If an aircraft flies into an area of lower pressure (without adjusting altimeter setting) its true altitude will be lower than the altimeter indicates.
    • altimeter reads high
    • High to low, look out below
  • If an aircraft flies into an area of higher pressure, its true altitude will be higher than the altimeter indicates.
    • altimeter reads low
    • Low to high, plenty of sky


DESCRIBE the effects of temperature deviations from the standard lapse rate on aircraft altimeters

  • Flying from standard temperature to lower temperature,  the aircraft will be lower than its indicated altitude.
    • altimeter reads high
    • High to low, look out below
  • Flying from standard temperature to higher temperature, the aircraft will be higher than its indicated altitude
    • altimeter reads low
    • Low to high, plenty of sky


EXPLAIN the term pressure gradient

The rate of pressure change over a horizontal distance, as indicated by the spacing of isobars on a surface analysis chart. This isobar spacing represents the size of the pressure gradient force (PGF). Closely spaced isobars indicate a steep (strong) PGF, which is the initiating force for all winds.


EXPLAIN and identify gradient winds and Buys Ballot's Law with respect to the isobars around pressure systems in the Northern Hemisphere

While the Pressure Gradient Force causes air to flow from high pressure to low pressure, the Coriolis effect (due to the earth's rotation) acts upon wind to divert the air to the right. Thus, gradient winds:

  • Which are found above 2000' AGL,
  • Flow parallel to the isobars,
  • Flow clockwise around highs, and
  • Flow counter-clockwise around lows.

Buys Ballot's Law states that if wind is at your back, the area of lower pressure will be to your left.

This pattern of flow exists in the Northern Hemisphere. The opposite is true in the Southern Hemisphere.


EXPLAIN and identify the surface wind direction with respect to the gradient winds in a pressure system in the Northern Hemisphere

For winds below 2000 ft AGL, surface friction plays a role in wind direction, in addtion to the PGF and Coriolis effect. Air flows at angles across the isobars from high pressure to low pressure. Surface winds still move clockwise around highs and counter-clockwise around lows, but since they blow across the isobars at a 45° angle, they also have a component of motion that moves air out of the high pressure and into the low.


DESCRIBE the jet stream

The jet stream is a narrow band of of strong winds of 50 knots or more that meanders vertically and horizontally around the hemisphere in wave-liike patterns.

  • Average wind speed: 100-150 knots, but may exceed 250 knots.
  • Found in segments:
    • 1000 to 3000 miles in length
    • 100 to 400 miles in width
    • 3000 to 7000 feet in depth.
  • Average height 30,000 feet MSL


DESCRIBE sea breezes

The difference in specific heat of land and water causes land surfaces to warm and cool more rapidly than water. Thus land is generally warmer than the ocean during the day. Rising air creates a low pressure over land, while descending air creates a high pressure over the water. The result is wind blowing from the sea, known as a sea breeze, with speeds sometimes reaching 15 to 20 knots.


DESCRIBE land breezes

At night, the circulation near the coast is reversed, so that the air movement is from land to sea, producing an offshore wind called the land breeze. It is typically not as strong as the sea breeze.


DESCRIBE mountain winds

At night, air in contact with the mountain slope is cooled by outgoing terrestrial radiation and becomes denser than the surrounding air. As the denser air flows downhill, from the top of the mountain, it is called the mountain wind.


DESCRIBE valley winds

In the daytime, mountain slopes are heated by the sun, and in turn, they heat the adjacent air. The air near the slope becomes warmer than air farther away at the same altitude, and since warm air is less dense, it begins to rise. It cools as it moves away from the ground and settles back to the valley floor. This downward motion forces the warmer air near the ground up the mountain, and since it is flowing from the valley, it is called a valley wind.


DEFINE saturation

The point where the air contains the maximum amount of water vapor it can hold for that temperature.


DEFINE dew point temperature

(TD) the temperature at which saturation occurs. The dew point is a direct indication of the amount of moisture present in the air.


DEFINE dew point depression

The difference between the air temperature and the dew point temperature (AKA dew point spread).


DEFINE relative humidity

(RH) The percent of saturation of the air. The air can be saturated either by the air cooling to the dew point or by evaporation adding moisture to the atmosphere, increasing the dew point.


DESCRIBE the relationship between air temperature and dew point temperature with respect to saturation

The difference between air temperature and dew point provides an indication of how close the atmosphere is to saturation. The dew point will always be the lower of the two. When the dew point spread reaches about 4°F (90% RH), water vapor will begin to condense into fog or clouds. Any further cooling of the air, or evaporation will produce precipitation.


DESCRIBE the three characteristics of precipitation

  • Showers: Sudden beginning and ending, abruptly changing intensity or sky conditions. Usually associated with cumuliform clouds.
  • Continuous: Steady. Intensity changes gradually. Associated with stratiform clouds.
  • Intermittent: Stops and restarts at least once during the hour. May be showery or steady. May be associated with cumuliform or stratiform clouds.