Private Pilot | Lesson 13 - Weather

TABLE OF CONTENTS:
13.1 CAUSES OF WEATHER, CONVECTIVE CURRENTS, AND THERMALS
13.2 AIR MASSES AND FRONTS
13.3 THUNDERSTORMS AND SQUALL LINES
13.4 FROST
13.5 ICING, FREEZING RAIN, AND ICE PELLETS
13.6 MOUNTAIN WAVE AND WIND SHEAR
13.7 TEMPERATURE/DEW POINT, FOG, AND FROST
13.8 CLOUDS
13.9 CHARACTERISTICS OF STABLE AND UNSTABLE AIR
13.10 TEMPERATURE INVERSIONS

13.1 Causes of Weather, Convective Currents, and Thermals

Every physical process of weather is accompanied by, or is the result of, heat exchanges.

The unequal heating of the Earth's surface causes differences in pressure and altimeter settings.

The Coriolis force, which is caused by the rotation of the earth, deflects winds to the right in the Northern Hemisphere. These deflections caused by Coriolis force are more pronounced at altitude, and less pronounced at the surface due to generally slower wind speeds at the surface.

Wind speeds are slower at the surface due to friction between the wind and the Earth's surface, which work to slow wind speeds.

Sea breezes and convective circulation patterns are the generally caused when cool, dense air moves inland off the water. Once inland, now over the warmer land mass, the air heats up and rises. Wind currents push the air out over the water again where it cools, descends, and blows inland, starting the pattern over again.

The development of thermals depends upon solar heating.

13.2 Air Masses and Fronts

A front is the boundary between two air masses of different density. One weather phenomenon that will always occur when flying across a front is a change in the wind direction.

Additionally, one of the most easiest recognizable changes when flying across a front is a change in temperature.

13.3 Thunderstorms and Squall Lines

Thunderstorms are produced by cumulonimbus clouds. They form when there is

1. High humidity (moist air),
2. An unstable lapse rate (unstable atmospheric conditions), and
3. An initial upward boost (lifting force) to start the process.

Thunderstorms have three phases in their life cycle:

Cumulus — (A) The building stage of a thunderstorm when there are continuous updrafts.
Mature — (B) The time of greatest storm intensity, when there are both updrafts and downdrafts (causing severe wind shear and turbulence). When rain begins to fall at the surface, this indicates the start of the mature stage of a thunderstorm.
Dissipating — (C) The end of the storm, when there are only downdrafts and the storm begins raining itself out.

A thunderstorm, by definition, has lightning. Lighting is what causes thunder. All thunderstorms produce wind shear turbulence, a hazard particularly for airplanes taking off and landing, or flying at low altitudes (less than 1,500 ft. AGL).
Embedded thunderstorms are obscured, and usually pilots cannot see them because they occur in very cloudy conditions or are buried deeply within other clouds.

A squall line is a nonfrontal narrow band of thunderstorms usually ahead of a cold front.

Squall line thunderstorms usually produce the most severe thunderstorm conditions (heavy hail, destructive winds, tornadoes, etc.).

13.4 Frost

Frost forms on exposed surfaces when both the collecting surface is below the dew point of the adjacent air AND the dew point is below freezing.

Frost is the direct sublimation of water vapor to ice crystal, that is, water vapor (gas) in the air sublimes directly in (solid) ice crystals bypassing the liquid state.

• Sublimation is the conversion of water from solid (frost, or ice) directly to gas (water vapor), skipping the liquid state. Or, in reverse, from gas (water vapor) sublimating directly to solid (frost, or ice).

Frost is considered hazardous to flight, and should be removed from the aircraft before flight. When frost forms on aircraft wings and and tail surfaces, it disrupts and spoils the smooth airflow over the wings causing a decrease in lifting capability. Because of these effects, frost may prevent the aircraft from becoming airborne at normal takeoff speeds.

13.5 Icing, Freezing Rain, and Ice Pellets

Structural icing requires two conditions:

1. Flight through visible moisture, and
2. Outside Air Temperature at or below freezing.

Freezing rain causes the greatest accumulation of structural ice.

Ice pellets are caused when rain droplets freeze at a higher altitude, i.e., freezing rain exists above.

13.6 Mountain Wave and Wind Shear

Mountain Wave

Lenticular clouds are almond or lens-shaped clouds, which appear stationary but may contain winds of 50 kts. or more, and are usually found on the leeward side of a mountain range.

