Updated: Dec 8, 2020
Weather Theory Lesson by wifiCFI
Weather Theory (PHAK C12)
Weather is an important factor that influences aircraft performance and flying safety.
It is the state of the atmosphere at a given time and place with respect to variables, such as temperature, moisture, wind velocity, visibility, and barometric pressure.
The term “weather” can also apply to adverse or destructive atmospheric conditions, such as high winds.
The Atmosphere (PHAK C12)
The atmosphere is a blanket of air made up of a mixture of gases that surrounds the Earth and reaches almost 350 miles from the surface of the Earth.
This mixture is in constant motion.
In any given volume of air, nitrogen accounts for 78 percent of the gases that comprise the atmosphere, while oxygen makes up 21 percent.
Argon, carbon dioxide, and traces of other gases make up the remaining one percent.
The first layer, known as the troposphere, extends from 4 to 12 miles over the northern and southern poles and up to 48,000 feet over the equatorial regions.
The vast majority of weather, clouds, storms, and temperature variances occur within this first layer of the atmosphere. Inside the troposphere, the average temperature decreases at a rate of about 2 °Celsius every 1,000 feet of altitude gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain.
At the top of the troposphere is a boundary known as the tropopause, which traps moisture and the associated weather in the troposphere.
The altitude of the tropopause varies with latitude and with the season of the year; therefore, it takes on an elliptical shape as opposed to round.
Location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence.
The temperature in the tropopause remains fairly constant with changing altitude.
Warm air rises because heat causes air molecules to spread apart.
As the air expands, it becomes less dense and lighter than the surrounding air.
As air cools, the molecules pack together more closely, becoming denser and heavier than warm air.
As a result, cool, heavy air tends to sink and replace warmer, rising air.
Solar heating causes higher temperatures in equatorial areas, which causes the air to be less dense and rise.
As the warm air flows toward the poles, it cools, becoming denser and sinks back toward the surface.
The unequal heating of the Earth’s surface not only modifies air density and creates circulation patterns; it also causes changes in air pressure or the force exerted by the weight of air molecules.
Although air molecules are invisible, they still have weight and take up space.
The actual pressure at a given place and time differs with altitude, temperature, and density of the air.
These conditions also affect aircraft performance, especially with regard to takeoff, rate of climb, and landings.
Areas of low pressure exist over the equatorial regions and areas of high pressure exist over the polar regions due to a difference in temperature.
The resulting low pressure allows the high pressure air at the poles to flow along the planet’s surface toward the equator.
The force created by the rotation of the Earth is known as the Coriolis force.
The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line.
The Coriolis force causes the general flow to break up into three distinct cells in each hemisphere.
Air deflects to the Right in the Northern Hemisphere due to the Rotation of the Earth.
Within 2,000 feet of the ground, the friction between the surface and the atmosphere slows moving air.
The wind is diverted from its path because of the frictional force.
Thus, the wind direction at the surface varies somewhat from the wind direction just a few thousand feet above the Earth.
Standard Atmospheric Pressure:
To provide a common reference, the International Standard Atmosphere (ISA) has been established.
These standard conditions are the basis for certain flight instruments and most aircraft performance data.
Standard sea level pressure is defined as 29.92 "Hg and a standard temperature of 59 °F (15 °C).
Atmospheric pressure is also reported in millibars (mb), with 1 "Hg equal to approximately 34 mb.
Standard sea level pressure is 1,013.2 mb.
Typical mb pressure readings range from 950.0 to 1,040.0 mb.
As altitude increases, atmospheric pressure decreases.
On average, with every 1,000 feet of increase in altitude, the atmospheric pressure decreases 1 "Hg. As pressure decreases, the air becomes less dense or thinner.
This is the equivalent of being at a higher altitude and is referred to as density altitude.
As pressure decreases, density altitude increases and has a pronounced effect on aircraft performance.
Standard Temperature and Atmospheric Pressure:
How temperature and pressure decrease with altitude.
Temperature decreases 2 Celsuis per 1,000 feet of altitude gain.
Pressure decreases 1” per 1,000’ feet of altitude gain.
Wind and Currents (PHAK C12)
Air flows from areas of high pressure into areas of low pressure because air always seeks out lower pressure.
The combination of atmospheric pressure differences, Coriolis force, friction, and temperature differences of the air near the earth cause two kinds of atmospheric motion:
Convective currents (upward and downward motion)
Wind (horizontal motion)
In the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure.
This is known as anticyclonic circulation.
