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Weather Theory (Part 2) (Private Pilot)

Atmospheric Stability

If the atmosphere is in a stable state vertical movement is very little or not at all. However, if the atmosphere is unstable, vertical movement increases becoming turbulent. In this lesson, we will cover:

  • Inversion

  • Relative Humidity

  • Temperature/Dew Point

  • Fog/Clouds

  • Ceilings

  • Visibility



As the air rises and expands the temperature decreases.

Inversion layers are shallow layers of smooth stable air close to the surface. An inversion is when the temperature of the air increases with altitude.

Surface based inversions can occur on clear cool nights when the air close to the ground is cooled.  The air on the surface becomes cooler than the air above it.


Relative Humidity:

Humidity is the amount of water vapor in the atmosphere at a curtain time. Whereas relative humidity is the actual amount of moisture in the air compared to total amount of moisture the air holds at a curtain temperature.

An example of this is if the relative humidity is at 65%. Therefore, it means the air can only hold 65% of the moisture at a curtain temperature.


Temperature/Dew Point:

Dew point, which is given in degrees of Celsius, is the temperature the air can no longer hold any more moisture.

When the temperature and the dew point are the same, the air has become completely saturated with moisture. This means that clouds, dew, fog, rain, snow, etc. is present.


Fog and Clouds:


There a five types of Fog. Fog is another type of cloud that is on or near the surface. It can be difficult for pilots by causing icing or IFR conditions. Fog is formed when the air temperature near the ground is cooled to its dew point. The water vapor in the air condenses and becomes fog. 

Five types of fog are classified by the way it is formed, they are:

  • Radiation fog

    • Radiation fog, is formed when the ground cools due to                                                                                                  terrestrial radiation, and the air temperature reaches the dew point.

    • Usually forms in low-lying areas such as mountain valleys.

    • Can form on clear nights, with no wind present.


  • Advection fog

    • Forms when warm moist air moves over a cold surface.

    • Common in coastal areas with a sea breeze.

    • Winds up to 15 knots is required to form advection fog.


  • Upslope fog

    • Moist stable air is forced up slope of a mountain.

    • Wind is required to move the air up slope.

    • May not burn off with the morning sun and can last for days.


  • Steam fog

    • Forms when cold dry air moves over warm water.                                                                                     When the water evaporates and rises, it resembles smoke.

    • Common over water during the coldest times of the year.

    • Low-level turbulence and icing are common.


  • Ice fog

    • Ice fog is formed when the weather is below freezing                                                                                  and water vapor forms directly into ice crystals.

    • Mostly found in cold or arctic regions where temperatures                                                                            are -25⁰F or colder, but can occur in middle latitudes during winter months.



Clouds are formed by adequate water vapor and a method for air to cool.  As air-cools, it reaches its saturation point.  This process is in the form of sublimation or condensation and attaches to a particle in the air such as dust, salt, or smoke in the air.

Cloud bases in aviation are called ceilings.

  • Broken (BKN)

  • Overcast (OVC)

Types of Clouds

Clouds are determined and classified according to the shape, characteristics, and height of the cloud base.

  • Low Clouds

    • Low to the Earth’s surface up to 6,500 feet AGL

    • Fog is classified as a Low Clouds

    • Made of water droplets and super-cooled

droplets (icing), VFR flight not recommended

  • Middle Clouds

    • 6,500 feet up to 20,000 feet AGL

    • Made of water droplets, super-cooled

    • droplets, and ice crystals

    • May contain moderate icing

    • Can be turbulence on cross country flights

  • High Clouds

    • 20,000 feet AGL and higher

    • Formed in stable air

    • Made of ice crystals 

    • No threat of icing or turbulence 

Extensive Vertical Development Clouds

Towering Cumulus or Cumulonimbus Clouds indicate instability in atmosphere and contain turbulence.

Cumulonimbus clouds contain a lot of moisture and unstable air and produce lightning, hail, tornadoes, gusty winds, wind shear.

Other Types of Clouds




Ceilings are used in aviation to give the lowest cloud layer reported, in either broken or overcast. Ceiling information can also be found on a METAR.

  • Broken means the sky is 5/8ths to 7/8ths covered with clouds.

  • Overcast means the sky is completely covered with clouds.

  • All other cloud covers such as few or scattered are not ceilings.



Is the horizontal distance an object can be seen by the naked eye. A METAR also gives current visibility, along with other weather reports.


Air Masses

Air masses are large bodies of air that take on temperature and moisture that classifies them by the region they are formed in. Such as a continental polar air mass is formed over, the Polar Regions, which brings cool dry air. Another example of an air mass is a maritime air mass that forms over warm tropical waters, which brings warm, moist air.

