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The Private Pilot. Class 7 - Weather. Objective: Weather Theory for pilots, A to Z. The Atmosphere . Atmospheric Circulation. Atmospheric circulation is the movement of air around the surface of the Earth.

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atmospheric circulation
Atmospheric Circulation
  • Atmospheric circulation is the movement of air around the surface of the Earth.
  • It is caused by uneven heating of the Earth’s surface and upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure.
  • EVERY physical process of weather is accompanied by or is the product of a HEAT EXCHANGE
  • The force created by the rotation of the Earth is known as Coriolis force.
  • This force is not perceptible to us as we walk around because we move so slowly and travel relatively short distances compared to the size and rotation rate of the Earth.
  • Atmospheric Circulation is air movement relative to the Earth’s Surface
temperature atmospheric pressure
Temperature & Atmospheric Pressure
  • Unequal heating of the Earth’s surface causes variations in altimeter settings between stations.
  • High to Low or Hot to Cold

“Look Out Below!”

Under which condition will pressure altitude be equal to true altitude?
  • When the atmospheric pressure is 29.92 inches Hg.
  • When standard atmospheric conditions exist.
  • When indicated altitude is equal to the pressure altitude.
If a flight is made from an area of low pressure into an area of high pressure without the altimeter setting being adjusted, the altimeter will indicate
  • the actual altitude above sea level.
  • higher than the actual altitude above sea level.
  • lower than the actual altitude above sea level.
If a flight is made from an area of low pressure into an area of high pressure without the altimeter setting being adjusted, the altimeter will indicate
  • lower than the actual altitude above sea level.

The aircraft will be at a higher true altitude above sea level than is indicated.

  • At lower altitudes, friction with the Earth’s surface deflects the wind.
  • Convective circulation patterns associated with the sea breezes are caused by cool, dense air moving inland from over water.
What causes variations in altimeter settings between weather reporting points?
  • Unequal heating of the Earth's surface.
  • Variation of terrain elevation.
  • Coriolis force.
The wind at 5,000 feet AGL is southwesterly while the surface wind is southerly. This difference in direction is primarily due to
  • stronger pressure gradient at higher altitudes.
  • friction between the wind and the surface.
  • stronger Coriolis force at the surface.
Convective circulation patterns associated with sea breezes are caused by
  • warm, dense air moving inland from over the water.
  • water absorbing and radiating heat faster than the land.
  • cool, dense air moving inland from over the water.
atmospheric stability
Atmospheric Stability
  • The amount of moisture present in the atmosphere is dependent upon the temperature of the air.
  • Water vapor is added into the atmosphere only by the processes of evaporation and sublimation. Evaporation is the changing of liquid water to water vapor.
atmospheric stability22
Atmospheric Stability
  • 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. Adiabatic heating, or adiabatic cooling, are the terms used to describe this temperature change.
atmospheric stability23
Atmospheric Stability
  • 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.
  • Stability of air can be measured by its ACTUAL lapse rate.
What measurement can be used to determine the stability of the atmosphere?
  • Atmospheric pressure.
  • Actual lapse rate.
  • Surface temperature.
atmospheric stability25
Atmospheric Stability
  • STABLE Air - Stratiform Clouds.
  • When moist stable air flows upslope, stratus type clouds form.
  • UNSTABLE AIR = turbulence, GOOD surface visibility.
What are characteristics of unstable air?
  • Turbulence and good surface visibility.
  • Turbulence and poor surface visibility.
  • Nimbostratus clouds and good surface visibility.
What are characteristics of a moist, unstable air mass?
  • Cumuliform clouds and showery precipitation.
  • Poor visibility and smooth air.
  • Stratiform clouds and showery precipitation.
A stable air mass is most likely to have which characteristic?
  • Showery precipitation.
  • Turbulent air.
  • Poor surface visibility.
Moist, stable air flowing upslope can be expected to
  • produce stratus type clouds.
  • cause showers and thunderstorms.
  • develop convective turbulence.
What would decrease the stability of an air mass?
  • Warming from below.
  • Cooling from below.
  • Decrease in water vapor.
What would decrease the stability of an air mass?
  • Warming from below.

Stability is altered by a change in the lapse rate of an air mass. Warming from below or cooling from above will increase the lapse rate and make the air LESS stable.