The opposite is true of low-pressure areas; the air flows toward a low and is deflected to create a counterclockwise or cyclonic circulation.
High Pressure = Clockwise/Anticyclonic Circulation
Low Pressure = Counterclockwise/Cyclonic Circulation
The Thumb Trick
Must be done with the Right Hand (does not work with the Left Hand)
Thumbs Down = High Pressure System
Thumbs Up = Low Pressure System
Convective Currents (PHAK C12)
Plowed ground, rocks, sand, and barren land absorb solar energy quickly and can therefore give off a large amount of heat; whereas, water, trees, and other areas of vegetation tend to more slowly absorb heat and give off heat.
The resulting uneven heating of the air creates small areas of local circulation called convective currents.
Convective currents cause the bumpy, turbulent air sometimes experienced when flying at lower altitudes during warmer weather.
While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar manner.
As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent.
Windshear (PHAK C12)
Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area.
Wind shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the aircraft.
Low Level Windshear:
Low-level wind shear is especially hazardous due to the proximity of an aircraft to the ground.
Low-level wind shear is commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong upper level winds.
Microbursts (PHAK C12)
The most severe type of low-level wind shear, a microburst, is associated with convective precipitation into dry air at cloud base.
Microburst activity may be indicated by an intense rain shaft at the surface but virga at cloud base and a ring of blowing dust is often the only visible clue.
A typical microburst has a horizontal diameter of 1–2 miles and a nominal depth of 1,000 feet.
The lifespan of a microburst is about 5–15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm) and headwind losses of 30–90 knots, seriously degrading performance. It can also produce strong turbulence and hazardous wind direction changes.
Atmospheric Stability (PHAK C12)
The stability of the atmosphere depends on its ability to resist vertical motion.
A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear.
In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity.
Instability can lead to significant turbulence, extensive vertical clouds, and severe weather.
Rising air expands and cools due to the decrease in air pressure as altitude increases.
The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed.
The Adiabatic Process (PHAK C12)
The adiabatic process takes place in all upward and downward moving air.
When air rises into an area of lower pressure, it expands to a larger volume.
As the molecules of air expand, the temperature of the air lowers.
As a result, when a parcel of air rises, pressure decreases, volume increases, and temperature decreases.
When air descends, the opposite is true.
The rate at which temperature decreases with an increase in altitude is referred to as its lapse rate.
As air ascends through the atmosphere, the average rate of temperature change is 2 °C (3.5 °F) per 1,000 feet.
Since water vapor is lighter than air, moisture decreases air density, causing it to rise.
Conversely, as moisture decreases, air becomes denser and tends to sink.
Since moist air cools at a slower rate, it is generally less stable than dry air since the moist air must rise higher before its temperature cools to that of the surrounding air.
The dry adiabatic lapse rate (unsaturated air) is 3 °C (5.4 °F) per 1,000 feet.
The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 feet.
Inversion (PHAK C12)
As air rises and expands in the atmosphere, the temperature decreases.
There is an atmospheric anomaly that can occur; however, that changes this typical pattern of atmospheric behavior.
When the temperature of the air rises with altitude, a temperature inversion exists. Inversion layers are commonly shallow layers of smooth, stable air close to the ground.
The temperature of the air increases with altitude to a certain point, which is the top of the inversion.
The air at the top of the layer acts as a lid, keeping weather and pollutants trapped below.
If the relative humidity of the air is high, it can contribute to the formation of clouds, fog, haze, or smoke resulting in diminished visibility in the inversion layer.
Relative Humidity (PHAK C12)
Humidity refers to the amount of water vapor present in the atmosphere at a given time.
Relative humidity is the actual amount of moisture in the air compared to the total amount of moisture the air could hold at that temperature.
For example, if the current relative humidity is 65 percent, the air is holding 65 percent of the total amount of moisture that it is capable of holding at that temperature and pressure.
Determining Cloud Bases (PHAK C12)
With an outside air temperature (OAT) of 85 °F at the surface and dew point at the surface of 71 °F, the spread is 14°.
Divide the temperature dewpoint spread by the convergence rate of 4.4 °F (2.5C), and multiply by 1,000 to determine the approximate height of the cloud base.
14/4.4 = 3.18
3.18 x 1,000 = 3,180 ft AGL.
Fog (PHAK C12)
Fog is a cloud that is on the surface.
It typically occurs when the temperature of air near the ground is cooled to the air’s dew point.
At this point, water vapor in the air condenses and becomes visible in the form of fog.
