Content:

1. Atmospheric Stability

2. Moitures

3. Clouds

4. Precipitation

5. Airmasses

6. Fronts

1. Atmospheric Stability

• Adiabatic Process

• Stability is the atmosphere's resistance to vertical motion

• Adiabatic cooling and adiabatic heating causes the temperature of an airmass to rise or fall without exchanging heat with surrounding air

• The pressure of the upper layers of the atmosphere compresses the layers below them

• In the process of adiabatic cooling:

• Air rises and pressure decreases

• Air expands and cools

• In the process of adiabatic heating

• Air sinks and pressure rises

• Air compresses and the temperature rises

• Lapse Rate

• The lapse rate is the rate at which air temperature decreases with increased altitude

• Avg. lapse rate = 2°C per 1,000 ft.

• Actual lapse rate varies, and determines the stability of the atmosphere

• The Effect of Temperature and Moisture on Stability

• Temperature and moisture influence atmospheric stability and the resulting weather in the following ways:

• Dry air is usually stable, and the weather is usually clear

• Moist air causes more vertical motion and more dynamic weather than dry air

• Warm moist air is the least stable, and the resulting weather can include precipitation and thunderstorms

• Causes of Temperature Inversion

• A temperature inversion is a condition in which the air temperature increases with altitude

• Causes:

• The ground cools and lowers the temperature of the adjacent air

• A frontal system forces cool air under warm air or warm air over cold air

• Temperature inversions develop in smooth, stable air. If wind blows into the area, it mixes the air enough to break up any inversion

• Temperature Inversion Hazards

• Trap weather and pollutants

• Restrict visibility

• Contribute to the formation of fog, haze, or low clouds when the inversion is near the ground

• Formation of Temperature Inversions

1. Terrestrial radiation inversions occur near the ground on clear, still nights

2. Frontal inversions occur when cool air is forced under warm air (cool fronts), or when warm air speeds over cold air (warm fronts)
   

2. Moitures

• Moisture is present in three states:

1. Solid

2. Liquid

3. Gas

• Each time moisture changes physical state, it releases or absorbs energy

• Latent heat is the amount of heat energy required to change the state of a substance without changing its temmperature

• There are six processes that cause moisture to change state

1. Evaopration - changes liquid water into water vapor when water absorbs latent heat

2. Condensation - changes water vapor into liquid water when water vapor releases latent heat

3. Sublimation - changes ice directly into water vapor when ice absorbs latent heat

4. Deposition changes water vapor directly into ice when water vapor releases latend heat

5. Melting - changes ice into water when ice crystals absorb latent heat

6. Freezing - changes water into ice when water releases latent heat

• Little heat is exchanged during melting and freezing, so these processes has little effect on weather

• Potential hazards caused by water vapor:

• Clouds

• Fog

• Precipitation

• Icing

• Humidity and Relative Humidity

• Humidity describes the amount of water vapor in the air

• The amount of moisture the air is capable of holding depends on air temperature

• Dewpoint

• The temperature at which the air reaches saturation and can hold no more water vapor

• Dewpoint and temperature are proportional: The higher the air temperature, the higher the dewpoint

• Condensation occurs when the temperature and dewpoint are equal, forming dew, fog or clouds

• Frost forms under the same conditions as dew at freezing temperature

• Calculating Cloud Bases

• The Effects of Frost on Aircraft

• Frost forms when the air is saturated and the surface temperature is below freezing

• Deposition changes water vapor directly into ice crystals on freezing surfaces

• Frost is a serious hazard because it interferes with the smooth airflow over the wings and can cause early airflow separation, resulting in a loss of lift

• Frost increases drag and weight, which, when combined with the loss of lift, can prevent an aircraft from becoming airborne at normal takeoff speed

• Federal Air Regulations require that your airplane is free of frost before you attempt to fly (FAR 91.527)

3. Clouds

• Cloud Formation

• The cloud formation process involves three events:

• Air cools to its dewpoint and becomes saturated

• Water vapor collects around condensation nuclei

• Condensation changes water vapor into clouds or fog

• The difference between temperature and dewpoint is referred to as the temperature dewpoint spread: A small spread indicates that clouds or fog are likely to form

• The Four Families of Clouds: Distinguished by their characteristics and their altitudes

1. Low clouds - made of droplets of liquid water and develop between ground level and about 6,500 ft. AGL

1. Stratus clouds - layered clouds that form as the earth cools the moist air above it or when moist, stable air flows up sloping terrain

