• The atmosphere, simply put, is the envelope of air which surrounds the Earth
  • The standard atmosphere was defined in order to provide a common denominator for atmospheric values
    • This enables us to work from average values of temperature, pressure, and density as it relates to us (altitude above mean sea level)
  • Weather is caused from the sun and unequal heating of the Earth's surface
    • In fact weather is a direct result from the Earth spinning
    • If the Earth were to stop rotating, there would be no weather
  • Unequal heating creates a temperature imbalance between the poles


  • The atmosphere is a mixture of gases in motion that surround the Earth
  • These gases remain relatively constant throughout all levels of the atmosphere:

Atmospheric Gases
Figure 1: Atmospheric Gases

Atmospheric Levels

  • Temperatures, chemical composition, movement, and density differences divide the atmosphere into four district atmospheric layers
    • Troposphere
    • Stratosphere
    • Mesosphere
    • Thermosphere
  • The trophosphere starts at the Earth's surface and extends approximately 36,000'
  • The stratosphere starts just above the troposphere and extends to approximately 160,000'
  • The mesosphere starts just above the stratosphere and extends to 280,000' (53 mi)
  • The thermosphere starts just above the mesosphere and extends to 1,848,000 ft (350 mi)

International Standard Atmosphere (ISA)

  • The International Civil Aviation Organization (ICAO) established the ICAO Standard Atmosphere as a way of creating an international standard for reference and performance computations
  • Instrument indications and aircraft performance specifications are derived using this standard as a reference
  • Because the standard atmosphere is a derived set of conditions that rarely exist in reality, pilots need to understand how deviations from the standard affect both instrument indications and aircraft performance
  • In the standard atmosphere, sea level pressure is 29.92" inches of mercury (Hg) and the temperature is 15° C (59° F)
  • The standard lapse rate for pressure is approximately a 1" Hg decrease per 1,000' increase in altitude
  • The standard lapse rate for temperature is a 2° C (3.6° F) decrease per 1,000' increase, up to the top of the stratosphere
  • Since all aircraft performance is compared and evaluated in the environment of the standard atmosphere, all aircraft performance instrumentation is calibrated for the standard atmosphere
  • Because the actual operating conditions rarely, if ever, fit the standard atmosphere, certain corrections must apply to the instrumentation and aircraft performance
    • For instance, at 10,000 ISA predicts that the air pressure should be 19.92" Hg (29.92" - 10" Hg = 19.92") and the outside temperature at -5°C (15° C - 20° C)
  • If the temperature or the pressure is different than the International Standard Atmosphere (ISA) prediction an adjustment must be made to performance predictions and various instrument indications


  • Atmospheric pressure is the force per unit area exerted against a surface by the weight of the air above that surface
  • Put another way, pressure is the average force exerted over a given area by a fluid
  • Standard pressure is 14.7 PSI or 29.92 in-Hg or 1013.2 mb
    • The U.S. standard is in-Hg while most of the world uses mb
    • To convert to in-Hg, divide mb by 33.86
  • Standard temperature 15°C or 59°F
  • Temperature and pressure vary directly
  • As altitude increases, pressure will decrease and by 18,000' the pressure has decreased by about half
  • ISA: International Standard Atmosphere
  • Air density is a result of the relationship between temperature and pressure
  • Air density is inversely related to temperature and directly related to pressure
  • For a constant pressure to be maintained as temperature increases, density must decrease, and vice versa
  • For a constant temperature to be maintained as pressure increases, density must increase, and vice versa
  • High Pressure:
    • A mass of air that is considered more dense than the air around it
    • Generally brings fair weather
    • Generally brings cooler temperatures and lower humidity
  • Low Pressure:
    • A mass of air that is considered less dense than the air around it
    • Generally brings bad whether
    • Generally brings warmer temperatures and higher humidity
  • Because air is less dense at higher altitudes, it causes:
    • Wings to produce less lift
    • Propellers to create less thrust
    • Engines to create less power