Fog is classified according to the manner in which it forms and is dependent upon the current temperature and the amount of water vapor in the air.
On clear nights, with relatively little to no wind present, radiation fog may develop.
Usually, it forms in low-lying areas like mountain valleys.
This type of fog occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dew point.
When a layer of warm, moist air moves over a cold surface, advection fog is likely to occur.
Unlike radiation fog, wind is required to form advection fog.
Winds of up to 15 knots allow the fog to form and intensify; above a speed of 15 knots, the fog usually lifts and forms low stratus clouds.
Advection fog is common in coastal areas where sea breezes can blow the air over cooler landmasses.
Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range.
This type of fog also requires wind for formation and continued existence.
Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun but instead can persist for days.
They can also extend to greater heights than radiation fog.
Steam fog, or sea smoke, forms when cold, dry air moves over warm water.
As the water evaporates, it rises and resembles smoke.
This type of fog is common over bodies of water during the coldest times of the year.
Low-level turbulence and icing are commonly associated with steam fog.
Ice fog occurs in cold weather when the temperature is much below freezing and water vapor forms directly into ice crystals.
Conditions favorable for its formation are the same as for radiation fog except for cold temperature, usually –25 °F or colder.
It occurs mostly in the arctic regions but is not unknown in middle latitudes during the cold season.
Clouds (PHAK C12)
Clouds are visible indicators and are often indicative of future weather.
For clouds to form, there must be adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled.
When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state.
Cloud type is determined by its height, shape, and characteristics.
They are classified according to the height of their bases as:
Clouds with vertical development
Low clouds are those that form near the Earth’s surface and extend up to about 6,500 feet AGL.
They are made primarily of water droplets but can include supercooled water droplets that induce hazardous aircraft icing.
Typical low clouds are stratus, stratocumulus, and nimbostratus.
Middle clouds form around 6,500 feet AGL and extend up to 20,000 feet AGL.
They are composed of water, ice crystals, and supercooled water droplets.
Typical middle-level clouds include altostratus and altocumulus.
High clouds form above 20,000 feet AGL and usually form only in stable air.
They are made up of ice crystals and pose no real threat of turbulence or aircraft icing.
Typical high level clouds are cirrus, cirrostratus, and cirrocumulus.
Clouds with Extensive Vertical Development:
Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds.
The bases of these clouds form in the low to middle cloud base region but can extend into high altitude cloud levels.
Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent.
These types of clouds often develop into cumulonimbus clouds or thunderstorms.
Cumulonimbus clouds contain large amounts of moisture and unstable air and usually produce hazardous weather phenomena, such as:
Heaped or piled clouds.
Formed in layers.
Ringlets, fibrous clouds.
Also high level clouds.
Common base with separate vertical development.
Lens-shaped, formed over mountains in strong winds.
Ragged or broken.
Air Masses (PHAK C12)
Air masses are classified according to the regions where they originate.
They are large bodies of air that take on the characteristics of the surrounding area or source region.
A source region is typically an area in which the air remains relatively stagnant for a period of days or longer.
During this time of stagnation, the air mass takes on the temperature and moisture characteristics of the source region.
Air mass passing over a warmer surface:
An air mass passing over a warmer surface is warmed from below, and convective currents form, causing the air to rise.
This creates an unstable air mass with good surface visibility.
Moist, unstable air causes cumulus clouds, showers, and turbulence to form.
Air mass passing over a colder surface:
An air mass passing over a colder surface does not form convective currents but instead creates a stable air mass with poor surface visibility.
The poor surface visibility is due to the fact that smoke, dust, and other particles cannot rise.
Fronts (PHAK C12)
As an air mass moves across bodies of water and land, it eventually comes in contact with another air mass with different characteristics.
The boundary layer between two types of air masses is known as a front.
A warm front occurs when a warm mass of air advances and replaces a body of colder air.
Warm fronts move slowly, typically 10 to 25 miles per hour.
The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area.
Warm fronts contain warm air that often has very high humidity.
As the warm air is lifted, the temperature drops and condensation occurs.
Generally, prior to the passage of a warm front, cirriform or stratiform clouds, along with fog, can be expected to form along the frontal boundary.
In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop.
A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.
Cold fronts move more rapidly than warm fronts, progressing at a rate of 25 to 30 mph.
However, extreme cold fronts have been recorded moving at speeds of up to 60 mph.
A typical cold front moves in a manner opposite that of a warm front.
It is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft.
The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds.