2. Nimbostratus clouds - gray or black. These low clouds can be several thousand feet thick and pose serious icing risks

3. Stratocumulus clouds - white, puffy clouds formed when stratus layers break up or cumulus clouds spread out

2. Middle clouds are made of liquid water droplets, ice crystals, or supercooled water droplets and develop between 6,500 ft. and 20,000 ft. AGL. These clouds pose turbulence and icing hazards

1. Altostratus clouds - flat, dense clouds that cover a wide area. They are gray or gray-white, and although they produce minimal turbulence, they can cause icing hazards

2. Altocumulus clouds - patchy gray or white clouds that form when altostratus clouds break up. They can contain supercooled water droplets and can produce light turbulence

3. High clouds are usually compsed of ice crystals and develop above 20,000 ft. AGL

1. Cirrus clouds - think, wispy clouds that form above 30,000 ft AGL. They can serve as a warning of approaching bad weather

2. Cirrostratus clouds - long bands or sheets of thin, white clouds against a deep blue background. They have low moisture content and pose no icing hazard

3. Cirrocumulus clouds are white, patchy clouds that resemble cotton. They can produce light turbulence

4. Vertical development clouds start as low or middle clouds, but they build vertically because of convection. They always indicate atmospheric instability. When other cloud formations obscure vertically-developed clouds, they are referred to as embedded clouds

1. Cumulus clouds - puffy white dome-shaped clouds with flat bases. They indicate a shallow layer of instability, and although they cause turbulence, they cause little icing or precipitation

2. Towering cumulus clouds - large mounts of cotton with cauliflower tops. Their color can vary from brilliant white at the top to gray near the bottom, and they indicate deep areas of unstable air. They contain moderate to severe turbulence with icing and often develop into thunderstorms

3. Cumulonimbus clouds are thunderstorms. They are gray-white or black clouds that contain large amounts of moisture and present many flying hazards

• What is Fog (Low Clouds)?

• Fog is a low-lying cloud that can form thick layers close to the ground

• Fog typically forms as soon as the temperature of the air approaches or reaches its dewpoint

• Its base is no more than 50 ft. AGL

• Ground fog does not extend very high above the surface

1. Radiation Fog

1. Develops in moist air over low, flat areas on clear, calm, humid nights, or in mountainous terrain in river valleys where cool air pools

2. Occurs in stable air in high-pressure area

3. Usually dissipates in warmer temperatures by mid-morning

2. Advection Fog

1. Exists most commonly along coastlines

2. Requires light wind to form

3. Develops when wind moves a low layer of warm, moist air over a cooler surface

3. Upslope Fog

1. Develops along coastlines or mountain valleys near lakes

2. Requires wind to form

3. Develops when winds force moist, stable air up a sloping land mass

4. Steam Fog (Sea Smoke)

1. Develops when cold, dry air moves over warmer water, which evaporates and rises

2. Composed entirely of water droplets that can freeze quickly and fall as ice particles, posing icing hazards

3. Because it forms in relatively unstable air, low-level turbulence is associated with steam fog

4. Precipitation

• Precipitation is a liquid or solid water particle that falls from the atmosphere

• Causes:

• Condensation forms water droplets that continue to grow until they fall from the cloud

• Deposition causes ice crystals to form directly from water vapor and grow until they are heavy enough to fall from a cloud

• Coalescence occurs when water droplets of different sizes collide and merge until they are large enough to fall from the cloud

• The ice crystal process causes ice crystals to acquire evaporating water vapor from supercooled water droplets. During the ice-crystal process, ice crystals form directly from water vapor and grow until they fall

• Rain

• Rain is .02 inches or greater in diameter. It falls at a steady rate and stops gradually

• Rain showers start suddenly, change intensity suddenly, and stop suddenly

• Drizzle is very small droplets that are less than .02 inches in diameter

• Warm rain and drizzle can develop into dense precipitation-induced fog

• Freezing rain and freezing dribble freeze upon contact with objects

• Virga - falling rain evaporates very quickly and never reaches the ground

• Ice pellets - the rain falls through a temperature inversion, so it can freeze and arrive at ground level

• Hail

• Hail is made up of large clumps of ice

• Hail forms when water droplets freeze in the upper cloud levels. As the freezing droplets start to fall, they collide and increase in size. Strong updrafts return them to the higher cloud levels where they collide with other ice pellets and get bigger

• Hailstones can grow into large ice chunks and pose serious flight hazards

• Snow

• Snow forms when temperature are below freezing and the temperatures of the air and dewpoint converge