What is a characteristic of stable air?
  • Stratiform clouds.
  • Unlimited visibility.
  • Cumulus clouds.
atmospheric stability33
Atmospheric Stability
  • 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.
atmospheric stability34
Atmospheric Stability
  • 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.
atmospheric stability35
Atmospheric Stability
  • The most frequent type of ground or surface-based inversion is that which is produced by terrestrial radiation on a clear, relatively still night.
  • Beneath a low-level temperature inversion, when the relative humidity is high, can be expected smooth air, poor visibility, fog, haze, or low clouds.
The most frequent type of ground or surface-based temperature inversion is that which is produced by
  • terrestrial radiation on a clear, relatively still night.
  • warm air being lifted rapidly aloft in the vicinity of mountainous terrain.
  • the movement of colder air under warm air, or the movement of warm air over cold air.
Which weather conditions should be expected beneath a low-level temperature inversion layer when the relative humidity is high?
  • Smooth air, poor visibility, fog, haze, or low clouds.
  • Light wind shear, poor visibility, haze, and light rain.
  • Turbulent air, poor visibility, fog, low stratus type clouds, and showery precipitation.
What feature is associated with a temperature inversion?
  • A stable layer of air.
  • An unstable layer of air.
  • Chinook winds on mountain slopes.
A temperature inversion would most likely result in which weather condition?
  • Clouds with extensive vertical development above an inversion aloft.
  • Good visibility in the lower levels of the atmosphere and poor visibility above an inversion aloft.
  • An increase in temperature as altitude is increased.
moisture dewpoint and temperature
MoistureDewpoint and temperature
  • The dewpoint, given in degrees, is the temperature at which the air can hold no more moisture. When the temperature of the air is reduced to the dewpoint, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, hail, or snow.
  • The Amount of water vapor which air can hold depends on the AIR TEMPERATURE
atmospheric stability moisture
Atmospheric Stability & Moisture
  • When lifted, unsaturated air cools at a rate of 5.4°F per 1,000 feet and the dewpoint temperature decreases at a rate of 1°F per 1,000 feet.
  • Convergence of temperature and dewpoint is at a rate of 4.4°F (2° C).
  • The convergence rate determines the height of the cloud base.
atmospheric stability moisture44
Atmospheric Stability & Moisture
  • Temperature (T) = 85°F
  • Dewpoint (DP) = 71°F
  • Convergence Rate (CR) = 4.4°

T – DP = (TDS) Temp-dewpoint spread

TDS ÷ CR = X

X × 1,000 feet = height of cloud base AGL

What is meant by the term 'dewpoint'?
  • The temperature at which condensation and evaporation are equal.
  • The temperature at which dew will always form.
  • The temperature to which air must be cooled to become saturated.
The amount of water vapor which air can hold depends on the
  • dewpoint.
  • air temperature.
  • stability of the air.
What is the approximate base of the cumulus clouds if the surface air temperature at 1,000 feet MSL is 70 °F and the dewpoint is 48 °F?
  • 4,000 feet MSL.
  • 5,000 feet MSL.
  • 6,000 feet MSL.
  • the lowest layer of clouds reported as being broken or overcast, or the vertical visibility into an obscuration like fog or haze.
  • Clouds are reported as broken when five-eighths to seven-eighths of the sky is covered with clouds. Overcast means the entire sky is covered with clouds.
Clouds, fog, or dew will always form when
  • water vapor condenses.
  • water vapor is present.
  • relative humidity reaches 100 percent.
  • Fog, by definition, is a cloud that begins within 50 feet of the surface.
  • It typically occurs when the temperature of air near the ground is cooled to the air’s dewpoint.
  • Radiation - warm, moist air lies over flatland areas on clear, calm nights.
  • Advection - warm air mass moves inland from coast in winter.
  • Upslope - occurs when moist, stable air is forced up sloping land features like a mountain range.
  • Steam - or sea smoke, forms when cold, dry air moves over warm water. (turbulence and icing)
  • Advection and Upslope require WIND!
If the temperature/dewpoint spread is small and decreasing, and the temperature is 62 °F, what type weather is most likely to develop?
  • Freezing precipitation.
  • Thunderstorms.
  • Fog or low clouds.
What situation is most conducive to the formation of radiation fog?
  • Warm, moist air over low, flatland areas on clear, calm nights.
  • Moist, tropical air moving over cold, offshore water.
  • The movement of cold air over much warmer water.
In which situation is advection fog most likely to form?
  • A warm, moist air mass on the windward side of mountains.
  • An air mass moving inland from the coast in winter.
  • A light breeze blowing colder air out to sea.
What types of fog depend upon wind in order to exist?
  • Radiation fog and ice fog.
  • Steam fog and ground fog.
  • Advection fog and upslope fog.
Low-level turbulence can occur and icing can become hazardous in which type of fog?
  • Rain-induced fog.
  • Upslope fog.
  • Steam fog.
visible moisture
Visible Moisture
  • 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.
atmospheric stability visible moisture
Atmospheric Stability & Visible Moisture
  • Through the processes of sublimation and condensation, moisture condenses or sublimates onto miniscule particles of matter like dust, salt, and smoke known as condensation nuclei.
  • The nuclei are important because they provide a means for the moisture to change from one state to another.
What are the processes by which moisture is added to unsaturated air?
  • Evaporation and sublimation.
  • Heating and condensation.
  • Supersaturation and evaporation.
atmospheric stability visible moisture62
Atmospheric Stability & Visible Moisture