Different Levels of the Atmosphere
Figure 2: Different Levels of the Atmosphere

Pressure Altitude

  • There are two measurements of the atmosphere that affect performance and instrument calibrations: pressure altitude and density altitude
  • Pressure altitude is the height above the Standard Datum Pressure (SDP) (29.92" Hg, sea level under ISA) and is used for standardizing altitudes for flight levels (FL)
  • Generally, flight levels are at or above 18,000' (FL 180), providing the pressure is at or above 29.92"Hg
  • For calculations involving aircraft performance when the altimeter is set for 29.92" Hg, the altitude indicated is the pressure altitude

Density Altitude

  • Density altitude is pressure altitude corrected for nonstandard temperatures, and is used for determining aerodynamic performance in the nonstandard atmosphere
  • Density altitude increases as the density decreases
  • Since density varies directly with pressure, and inversely with temperature, a wide range of temperatures may exist with a given pressure altitude, which allows the density to vary
  • However, a known density occurs for any one temperature and pressure altitude combination
  • The density of the air has a significant effect on aircraft and engine performance
  • Regardless of the actual altitude above sea level an aircraft is operating at, its performance will be as though it were operating at an altitude equal to the existing density altitude
  • If a chart is not available the density altitude can be estimated by adding 120' for every degree Celsius above the ISA
    • For example, at 3,000' pressure altitude (PA), the ISA prediction is 9° C (15° C - [lapse rate of 2° C per 1,000' x 3 = 6° C])
    • However, if the actual temperature is 20° C (11° C more than that predicted by ISA) then the difference of 11° C is multiplied by 120' equaling 1,320
    • Adding this figure to the original 3,000' provides a density altitude of 4,320' (3,000' + 1,320')

Standard Lapse Rate:

  • In the troposphere (sea level to 36,089'), for each 1000' you climb, temperature drops:
    • 1.00 in-Hg
    • 2°C
    • 4°F (3.6°F specifically)
  • Example:
    • So by 10,000' you've lost:
      • 10.00 in-Hg
      • 20°C
      • 40°F (36°F specifically)
  • In the stratosphere (36,090' to 82,020') temperature is constant (isothermal at 70°F (-57&degC)
  • Note that standard lapse rates are for a "standard atmosphere
  • The real atmosphere will contain inversions and higher or lower lapse rates
  • Additionally, seasonal effects will raise or lower the start of the isothermal stratosphere

Pressure Gradient Force:

  • Horizontal spacing of isobars affecting wind


  • Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path as seen by an observer on Earth
  • Accounts for the Earths rotation in the movement of air
  • Acts 90° to the right of the wind in the northern hemisphere
  • Coriolis force only affects direction, not speed
    • Coriolis force depends on wind speed in that the faster the air is blowing, the more it is deflected
  • Changes in latitude affect Coriolis force
    • Coriolis force is zero at the equator and greatest at the poles
  • Changes in altitude affect Coriolis force
    • Below 2,000' friction disrupts the Coriolis force
  • Geostrophic Balance:
    • Pressure gradient force and Coriolis force general cancel each other out
    • As a result, winds generally flow parallel to isobars
  • Pressure always moves from high to low but in the northern hemisphere this will produce a right deflection of the air while the opposite is true in the southern hemisphere

Bay's Ballots Law

  • The 4Hs have a negative effect on performance:
    • Hot
    • Heavy
    • Humid
    • High

Heat vs. Temperature:

  • Heat: bouncing molecules
  • Temperature: vibrating molecules


  • Worse the faster you go
  • Most dangerous weather phenomenon to aircraft, hail being second


  • The circulation process by which cool dense air replaces warm light air
  • Air at the poles is cool and dense, and "sinks" toward the equator to replace warmer air
  • The warmer air at the poles expands and rises, to move toward the poles
  • This system created a general circulation pattern
  • As the earth rotates, the general circulation pattern gives way to the three cell circulation pattern, and pressure systems

Pressure Systems
Figure 2: Pressure Systems

Pressure Systems:

