These notes: 1) highlight portions of the text I felt were most important; and 2) give information related to the chapter topic that the text does not cover. You may find these notes useful as a review of, and supplement to the text.

1. Adiabatic Temperature Changes *

2. Atmospheric Stability *

3. Cloud Formation *

4. Precipitation *

• A temperature change that does NOT involve heat exchange –
• No heat added or subtracted
• Cooling caused by expansion (lifting) of air
• Work done by the parcel of air
• Warming caused by compression (sinking ) of air
• Work done on the parcel

• The concept of atmospheric stability involves the tendency of a parcel that is displaced from a specific location or level to return to its original position or to continue to move in the direction of displacement.

• Stability: Tendency of a displaced parcel to return to its original position.
• Stable: Returns to original position

• Unstable: Continues in direction of displacement

• Neutral: Stays at the location of displacement

• Stability is related to the difference in temperature between a lifted parcel (a bubble or balloon of air) and the environmental air around it. T_E - T_L

• Three lapse rates:
• Environmental – on AVG. the air cools off at 6.5 C/km but this value can vary from +11 to –20 C/km .G avg= 6.5 C/km

• Dry air (unsaturated) cools at 10 C for every 1 km of lifting. "Dry" air cooling is always at G dry = 10.0 C/km

• Moist air (saturated) cools at 5 C for every 1 km of lifting.
• "Moist" air cooling is always at G wet = 5.0 C/km

• Conditional Stability

• Conditionally stable when dry (unsaturated)

• Conditionally unstable when moist (saturated)

• LIFTING is primary way to saturate the air

• Air at a constant temperature that has more moisture to it will become less dense or lighter than the dryer air. Why should this be?

• When air is saturated, the excess water vapor condenses to form clouds or fog.
• Air becomes saturated by:
• addition of water vapor
• cooling the air – usually through lifting

• A common way for air to become saturated is for it to be lifted and adiabatically cooled via one of the four methods of lifting
• convective lifting
• orographic lifting
• low level convergence (upper level divergence)
• frontal wedging

• Condensation nuclei: Tiny dust, dirt, or smoke particles that act as a surface for the water to condense onto.
• Hygroscopic nuclei: Not all particles in the atmosphere are good condensation nuclei. Only those that can absorb water (called hygroscopic) are effective as condensation nuclei.
• Supersaturation: If the air contains few condensation nuclei, than the relative humidity must be greater than 100% (supersaturation) for condensation to occur.
• • Supercooling: If the air contains few ice ‘condensation" nuclei (freezing nuclei), than the cloud droplet remain liquid at temperatures well below freezing to as low as –40 º C.
• Dew: Condensation can also occur on large surfaces, such as grass, cars, and windows. This is known as dew.
  

• Clouds are made of a large number of very small droplets of liquid water and/or ice crystals.
• Clouds are NOT water vapor.
• The droplets are so small that they do not fall, but remain suspended in the air.
• Size. Typical cloud droplet diameter 0.02 mm - 0.05 mm, Rain drop 2.0 mm, CCN 0.0002 mm. Avg. rain drop has 1 million times the volume (mass of water) of typical cloud drop
• Typical cloud droplet concentration:
• 300 hundred per cubic centimeter.
• 500,000 droplets would fit in a 2 liter soft-drink bottle!

• Clouds are classified in two ways-Height & Form
• Height
• High clouds – bases above 20,000 ft
• Middle clouds – 6500 to 20,000 ft
• Low clouds – bases below 6500 ft
• Clouds of vertical development – Tend to be about as tall as wide - some form of cumulus

• Form
• Cirriform – Cirriform clouds: very high, thin, wispy. Mostly ice crystals.
• Cumuliform – Puffy, develop vertically. Generally have flat bottoms. Often
• individual cloud units. Associated with unstable atmospheres.
• Stratiform – Generally flat, spread out (sheet like). May be breaks in the clouds, but no distinct, individual clouds. Associated with stable conditions.
• Hybrid clouds – Part puffy and part flat (Stratocumulus)

• High clouds
• Cirrus – delicate, icy filaments. Often form "mare’s tails" (Uncinus clouds)
• Cirrostratus – transparent veil, often smooth and covering much of the sky. This cloud produces a halo around the sun or moon.
• Cirrocumulus – white patches with very small cells or ripples. Often has a regular pattern. Gives a "mackerel sky" (looks like fish scales).

• Middle clouds
• Altocumulus – similar to cirrocumulus, but are lower, have larger cells, and are composed of water drops rather than ice crystals.
• Altostratus – grayish smooth clouds covering most of the sky. Sun is sometimes visible, but not distinct, as though you are looking at it through frosted glass. Can be a "corona" but there is no halo!

