Chapter 3: Weather (Meteorology)

Explain the difference between a stable atmosphere and an unstable atmosphere. Why is the stability of the atmosphere important?

Stable Atmosphere is the ability to resist vertical motion.

Unstable Atmosphere is when small vertical air movements tend to become larger. Results in:

• Turbulence

• Severe Weather

• Vertical Clouds

How can you determine the stability of the atmosphere?

Comparing temperature lapse rate (changes with altitude).

• If temperature decreases rapidly as you increase altitude the air tends to be unstable air.

• If temperature remains unchanged or decreases slightly with altitude the air tends to be stable.

What are the effects of Stable Air on clouds/turbulence/precipitation/visibility?

Clouds: Stratiform

Turbulence: Smooth

Precipitation: Steady

Visibility: Fair to poor

What are the effects of Unstable Air on clouds/turbulence/precipitation/visibility?

Clouds: Cumuliform

Turbulence: Rough

Precipitation: Showery

Visibility: Good

What types of Weather Information will help you examine if wind shear conditions might affect your flight?

Terminal Forecasts

METARs

SIGMETs

Low Level Windshear Alert System (LLWAS)

PIREPs

While on a Cross-Country flight you notice a lens-shaped cloud over a mountainous area along your route of flight. What does the presence of this cloud indicate?

There’s a presence of turbulent air and mountain wave activity.

These clouds form when:

• Moist, stable air flows over mountains.

• Then it rises, cools and forms a smooth, lens shape.

• The air sinks again and the clouds disappear.

The amount of moisture in the air is dependent on what factor?

The temperature of the air.

• When the temperature goes up by 20°F, the air can hold twice as much moisture.

• When the temperature drops by 20°F, the air can hold half as much moisture.

What is Relative Humidity?

How much moisture is in the air compared to how much it could hold at a certain temperature (shown as a percentage). When air is fully saturated it cannot hold anymore moisture.

This Affects:

• Visibility

• Clouds

• How the engine performs

What are 3 ingredients necessary for precipitation to form?

• Water Vapor

• Sufficient lift to condense the water vapor into clouds

• Growth process allowing cloud droplets to grow large and heavy enough to fall as precipitation

If your route of flight takes you toward a low-pressure system, in general what kind of weather can you expect? What if you were flying toward a high-pressure system?

Low pressure

• Rising air causes clouds

• Rain

• Bad Weather

High Pressure

• Sinking air / clear skies

• Leading to good weather

What are the characteristics of a Cold Front?

As it’s passing,

• Expect towering cumulus or cumulonimbus clouds

• Warm Temperatures

• Falling atmospheric pressure/temperature

After it passes

• Cooler/drier air

• Rising atmospheric pressure

What are the characteristics of a Warm Front?

1. As the front passes

2. Stratiform clouds

3. Drizzle/Low ceilings

4. Poor visibility

5. Rise in temperature

What does a cloud consist of and why do they form?

Made of tiny water droplets or ice particles you can see in the sky.

• Warm rising air cools and water vapor condenses.

• Strong rising air is needed to form tall clouds that make heavy rain.

What favor primarily determines the type of vertical extent of clouds?

The stability of the atmosphere.

Describe the four basic cloud forms in the Earth’s Atmosphere.

1. Cirriform

2. Nimbo-form

3. Cummuliform

4. Stratiform

Characteristics of Cirriform Clouds

High level clouds (20,000ft).

Thin and White.

Characteristics of Nimbo-form Clouds

Rain clouds

Steady precipitation.

(7,000-15,000ft).

Characteristics of Cummuliform Clouds

Vertical motion and Thermal Uplift.

Most humidity = lower cloud base.

Top of cloud can reach 60,000ft.

What are 3 primary causes of turbulence?

• Convective currents (convective turbulence)

• Obstructions of wind flow (mechanical turbulence)

• Wind Shear

What are the four intensity levels of turbulence?

Light

Moderate

Severe

Extreme

What is Light Turbulence?

causes slight, erratic changes in altitude and or attitude pitch, roll, yaw

What is Moderate Turbulence?

changes in altitude or attitude occur aircraft always remains in control.

What is Severe Turbulence?

Causes Large abrupt changes in altitude and attitude.

Causes large variations in indicated airspeed.

What is Extreme Turbulence?

Aircraft is violently tossed about and is impossible to control.

May cause structural damage.

Define the term Clear Air Turbulence.

• CAT

• Sudden, severe turbulence occurring in cloudless regions

• Higher altitude turbulence (above 15,000ft)

• Can find in PIREPs

What is a Single Cell Thunderstorm?

One cell thunderstorm.

Easy to avoid except at night.

Rare and almost all thunderstorms are multi-celled.

What is a Multicell Thunderstorm?

Cluster of cells forming near each other. Each cell is at a different stage.

• one cell dies and another forms

• can bring heavy rain, hail, strong winds

What is a Supercell Thunderstorm?

Most powerful thunderstorm bring

• large hail, strong winds and tornadoes

• usually isolated, long lasting and very organized

Main types of Icing an aircraft may encounter?

1. Structural (clear, rime, mixed)

2. Induction System

3. Instrument Icing

What is the definition of the term freezing level, and how can you determine where that level is?

The lowest altitude where the air temperature is 0°C (32°F).

To find it, pilots use:

• Freezing level charts

• Icing forecasts (CIP and FIP)

• PIREPs, AIRMETs, SIGMETs

• Surface/upper-level weather charts

What action is recommended if you inadvertently encounter icing conditions?

Leave the icing area by:

• Leaving the clouds or precipitation/climbing

• Descending to a temperature above freezing.

• If you can’t do either, land ASAP.

Is frost considered to be hazardous to flight? Why or why not?

Yes, even a thin layer of frost can distort the smooth airflow over the wings.

This results in:

• Less lift

• Longer Takeoff roll

• Higher risk of stall.

Describe several types of obstructions to visibility that may occur in the atmosphere.

Weather and obstructions to visibility include:

• Fog

• Mist

• Haze

• Smoke

• Precipitation

• blowing snow

• dust storm

• sandstorm

• volcanic ash.