Fog & Atmospheric Pressure

Private Pilot License (PPL) Notes

Understanding weather is essential for safe flying. This section covers fog, atmospheric pressure, and their effects on flight. As a new pilot, you should learn to recognize, respect, and refrain from flying near marginal or hazardous weather conditions.

Flying and Weather:

1. Weather and flying are inseparable.
   1. Appreciate good weather.
   2. Apply the "3 Rs": Recognize, Respect, and Refrain from flying near hazardous weather.
2. Vertical dimension of weather.
   1. Conditions on the ground may differ from those at altitude.
   2. Clouds become significant indicators when flying.

Clouds and Their Significance:

1. Clouds are collections of minute ice or water particles.
   1. They act as weather signposts in the sky.
2. Types of clouds:
   1. Layered clouds (Stratus and Nimbostratus):
      1. Indicate stable atmospheric conditions.
      2. Expect smooth flight with fair to poor visibility.
      3. Precipitation is widespread, light to moderate.
      4. Fog is a type of layered cloud at or near the surface.
   2. Cumulus clouds (Cumulonimbus if producing precipitation):
      1. Indicate unstable air with vertical development.
      2. Expect bumpy flight with high visibility.
      3. Precipitation is heavy and localized (showery).

Atmospheric Layers:

1. The atmosphere consists of layers:
   1. Troposphere: Surface up to approximately 40,000 feet.
      1. Most weather and flying occur here.
      2. Temperature decreases uniformly with altitude.
   2. Tropopause: Boundary between the troposphere and stratosphere.
   3. Stratosphere: Above the tropopause.
      1. Little temperature change with altitude.
2. Temperature Inversion:
   1. Occurs when temperature increases with altitude.
   2. Indicates a stable layer of the atmosphere.
   3. Traps pollutants close to the surface, causing low visibility.

Moisture in the Atmosphere:

1. Water exists in three states: vapor, liquid, and ice.
   1. Changes between states involve the absorption or release of heat.
2. Relative Humidity:
   1. The ratio of current water vapor to the maximum possible at that temperature and pressure.
   2. Cooling air increases relative humidity.
3. Dew Point:
   1. The temperature at which air becomes saturated (100% relative humidity).
   2. Condensation can occur, forming clouds or fog.
4. Condensation Nuclei:
   1. Particles that provide surfaces for water vapor to condense.
   2. Essential for cloud and fog formation.

Fog:

1. Fog is a cloud that forms at or near the surface.
   1. Major aviation hazard during takeoffs and landings.
2. Types of Fog:
   1. Radiation Fog:
      1. Forms on clear nights over flat land.
      2. Caused by ground cooling the air to the dew point.
      3. Light winds deepen the fog.
      4. Usually dissipates by noon.
      5. Rarely forms over water bodies.
   2. Advection Fog:
      1. Occurs when warm, moist air moves over a colder surface.
      2. Common along coasts and large lakes.
      3. Can occur day or night and persists longer.
      4. Light to moderate winds deepen the fog.
   3. Upslope Fog:
      1. Formed by moist, stable air being forced uphill.
      2. Common on the eastern slopes of mountains.
      3. Dense and persists until the upslope wind ceases.
   4. Precipitation-Induced Fog:
      1. Caused by evaporation of rain or drizzle.
      2. Occurs with frontal activities and non-frontal precipitation.
      3. Can form rapidly over large areas and persist.
   5. Ice Fog:
      1. Forms in extremely cold temperatures.
      2. Water vapor sublimates directly into ice crystals.
3. Conditions Favorable for Fog Formation:
   1. Small temperature-dew point spread.
   2. Calm or light winds.
   3. Precipitation adding moisture to the air.
   4. Abundance of condensation nuclei (e.g., urban areas).
   5. Cooling processes (e.g., radiational cooling, onshore winds, upslope winds).

Atmospheric Pressure and Altimeter Readings:

1. Atmospheric Pressure Systems:
   1. High Pressure (Highs):
      1. Center of pressure surrounded by lower pressure.
      2. Can be visualized as a hill of pressure.
   2. Low Pressure (Lows):
      1. Center of low pressure surrounded by higher pressure.
      2. Can be visualized as a valley of pressure.
   3. Ridges: Elongated areas of high pressure.
   4. Troughs: Elongated areas of low pressure.
2. Effect of Pressure on Altimeter Readings:
   1. Altimeters measure atmospheric pressure to indicate altitude.
   2. Pressure decreases approximately 1 inch Hg per 1,000 feet ascent.
   3. Flying from high to low pressure without adjusting altimeter causes actual altitude to be lower than indicated ("High to Low, Look Out Below").
   4. Flying from low to high pressure without adjustment results in actual altitude higher than indicated.
   5. Adjust altimeter settings regularly when flying across different pressure areas.
3. Effect of Temperature on Altimeter Readings:
   1. Temperature affects atmospheric density and pressure levels.
   2. Cold temperatures cause the atmosphere to contract, lowering pressure levels.
   3. Flying from warm to cold temperatures without correction results in actual altitude lower than indicated ("From Hot to Cold, Look Out Below").
   4. Flying from cold to warm temperatures without adjustment results in actual altitude higher than indicated ("From Cold to Hot, Look to the Sky").
   5. No adjustment on altimeter for temperature; pilots must be aware of the effects.

Remember, a thorough understanding of weather conditions and their impact on flight is crucial for safe and efficient flying. Always be alert to changes in weather and how they can affect your flight.