• Low clouds
• Stratus – Low, uniform cloud that covers much of the sky. It may produce light precipitation.
• Stratocumulus – Hybrid between stratus and cumulus clouds.
• Nimbostratus – Forms when stable air is forced to rise. A dark, low, uniform cloud, similar to stratus, but with long, continuous precipitation.

• Clouds of vertical development
• Cumulus – Individual, puffy masses that can grow vertically into towers or domes.
• Cumulonimbus – Cumulus clouds with great vertical development (usually fills the entire troposphere). Produces rain, hail, and lightning. An anvil head is often formed at the top where the cloud presses against the tropospause.

• The more unstable the atmosphere, the higher the cumulus cloud will develop.
• Stable layers aloft will cap cumulus cloud growth

• Fractus – Stratus or cumulus clouds that are broken into smaller, ragged pieces, usually underneath.
• Cap Cloud – Cloud that forms near the top of mountains on an otherwise clear day.
• Lenticular – Lens shaped, clouds sometimes look like "flying saucers" that form downwind (leeside) of mountains. Associated with strong turbulence. Pilots beware!
• Billow Cloud – A wavelike cloud caused by strong wind shear.
• Mammatus – Rounded clouds on undersides of cumulonimbus clouds. A sign of severe storms.
• Virga – Precipitation that evaporates before it hits the ground.
• Vapor Trail or Contrail - "Pseudo" cirrus cloud caused by the vapor trail of jet aircraft.

• Fog – defined as a cloud base on the ground. Usually like a stratus cloud.
• There are five types of fog. They all look the same, but they are formed differently.
• 3 Fogs formed by cooling
• Radiation fog – results from radiation cooling of the ground and air next to the ground
• Advection fog – results from warm, moist air moving (advecting) over a cooler surface
• Upslope fog – results from air being lifted and cooled orographically
• 2 Fogs formed by evaporation
• Steam fog – results when cool air moves over warm water. Similar to the steam formed over a cup of hot coffee. Sometimes called "sea smoke".
• Frontal fog – formed from rain falling through cool air and evaporating.
  

• Dew: Dew is formed by condensation onto a surface that has cooled below the dew point of the surrounding air.
• Dew forms first on grass because the grass also releases moisture through transpiration.
  
• Frost: If the dew point is below freezing, then instead of condensing, the water vapor undergoes deposition and forms frost without entering the liquid state. Frost is NOT frozen dew.

• Human Observation - from below
• Ceilometer - from below
• Satellite - from above

• Geosynchronous (GOES) Satellite

- Must be over the equator and over same spot "stationary"

- Must be 36,000 km (22,300 miles) high

- Always covers same area does NOT cover north/south of 60 degrees latitude

• Cloud droplets:
• Must form first
• Small (diameter of about 0.02 mm),
• Do not fall fast.
• Precipitation
• cloud droplets must grow heavy enough to fall.
• raindrop diameter of ~2 mm a million times more volume than cloud droplet.
• Growth from cloud to rain drop occurs in two ways:
• Bergeron process - starts as snow ("cold rain process")
• collision-coalescence process (always liquid - "warm rain process")

• Facts important to formation of precipitation in cold clouds:
• Super cooled clouds: Liquid cloud water drops exists in the atmosphere at temperatures as low as -40 ºC.
• Freezing doesn't occur unless there are freezing (ice) nuclei present.
• Freezing (ice) nuclei are not very abundant.
• The saturation vapor pressure over ice is less than that over liquid water.
• Causes "Resistance" to condensation or depositions
• Ice crystal forms on ice nuclei
• Once ice crystal forms in the presence of supercooled liquid water, the ice crystals grow at the expense of the cloud droplets.
• The ice crystals continue to grow until they are heavy enough to fall to the ground.
• If the temperature at the ground is near freezing, the snowflakes reach the ground as snow.
• If the temperature at the ground is above 39 degrees F, the snowflakes will melt to form rain.
• The Bergeron process is the primary mechanism for forming precipitation in storms in the middle latitudes.
• Small cloud droplets exert more vapor pressure than a flat surface because they have more surface area per volume.