As an air mass passes over warm surface convective currents form which cause the air to rise. An unstable air mass is created which bring good visibility, cumulus clouds, showers, and turbulence.

On the opposite side, as an air mass passes over a cold surface, no convective currents are formed. This creates a stable air mass to form bring poor visibility, ad low stratus clouds or fog.



As two different air masses meet with each other over water or land, the line between the two different types of air masses is called a front.

When a front is approaching an area, it is a sign that the weather is going to change.

There are four types of fronts:

  • Warm

  • Cold

  • Stationary

  • Occluded

Warm fronts:

Warm fronts usually replace a cold front and bring warm air. Prior to the warm front arriving cirriform clouds or stratiform clouds, and fog will be present.

Thunderstorms are associated with warm fronts during the summer. If the warm front slows down it can cause several days with low visibility and IFR conditions.

Cold fronts:

Cold fronts are cold dense air masses pushing warm air masses upward and away. Remember cold air is denser, and replaces warm less dense air.

Cold fronts bring cumulus and cumulonimbus clouds. They are also associated with squall lines, thunderstorms, turbulence along the frontal line.

Stationary fronts:

These fronts are actually two different air masses a cold and warm air mass pushing and pulling, which causes it to stop moving across the surface.

This front can bring thunderstorms. Stratus clouds and light precipitation or drizzle with calm winds are most common, and can cover a large area with IFR conditions.

Occluded fronts:

These fronts are unstable and not fun for flying. Occluded fronts are a combination of two fronts over the same area. Two fronts catch up and over take a warm front. One front is near the surface and the other front is aloft or right on top.

Since they are unstable fronts, they are associated with clouds, precipitation, thunderstorms.  Thunderstorms might be embedded within clouds.

Chart symbols:

The symbols for all four fronts are found on surface analysis charts.




For thunderstorms to form there are three sufficient things that must be present. They are:

  • Sufficient water vapor

  • Unstable conditions

  • Initial lifting action


Thunderstorms can reach altitudes as high as 50,000 – 60,000 feet. It is not usual for thunderstorms to grow as high as 75,000 feet, especially in the tropic areas. The taller the thunderstorm is the more dangerous and intense it is.

Lift Cycle

There are three stages to a thunderstorm life cycle, they are:

  • Cumulus stage – The beginning stage of the storm has

strong updrafts or the lifting action of moist air to create

the formation of cumulonimbus clouds.

  • Mature stage – It takes approximately 15 minutes to

reach this stage. Moisture falls in the form of rain or

hail, along with downward and updraft air creating

strong turbulence. When vertical motion of air

begins to slow down the top forms an anvil shape

and begins the next stage.

  • Dissipating stage – The final stage of the storm has

downdrafts or downward motion of air, which

intensifies and spreads out. It is also the sign that the

storm is dying out. 


Small aircraft cannot fly over a thunderstorm. Flying under a thunderstorm is NEVER a good idea, due to strong downdrafts, violent turbulence, lightning, hail, etc. Always fly around a thunderstorm at least 20NM. The safest way to avoid thunderstorms is to, NOT fly at all and just wait it the storm out on the ground. 



Thunderstorms have updrafts within them; these updrafts support and carry large amounts of water droplets, which becomes super cooled above the freezing level. When the temperature is below -15⁰C, the water droplets sublimate into ice crystals.


Sublimation is the process when a solid material such as ice, changes directly into a vaper by skipping the liquid stage. An example of this is Dry ice.

Super cooled water can also freeze on impact with an aircraft. Clear ice can occur at any altitude above the freezing level. At higher altitudes, rime or mixed ice can occur from small water droplets.

These types of icing conditions effect aircraft:

  • Clear ice

  • Rime ice

  • Mix ice (combination of clear and rime)


Pilots should be aware of icing condition when flying in visible moisture (rain or clouds), and the temperature is 0˚C and below.


A more in depth discussion about aircraft icing will be cover in a later lesson.



Private Pilot ACS PA.I.C.K3a – PA.I.C.K3I

PHAK Ch. 12

AC 00-6

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Stable vs. Unstable Air
Lenticular Clouds
Wave Clouds
Basic Cloud Types
Front symbols.png
thunderstorm stages.jpg
Thunderstorm anvil.png
Mammatus Clouds.jpg
Cumulonimbus (nimbus means rain)
Mammatus (avoid flying near)
Advection fog.jpg
Radiation fog.jpg
Steam fog.jpg
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