You should anticipate possible mountain wave turbulence when the air is stable and winds of 40 kt. or greater blow across a mountain or ridge.

Wind Shear

Wind shear can occur at any altitude and be horizontal and/or vertical shear.

Hazardous wind shear may be expected in areas of low-level temperature inversions, frontal zones, and clear air turbulence. You should expect wind shear in a temperature inversion whenever wind speed at 2,000 to 4,000 ft. AGL is 25 kt. or more.

13.7 Temperature/Dew Point, Fog, and Frost

Dew Point is the temperature at which the air will have 100% humidity. So, air temperature determines how much water vapor can be held by the air. And, water vapor becomes visible as it condenses into clouds, fog, or dew.

Knowing this, it becomes easier to predict when clouds or fog will form - whenever the temperature and dew point are close. As a general rule, whenever the air temperature is within 5° of the dew point (and that spread is decreasing), you can expect fog and low clouds.

Types of Fog

Radiation fog — (shallow fog) is most likely to occur when there is a clear sky, little or no wind, and a small temperature/dew point spread.

Advection fog — forms as a result of moist air condensing as it moves over a cooler surface.

Upslope fog — results from warm, moist air being cooled as it is forced up sloping terrain.

Precipitation-induced fog — occurs when warm rain or drizzle falls through cool air and evaporation from the precipitation saturates the cool air and forms fog. Precipitation-induced fog is usually associated with fronts. Because of this, it is in the proximity of icing, turbulence, and thunderstorms.

Steam fog — forms in winter when cold, dry air passes from land areas over comparatively warm ocean waters and is composed entirely of water droplets that often freeze quickly. Low-level turbulence can occur and icing can become hazardous in steam fog.

13.8 Clouds

Clouds are divided into four families based on their height:

• High clouds
• Middle clouds
• Low clouds
• Clouds with extensive vertical development

Cumulonimbus clouds possess the greatest turbulence. (The suffix "Nimbus" means rain cloud.)

Towering cumulus are early stages of cumulonimbus; they usually indicate convective turbulence. Remember, lifting action, unstable air, and high humidity are the ingredients for the formation of a thunderstorm (cumulonimbus clouds).

Lapse Rate is the decrease in temperature with increase in altitude. When air rises, it cools at the rate of 5.4°F/1,000 ft., and its dew point decreases by 1°F/1,000 ft. Therefore, the temperature and dew point converge at 4.4°F/1,000 ft. Because clouds form when the temperature/dew point spread is 0°, you can use this rate convergence rate to estimate the bases of cumulus clouds.

The surface temperature/dew point spread divided by 4.4°F equals the bases of cumulus clouds in thousands of feet above ground level (AGL).

EXAMPLE:
A surface dew point of 56°F and a surface temperature of 69°F results in an estimate of cumulus cloud bases at 3,000 ft. AGL: 69°F – 56°F = 13°F temperature/dew point spread; 13°F/4.4°F = approximately 3,000 ft. AGL.

13.9 Characteristics of Stable and Unstable Air

As a general rule of thumb, stable air tends to display the following characteristics

And, conversely, the general characteristics of unstable air are:

When air is warmed by the surface from below, the air rises and causes instability. The lapse rate is the decrease in temperature that occurs as a given volume of air increases in altitude. As the lapse rate increases (air cools more and more as it increases in altitude), the air becomes more and more unstable (turbulent). So, lapse rate can be used to determine the stability of air masses.

Turbulence and clouds with extensive vertical development (thunderstorm clouds - cumulonimbus) result when unstable air rises.

Moist, stable air moving up a mountain slope produces stratus type clouds as it cools.

Steady precipitation preceding a front is usually an indication of a warm front, which results from warm air being cooled from the bottom by colder air. This results in stable air with stratiform clouds and little or no turbulence.

13.10 Temperature Inversions

Normally, temperature decreases as altitude increases; or, the higher your altitude, the cooler the air temperature. A temperature inversion occurs when temperature increases as altitude increases; or, it gets warmer the higher you go.

Temperature inversions usually result in a stable layer of air through the inversion.

Temperature inversions often develop near the ground on clear, cool nights when the winds are light. These types of low-level inversions are caused by terrestrial radiation - the warm earth heats the cool air that is close to the ground.

Smooth air with restricted visibility is usually found beneath a low-level temperature inversion.

Lesson 13 - Weather eFlash Cards

Lesson 13 - WeatherStudy Quiz