Cloud type is determined by its height, shape, and behavior. They are classified into families according to the height of their bases as:


middle, or

high clouds, as well as

clouds with vertical development.

cloud classifications
Cloud Classifications

• Cumulus — Heaped or piled clouds.

• Stratus — Formed in layers.

• Cirrus — Ringlets; fibrous clouds; also high-level clouds above 20,000 feet.

• Castellanus — Common base with separate vertical development; castle-like.

• Lenticularus — Lens shaped; formed over mountains in strong winds.

• Nimbus — Rain bearing clouds.

• Fracto — Ragged or broken.

• Alto — Meaning high; also middle-level clouds existing at 5,000 to 20,000 feet.

Clouds are divided into four families according to their
  • outward shape.
  • height range.
  • composition.
An almond or lens-shaped cloud which appears stationary, but which may contain winds of 50 knots or more, is referred to as
  • an inactive frontal cloud.
  • a funnel cloud.
  • a lenticular cloud.
Crests of standing mountain waves may be marked by stationary, lens-shaped clouds known as
  • mammatocumulus clouds.
  • standing lenticular clouds.
  • roll clouds.
Possible mountain wave turbulence could be anticipated when winds of 40 knots or greater blow
  • across a mountain ridge, and the air is stable.
  • down a mountain valley, and the air is unstable.
  • parallel to a mountain peak, and the air is stable.
Possible mountain wave turbulence could be anticipated when winds of 40 knots or greater blow
  • across a mountain ridge, and the air is stable.

Mountain waves are formed when strong winds flow across a barrier. When air is stable, the flow is laminar, or layered and creates a series of waves.

What cloud types would indicate convective turbulence?
  • Cirrus clouds.
  • Nimbostratus clouds.
  • Towering cumulus clouds.
What clouds have the greatest turbulence?
  • Towering cumulus.
  • Cumulonimbus.
  • Nimbostratus.
CLOUDS with Vertical development and associated turbulence can be expected when UNSTABLE air is forced upwards.

Moist unstable airmasses are characterized by “cumuliform” clouds and showery precipitation.

What are characteristics of a moist, unstable air mass?
  • Cumuliform clouds and showery precipitation.
  • Poor visibility and smooth air.
  • Stratiform clouds and showery precipitation.
If an unstable air mass is forced upward, what type clouds can be expected?
  • Stratus clouds with little vertical development.
  • Stratus clouds with considerable associated turbulence.
  • Clouds with considerable vertical development and associated turbulence.
nimbus means rain
“nimbus” means RAIN
  • Precipitation can mean anything from light rain, which you can fly right through to freezing rain, which can be very bad for ALL aircraft.
  • Freezing rain and temperature inversion can be indicated by ice pellets on the surface.
  • Induction
  • Structural:
    • Visible Moisture
    • Collecting Surface Near Freezing
One in-flight condition necessary for structural icing to form is
  • small temperature/dewpoint spread.
  • stratiform clouds.
  • visible moisture.
In which environment is aircraft structural ice most likely to have the highest accumulation rate?
  • Cumulus clouds with below freezing temperatures.
  • Freezing drizzle.
  • Freezing rain.
The presence of ice pellets at the surface is evidence that there
  • are thunderstorms in the area.
  • has been cold frontal passage.
  • is a temperature inversion with freezing rain at a higher altitude.
  • Much like Ice:
    • If the temperature of a collecting surface is at or below the dewpoint of the adjacent air, and the dewpoint is below freezing, frost will form and…
Frost on wings affects takeoff performance by disrupting the smooth airflow over airfoils, adversely affecting lifting capacity!
How will frost on the wings of an airplane affect takeoff performance?
  • Frost will disrupt the smooth flow of air over the wing, adversely affecting its lifting capability.
  • Frost will change the camber of the wing, increasing its lifting capability.
  • Frost will cause the airplane to become airborne with a higher angle of attack, decreasing the stall speed.
Which conditions result in the formation of frost?
  • The temperature of the collecting surface is at or below freezing when small droplets of moisture fall on the surface.
  • The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dewpoint is below freezing.
  • The temperature of the surrounding air is at or below freezing when small drops of moisture fall on the collecting surface.
air masses and fronts
Air Masses and Fronts

Air masses 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.

air masses and fronts84
Air Masses and Fronts

As air masses move across bodies of water and land, they eventually come in contact with another air mass with different characteristics. The boundary layer between two types of air masses is known as a front.

air masses and fronts85
Air Masses and Fronts

Fronts are named according to the temperature of the advancing air as it relates to the temperature of the air it is replacing.