• Precipitation formation in warm clouds (above 32 F).
• A few cloud droplets must grow to 20 microns (.02 mm) for the collision-coalescence process to work
• Larger drops fall faster and collide with smaller droplets and some will stick, or coalesce to become raindrops.
• The drop continues to grow through collisions. As drop reaches a size of 4 mm, it breaks apart and starts the process again
• The collision-coalescence process is the main means of precipitation formation in:
• Low stratus clouds
• In the Tropics

• There are two general classifications for lifting the atmosphere:
• Convective which is the result of strong nearly vertical lifting such as we see in a thunderstorm. (Cumulonimbus clouds)
• Stratiform which is the result of more gradual slanted lifting such as we see ahead of a warm front. (Nimbostratus)

• Liquid:
• Rain - Drops of water falling from a cloud, and having a diameter of greater than 0.5 mm
• Drizzle - Liquid water drops having a diameter of less than 0.5 mm. You can often tell the difference between rain and drizzle because drizzle usually doesn't cause ripples in standing water puddles. Drizzle reduces visibility more than rain.
• Frozen:
• Snow - Ice crystals or aggregates of ice crystals. The shape of snowflakes varies with the temperature at which they are formed.
• Sleet - Sleet or ice pellets form when raindrops fall through a colder layer of air and freeze into ice pellets.
• Snow Grains - are small, opaque grains of ice, the solid equivalent of drizzle. They are flat and elongated, with very small diameters.
• Graupel (Snow Pellets) - Graupel or snow pellets will start as a snowflake and pick up supercooled droplets through riming, becoming a larger, rounded pellet that are round, white, opaque grains of ice surrounding a center of snow. Snow pellets are brittle, crunchy, and bounce upon hitting a hard surface.
• Freezing:
• Freezing Rain (Glaze Ice) - Freezing rain goes by many names, glaze ice, black ice, clear ice, etc. It forms when supercooled raindrops strike an object and nearly instantly freezes on impact. Because of size it spreads out to a uniform-smooth clear glaze.
• Freezing Drizzle (Rime Ice)- Forms in a manner similar to glaze, only it is caused by the freezing of small supercooled small cloud droplets or drizzle size drops rather than supercooled raindrops. It will form more bumpy or feathery somewhat frost like ice crystals on trees and other surfaces.
• Hail - Hail is unique. It is NOT the same as sleet!!! It is a WARM season phenomena. It rarely if ever falls in cold season. Hail begins as a snowflake that partially or completely melts, and then refreezes. But, instead of immediately falling to the ground, it gets caught in an updraft and can make several trips up and down through the cloud, each time accumulating more ice. Hail is only formed in very strong thunderstorms (cumulonimbus clouds).

• Precip. can be measured directly through capturing the precipitation in a container then:
• Measuring with a ruler
• Or use an electronic device.
• Radar can also be used to estimate precipitation rates and amounts through strength of the returned power of the radar beam.

• Rainfall Measurements
• Rainfall is measured in inches (or millimeters) per hour. Any flat-bottomed, vertically sided container can be used as a rain gauge.
• Rainfall rate is classified as
• Trace - less than 0.01 inches per hour
• Light - between 0.01 and 0.1 inches per hour
• Moderate - between 0.1 and 0.3 inches per hour
• Heavy - greater than 0.3 inches per hour
• 19.Snowfall
• Snowfall is measured in one of two ways. (1)Either the depth of the snow, or (2) the depth of the liquid water content.
• Liquid water content is measured by melting the snow and then measuring the height of the resulting water.
• On average, 10 inches of snow is equivalent to 1 inch of rain. But, heavier snow has more water, and light, powdery snow has less water.
• The temperature at which the snowflakes formed determines how much water they contain.

• Cyclones (Low Pressure Systems) - large comma shaped cloud and precipitation patterns
• Fronts
• Warm - large shield ahead of warm front
• Cold - narrow band along cold front.
• We also must consider the role of High Pressure Systems

• Cloud seeding and cloud/fog dispersal
• Attempts have been made to "seed" cold clouds with dry ice or silver iodide crystals in order to enhance the Bergeron process.
• This is done to either enhance precipitation, or to disperse fog.
• It can be somewhat successful on a small scale (such as trying to clear fog at an airport).
• Warm clouds are seeded with salt particles in an effort to get a few large drops to form to initiate the collision-coalescence process.
• Inadvertent weather modification occurs when man's activities seed clouds. There is evidence that this is occurring near industrial areas.
• The jury is still out on how effective cloud seeding is. There are also legal and ethical issues involved.

• Fog dispersal can also be accomplished for some low-lying fogs through the use of large fans (aircraft propellers or exhaust) and/or heating the runway area.

• Hail suppression
• Hail suppression is also done through seeding processes.

• Frost prevention
• Turbines, heaters, and water spray are all used to prevent frost damage to crops

• Inadvertent - urban weather modification
• Pollution affects modify both temperature and precipitation patterns
• Urbanization (converting forests to parking lots and buildings) affect precipitation patterns, the water cycle and temperatures.