  • An area of low pressure occurs when you have a convergence of air at the surface with a divergence of air aloft
  • An area of high pressure occurs when you have a divergence of air at the surface with a convergence of air aloft
  • In the general circulation pattern, high pressure dominates the poles, with low pressure at the equator
  • Atmospheric pressure decreases more rapidly in cold air than warm air
  • Temperature errors are smaller at sea level but increases with increased altitude
  • High Pressure:
    • Air flows clockwise, outward, and descends
    • Descending air warms and radiates outward
    • High pressure is general associated with good weather
  • Low Pressure:
    • Air flows counterclockwise, inward, and rises
    • Rising air cools and radiates outward
    • Low pressure is generally associated with poor weather
  • Points of equal pressure called isobars
  • These different pressures create a pressure gradient, which is the source of wind
    • Closely spaced isobars represent a strong gradient where the wind speeds will be higher than widely spread isobars
    • Generally, air flows from high pressure areas to low pressure areas
    • The rising air of a low leaves a void filled by the descending air of the high


  • Pressure Gradient Force:
    • The change in pressure measured across a given distance
    • Air will travel from high to low because of PGF
    • Resultant of the spacing of isobars
    • Responsible for triggering the initial movement of air
  • Coriolis Force:
    • Described above
  • Centrifugal Force:
    • Its influenced upon the wind is dependent upon the linear velocity of the air particles and the radius of the curvature of the path of the air particles
    • Winds produced by a combination of the pressure gradient force, Coriolis force, and centrifugal force flow parallel to the curved isobars
  • Geostrophic Wind: When wind is blowing parallel to the isobars
  • Gradient Wind: Wind blowing across isobars because the effects of PGF and Coriolis force cancel each other out when there is no frictional drag with the surface
  • Surface Wind: Friction reduces the surface wind speed to about 40% of the velocity of the gradient wind and so it causes t he surface wind to flow across the isobars instead of parallel to them
  • Jet Stream: Relatively strong wind concentrated within a narrow stream in the atmosphere, typically embedded in the mid-latitude westerlies' and is concentrated in the upper trophosphere
  • Sea & Land Breezes:
    • The difference in the specific heat of land and water causes land surface to warm and cool more rapidly than water surfaces through isolation and terrestrial radiation
    • Therefore land is normally warmer than the ocean during the day and cooler at night
    • Sea Breeze:
      • During the day the pressure over the warm land becomes lower than over the cooler water
      • The cool air over the water moves toward the lower pressure, replacing the warm air over the land that moved upward
    • Land Breeze:
      • At night, the pressure over the cooler land becomes higher than over the warmer water
      • The cool air over the land moves toward the lower pressure, replacing the warm air over the water that moved upward
  • Valley & Mountain Breezes:
    • On warm days, winds tend to ascend the slopes during the day and descend the slopes at night
    • Valley Breeze:
      • In the daytime, mountain slopes are heated by the sun's radiation, and in turn, they heat the adjacent air through conduction
      • This air usually becomes warmer than air farther away from the slope at the same altitude and, since warmer air is less dense, it begins to rise
      • The air cools while moving away from the warm ground, increasing its density
      • It then settles downward, toward the valley floor, which then forces the warmer air that is near the ground up the mountain again
    • Mountain Breezes:
      • At night, the air in contact with the mountain slope is cooled by outgoing terrestrial radiation and becomes more dense than the surrounding air
      • The denser air flows down, from the top of the mountain, which is a circulation opposite to the daytime pattern
  • Convective Currents:
    • Uneven heating of the air creates a small area of local circulation called a convective current
    • Caused by variations in surface heating
    • Some surfaces give off heat (plowed ground, pavement)
    • Some surfaces absorb heat (water, trees)
    • Convective currents are most likely to be felt in areas containing a land mass directly adjacent to a large body of water, at low altitudes, and on warm days
  • Obstructions:
    • Structures, mountains or canyons can cause wind to rapidly change direction and speed
    • Across mountains, air flows up the windward side and on the leeward side becomes turbulent

Frictional Force:

  • Caused by features on the Earth's surface
    • Will be more pronounced over mountainous terrain than over the ocean
  • Affects wind up to 2,000' above the surface
  • Slows wind speed reducing Coriolis force, but not PGF
    • As a result, surface winds tend to flow perpendicular to the isobars