As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky.
When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days.
This front is called a stationary front.
The weather associated with a stationary front is typically a mixture that can be found in both warm and cold fronts.
An occluded front occurs when a fast-moving cold front catches up with a slow-moving warm front.
As the occluded front approaches, warm front weather prevails but is immediately followed by cold front weather.
Thunderstorms (PHAK C12)
The 3 Stages:
It begins with the cumulus stage, in which lifting action of the air begins.
If sufficient moisture and instability are present, the clouds continue to increase in vertical height.
Continuous, strong updrafts prohibit moisture from falling.
Within approximately 15 minutes, the thunderstorm reaches the mature stage, which is the most violent time period of the thunderstorm’s life cycle.
At this point, drops of moisture, whether rain or ice, are too heavy for the cloud to support and begin falling in the form of rain or hail.
This creates a downward motion of the air.
Warm, rising air; cool, precipitation-induced descending air; and violent turbulence all exist within and near the cloud.
Once the vertical motion near the top of the cloud slows down, the top of the cloud spreads out and takes on an anvil-like shape.
At this point, the storm enters the dissipating stage.
This is when the downdrafts spread out and replace the updrafts needed to sustain the storm.
Thunderstorms (PHAK C12)
A good rule of thumb is to circumnavigate thunderstorms identified as severe or giving an extreme radar echo by at least 20 nautical miles (NM) since hail may fall for miles outside of the clouds.
If flying around a thunderstorm is not an option, stay on the ground until it passes.
Squall Lines (PHAK C12)
A squall line is a narrow band of active thunderstorms.
Often it develops on or ahead of a cold front in moist, unstable air, but it may develop in unstable air far removed from any front.
The line may be too long to detour easily and too wide and severe to penetrate.
It often contains steady-state thunderstorms and presents the single most intense weather hazard to aircraft.
It usually forms rapidly, generally reaching maximum intensity during the late afternoon and the first few hours of darkness.
Tornadoes (PHAK C12)
Tornadoes occur with both isolated and squall line thunderstorms.
Reports for forecasts of tornadoes indicate that atmospheric conditions are favorable for violent turbulence.
An aircraft entering a tornado vortex is almost certain to suffer loss of control and structural damage.
Since the vortex extends well into the cloud, any pilot inadvertently caught on instruments in a severe thunderstorm could encounter a hidden vortex.
Turbulence (PHAK C12)
Potentially hazardous turbulence is present in all thunderstorms, and a severe thunderstorm can destroy an aircraft.
Strongest turbulence within the cloud occurs with shear between updrafts and downdrafts.
Outside the cloud, shear turbulence has been encountered several thousand feet above and 20 miles laterally from a severe storm.
A low-level turbulent area is the shear zone associated with the gust front.
Often, a “roll cloud” on the leading edge of a storm marks the top of the eddies in this shear, and it signifies an extremely turbulent zone.
Icing (PHAK C12)
Updrafts in a thunderstorm support abundant liquid water with relatively large droplet sizes.
When carried above the freezing level, the water becomes supercooled.
Supercooled water freezes on impact with an aircraft.
Clear icing can occur at any altitude above the freezing level, but at high levels, icing from smaller droplets may be rime or mixed rime and clear ice.
The abundance of large, supercooled water droplets makes clear icing very rapid
Pilots should be alert for icing anytime the temperature approaches 0 °C and visible moisture is present.
Types of Icing:
Clear in color.
Mostly develops from large water droplets.
Freezes like an icicle, as it runs back along the wing.
Milky in color.
Mostly develops from smaller water droplets.
Water droplets freeze on impact with the aircraft.
Mixture of Clear and Rime ice.
Hail (PHAK C12)
Hail may be encountered in clear air several miles from thunderstorm clouds.
As hailstones fall through air whose temperature is above 0 °C, they begin to melt and precipitation may reach the ground as either hail or rain.
Rain at the surface does not mean the absence of hail aloft.
Possible hail should be anticipated with any thunderstorm, especially beneath the anvil of a large cumulonimbus.
Hailstones larger than one-half inch in diameter can significantly damage an aircraft in a few seconds.
Lightning (PHAK C12)
A lightning strike can puncture the skin of an aircraft and damage communications and electronic navigational equipment.
Nearby lightning can blind the pilot, rendering him or her momentarily unable to navigate either by instrument or by visual reference.
Nearby lightning can also induce permanent errors in the magnetic compass.
Lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies.
FAA Sources Used for this Lesson
Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 12