  • Warm Front
  • Cold Front
  • Stationary Front
  • Occluded Front
air masses and fronts86
Air Masses and Fronts
  • 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. Light to moderate precipitation is probable, usually in the form of rain, sleet, snow, or drizzle, punctuated by poor visibility.
Steady precipitation preceding a front is an indication of
  • stratiform clouds with moderate turbulence.
  • cumuliform clouds with little or no turbulence.
  • stratiform clouds with little or no turbulence.
One of the most easily recognized discontinuities across a front is
  • a change in temperature.
  • an increase in cloud coverage.
  • an increase in relative humidity.
One weather phenomenon which will always occur when flying across a front is a change in the
  • wind direction.
  • type of precipitation.
  • stability of the air mass.

Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds.

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 that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.

thunderstorm recipe
Thunderstorm Recipe:

Humidity • Lifting Action• Unstable Air

The conditions necessary for the formation of cumulonimbus clouds are a lifting action and
  • unstable air containing an excess of condensation nuclei.
  • unstable, moist air.
  • either stable or unstable air.
What conditions are necessary for the formation of thunderstorms?
  • High humidity, lifting force, and unstable conditions.
  • High humidity, high temperature, and cumulus clouds.
  • Lifting force, moist air, and extensive cloud cover.
During the life cycle of a thunderstorm, which stage is characterizedpredominately by downdrafts?
  • Cumulus.
  • Dissipating.
  • Mature.
Thunderstorms reach their greatest intensity during the
  • mature stage.
  • downdraft stage.
  • cumulus stage.
Upon encountering severe turbulence, which flight condition should the pilot attempt to maintain?
  • Constant altitude and airspeed.
  • Constant angle of attack.
  • Level flight attitude.
Thunderstorms which generally produce the most intense hazard to aircraft are
  • squall line thunderstorms.
  • steady-state thunderstorms.
  • warm front thunderstorms.
A nonfrontal, narrow band of active thunderstorms that often develop ahead of a cold front is a known as a
  • prefrontal system.
  • squall line.
  • dry line.
Which weather phenomenon signals the beginning of the mature stage of a thunderstorm?
  • The appearance of an anvil top.
  • Precipitation beginning to fall.
  • Maximum growth rate of the clouds.
What feature is normally associated with the cumulus stage of a thunderstorm?
  • Roll cloud.
  • Continuous updraft.
  • Frequent lightning.
Which weather phenomenon is always associated with a thunderstorm?
  • Lightning.
  • Heavy rain.
  • Hail.
If there is thunderstorm activity in the vicinity of an airport at which you plan to land, which hazardous atmospheric phenomenon might be expected on the landing approach?
  • Precipitation static.
  • Wind-shear turbulence.
  • Steady rain.
wind shear
Wind Shear
  • a sudden, drastic change in windspeed 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 is obviously not so good, but:
Where does wind shear occur?
  • Only at higher altitudes.
  • Only at lower altitudes.
  • At all altitudes, in all directions.
wind shear microburst
Wind Shear - Microburst
  • occurs in a space of less than 1 mile horizontally and within 1,000 feet vertically
  • lifespan is about 15 minutes
  • can produce downdrafts of up to 6,000 feet per minute. It can also produce a hazardous wind direction change of 45 knots or more
wind shear microburst110
Wind Shear - Microburst
  • an inadvertent takeoff into a microburst
  • the plane first experiences a performance-increasing headwind,
  • followed by performance-decreasing downdrafts.
  • Then the wind rapidly shears to a tailwind,
  • and can result in terrain impact or flight dangerously close to the ground.
When may hazardous wind shear be expected?
  • When stable air crosses a mountain barrier where it tends to flow in layers forming lenticular clouds.
  • In areas of low-level temperature inversion, frontal zones, and clear air turbulence.
  • Following frontal passage when stratocumulus clouds form indicating mechanical mixing.
A pilot can expect a wind-shear zone in a temperature inversion whenever the windspeed at 2,000 to 4,000 feet above the surface is at least
  • 10 knots.
  • 15 knots.
  • 25 knots.