  • Defined: the atmosphere's resistance to vertical motion
  • Stability is the primary determinant of cloud development
  • 5 primary causes of vertical lifting:
    1. Convergence: a net inflow of air usually associated with a low
    2. Divergence: a net outflow of air usually associated with a high
    3. Orographic: where air is forced up or down by terrain features
    4. Fronts: act like orographic lifting by forcing air up/down the front
    5. Convective: if air is warmer than its surroundings, it will rise until reaching an equilibrium temperature
  • Vertical motion of air causes pressure changes within the parcel of air that is moving
    • As air rises, it expands and cools
    • As air sinks, it compresses and warms
  • Pressure changes are accompanied by temperature changes
    • Temperature changes (expansion/cooling - compression/warming) are called dry adiabatic processes
      • Adiabatic cooling always accompanies upward motion
      • Adiabatic heating always accompanies downward motion
    • The rate at which temperature changes with respect to altitude is called lapse rate
    • The average lapse rate is 2° per 1,000'
    • Lapse rate is affected by moisture content of the air
      • The dry lapse rate is 3° per 1,000'
      • The moist lapse rate is between 1.1 and 2.8° per 1,000'

Stable Air:

  • Limits vertical motion
  • Usually has smoother air, less cloud development
  • Visibility is usually restricted by smoke/fog/haze
  • Sinking air tends to have a stabilizing effect

Unstable Air:

  • Encourages vertical motion
  • Usually has turbulent air, with significant cloud development
  • Visibility is usually good
  • Rising air tends to have a destabilizing effect

Water Vapor:

  • 3 states of water: vapor, liquid, gas
  • Each change of state requires an absorption or release of heat called latent heat


  • The amount of moisture present in the air is called humidity
    • Relative humidity is the actual amount of moisture in the air compared to the total that could be present at that temperature
    • Saturation is the point at which air holds the maximum amount of water it can
  • Since water vapor weighs less than normal air, it can displace air and decrease density which increases density altitude
  • Humidity Rule of Thumb: When ambient temperature is over 70°F, density altitude increases 100' for every 10% of humidity

Dew point:

  • Defined as the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur
  • The difference between the temperature and dew point is called the temperature dew point spread
    • When the spread is small, relative humidity is high and precipitation is likely
    • As temperature decreases, air's ability to hold water vapor also decreases


  • Defined as any form of water particles that fall from the atmosphere that reach the ground
  • Can reduce visibility, decrease engine performance, increase braking distance and cause significant shifts in wind direction and velocity (shear)
  • Also, precipitation can freeze (ice), which is dangerous
  • Causes:
    • Water/ice particles too large for atmosphere to support
    • Coalescence: when larger water droplets overtake and absorb smaller ones
    • Ice-crystal process: using super-cooled water droplets to form larger heavy water partial in a relatively short time


  • A change in the flow of air currents over a short distance, OR, more appropriately bumpiness of flight
  • Convective:
    • Low altitude, with updrafts 200-2,000 FPM
    • Common on warm summer afternoons, turbulence increases as surface heating rises
    • Typically short distance, with possibly severe turbulence below the clouds
    • Varies widely with terrain
  • Frontal:
    • Occurs in a narrow zone, just ahead of a fast-moving cold front
    • Updrafts of 1,000 FPM, with significant turbulence
  • Mechanical:
    • Occurs over rough terrain and obstacles
    • Can form in stable or unstable air
    • Presents landing hazards
  • Mountain Wave:
    • Strong hazardous wind conditions
    • Downdrafts on the lee side of the mountain, updrafts on the windward side
    • Up/downdrafts increase as wind speed increases
    • Stable air and wind speeds between 25-40 knots foster turbulence
  • Clear Air:
    • Typically above 15,000 and in vicinity to the jet steam
    • Drastic changes in wind speed and velocity

Wake Vortex:

  • Anytime an airplane produces lift
  • Most prevalent when aircraft is heavy, clean, and slow
  • Engine blast is a realistic threat when taxiing behind/near large aircraft
  • Takeoff and Landing Precautions must be exercised

Wind Shear:

  • A sudden drastic change in wind speed and direction that can occur vertically/horizontally at any level in the atmosphere
  • Can be associated with convection, fronts, temperature inversions, etc.