• Ice crystal form by deposition and grow quickly as they acquire the water vapor that is evaporating from supercooled water droplets. The ice crystal must become heavy enough to fall from the cloud as snow

• Snow falls - at a relatively steady rate and stops gradually

• Snow showers - start suddenly, change intensity suddenly, and stop suddenly

• Snow grains - the solid equivalent of drizzle. They are very small, white, opaque particles of ice

• Fallstreak - similar to virga, but the ice particles change from ice into vapor as they fall

• sublimation -?the transformation of ice crystals directly into vapor as they fall

5. Airmasses

• An airmass is a large body of air that

• Contains uniform temeprature and moisture

• Acquires the overall characterstics of its source region

• Spans a few thousand miles and exists for several days to more than a week

• Classification of Airmasses?

• Continental airmasses develop over land and are typically dry

• Maritime airmasses develop over large bodies of water and contain large amounts of moisture

• Polar airmasses develop over polar regions and have cool temperatures

• Tropical airmasses develop near the tropics and have warm temperatures

• Continental-polar airmasses originate over polar land masses and contain cold, dry, stable air

• Maritime-tropical airmasses originate over water and contain warm, moist, unstable air

• Airmass Modification

• The degree to which an airmass changes depends on several factors, including the temperature difference between the airmass and the new surface

• Stable airmasses display:

• Generally smooth air

• Layered or stratiform clouds

• Widespread areas of clouds, steady rain, or drizzle

• When an airmass moves over a warmer surface, its lower layers are warmed from below, causing vertical movement and instability

• When an airmass flows over a cooler surface, its lower layers are cooled from below and the airmass stabilizes

• The moisture content of airmass affects the characterstics it acquires as it moves from its source region

• Prolonged cooling from below causes the air to cool to its dewpoint, forming low clouds or fog that can persist for weeks

6. Fronts

• The Characterestics of Fronts

• When airmasses leave their source regions, they come into contact with other airmasses that have different characteristics. These difference in airmass characteristics are called discontinuities

• The boundary that separates discontinuous airmasses is called a?front

• The major frontal discontinuities influence weather:

• A change in temperature - usually quite noticeable and abrupt

• Shift in wind direction or speed?

• A decrease in pressure at the boundary of the fronts - update your altimeter setting as soon as possible after you cross a front

• Frontal discontinuities across fronts can pose serious hazards to flight

• The characteristics of Cold Fronts

• A cold front:?

• Separates an advancing mass of cold,dense, and stable air from a warmer, lighter, less stable airmass

• Is accompanied by severe turbulence, strong winds, and precipitation

• The cool, dense air of a cold front forces the less dense, warmer air upward

• Cumuliform clouds and showers are common near cold fronts

• Fast-moving cold fronts

• Are pushed by intense, high-pressure system behind the front

• Have a steep leading edge and extreme discontinuities that makes them more hazardous than slow-moving cold fronts

• Slow-moving cold fronts

• Have a shallow leading edge

• Are less hazardous than fast-moving cold fronts

• Produce clouds that extend far behind the surface front

• Cold fronts approach quickly and can change weather completely within a few hours

• Phases of Cold Fronts

• Prior to

• During Passage

• After Passage

• Characteristics of Warm Fronts

• Warm air overtakes and replaces cooler air

• Less severe than that associated with cold fronts and tends to remain in a region for days

• Warm air can extend for several hundred miles ahead of the front

• Cirrostratus, altostratus, and stratus clouds often precede a slow-moving warm front

• Cumulus and stratocumulus clouds can develop along, ahead of, and behind the front

• Steady precipitation with little turbulence usually precedes a warm front

• Warm fronts can remain in a region for days and influence wather

• Phases of Warm Fronts

• Prior to

• During Passage

• After Passage

• Characteristic of Stationary Fronts

• Opposing forces between warm and cold airmasses are balanced and move very slowly

• Seperate two airmasses that might not move for several days

• A mixture of weather characteristics of both warm and cold fronts

• Characteristic of Occluded Fronts

• Occur when a warm front and cold front converge and the cold front overtakes the warm front

• Cold Front Occlusions - when the fast-moving cold front is colder than the air ahead of the slow-moving warm front

• Warm Front Occlusions - when the air ahead of a slow-moving warm front is colder than the air within a fast-moving cold front

• Phases of Occluded Fronts

• Prior to
  

• During Passage

• After Passage