AS 120 Principles of Aeronautical Science

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WEAX-201Chapter 6--Condensation, Dew, Fog, and
Clouds
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CondensationWITH STYLE!!
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A little less eye-catching, but the same
principle

• A cold drink sweats because warm, moist air
  comes in contact with the cold surface
• The air cools to below its dew point temperature
• Condensation occurs

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Formation of Dew and Frost

• Dew forms when the temperature cools to the
  dew-point temperature
• If T Td lt 32F, frost forms instead of dew
• Dew/frost often forms close to the ground, and
  not on objects just above the ground
• Why?   
• Dew and frost most often form on clear, calm
  nights
• Why? 
• Dew can be an important source of moisture during
  periods of low rain fall.

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Formation of Haze, Fog, and Clouds Condensation
Nuclei

• The process of condensation of vapor to form a
  cloud drop is not as simple as dew or frost
  formation
• Must have Cloud Condensation Nuclei (CCN) to form
  cloud drops
• CCN are small particles in the atmosphere
• Dust, volcanoes, factory smoke, forest fires
• Ocean salt, sulfate particles from phytoplankton
  in ocean
• They are most abundant in lower troposphere over
  urban areas
• They are quite small compared to a rain drop or
  cloud droplet

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Sizes and Amounts of CCN

• Total mass of CCN put into atmosphere each year
  is about 2x1012 kg
• Two types of CCN
• Hydroscopic (water seeking)
• Water vapor readily condenses on these
• Ocean salt is a good example (sticky salt shaker
  when humid)
• Hydrophobic (water repelling)
• Water vapor does not readily condense on these
  (wax on car)

• Note 1cm3 is about the size of your thumb

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Formation of Haze

• Two types of haze
• Dry haze - large/giant particles in the air
  (smoke, smog, dust)
• Wet haze - H2O condenses onto hydroscopic CCN
  can
• Can occur at RH as low as 75
• Wet haze has a dull gray, white color

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Formation of Fog

• Fog forms as the RH increases to 100
• Haze particles grow into fog (cloud) droplets
  near the ground
• Fog is really a cloud near the ground
• International definition Visibility less than 1
  km
• National Weather Service definition Visibility
  is less than or equal to 6 miles and T-Td lt 5F
• Fog in heavily polluted areas can be a health
  problem since it becomes acidic

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Types of Fog

• Radiation Fog
• Advection Fog
• Upslope Fog
• Steam Fog

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Formation of Radiation Fog

• Conditions needed
• Moist air near surface of the ground
• Clear and calm nights
• Light winds to bring a larger volume of air in
  contact with the cooler ground
• Radiational cooling allows the air temperature to
  drop to the dew point temperature.

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Formation of Radiation Fog

• Once the T reaches Td, radiation fog begins to
  develop
• Common in the fall - especially when weather is
  dominated by high pressure
• Often forms in valleys first since this is where
  the coldest air is. This is called valley fog

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Valley Fog
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Advection Fog

• Common off the west coast of the U.S.
• Cold current along coastline
• Warm water further to the west

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Advection Fog

• Westerly winds advect warm moist air over colder
  water
• Warm, moist air to the cold water via conduction
• The parcel reaches saturation. Fog forms, and is
  advected onshore
• Need a light breeze for this process to occur

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Advection Fog

• Advection fog can be an important source of
  moisture for plant life along the California-it
  rarely rains there during the summer months
• Why are advection fogs rare in the tropics?

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Upslope Fog

• A parcel of warm, moist air climbs from the Gulf
  of Mexico as it is advected toward Denver
• As the parcel ascends up the slope, it expands,
  and the temperature cools to the dew point

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Upslope Fog

• As the parcel ascends, it expands and cools to
  the dew point (lapse rate 10oC per 1000 meters)
• Upslope fog/clouds then form
• Neccessary ingredients
• Moist air
• Winds that move the air up the slope
• A slope

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Steam Fog

• Common here in late fall and winter
• Seen over lakes or heated pools in winter
• Need cold air over a warm body of water

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Steam Fog

• Heat and moisture are transferred from the warm
  water to the cooler, drier air
• This occurs in a shallow layer near the lakes
  surface
• This is an unstable situation with warm,
  saturated air at the surface below cooler air
• Whisps of warmer, moister air rise into the
  cooler airsteam fog is formed
• On a cold morning, you can see your breath. Why?

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Foggy Weather

• Where is it foggy????
• Pacific Coast
• Appalachian highland region
• New England
• Foggiest spot in the U.S. Cape Disappointment,
  WA
• it's foggy for 2556 hours per year, or about 107
  days.
• Fog is a significant weather problem for aviation
  ops

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Introduction to Cloud Types- Know the cloud
types- Be able to identify clouds
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Introduction to Cloud Types

• Clouds are comprised of liquid droplets of
  various sizes and/or ice crystals
• They are characterized according to their height
  location in the atmosphere and their vertical
  development
• High clouds
• Middle clouds
• Low clouds
• Vertically developed clouds

• NOTE cloud names come from Latin words
• cirrus - curl
• stratus - layer
• cumulus - heap
• nimbus - violent rain

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High Clouds - Cirriform

• High clouds are comprised largely of ice
• Cloud-base heights for high clouds
• Tropical Region//Middle Latitudes//Polar Regions
• 6-18 km 5-13 km 3-8 km

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High Clouds - Cirrus (Ci)
Cirrus Clouds (Ci)- high, thin wispy clouds at
jet stream level in the upper troposphere
Associated with fair weather
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High Clouds - Cirrostratus (Cs)

• High, thin, sheet-like clouds
• Produce halos around the sun/moon
• Many of the optical phenomenon we learned a
  couple of weeks ago are caused by Cs
• A sign that poor weather is often approaching
  (12-36 hours away)

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Cirrocumulus Clouds (Cc)

• High clouds
• Resemble fish scales or small rounded white puffs
• About the size of your thumbnail

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Middle Clouds Alto__

• Middle clouds are composed of water and/or ice
• Cloud-base heights for middle clouds
• Tropical Region//Middle Latitudes//Polar Regions
• 2-8 km 2-7 km 2-4 km

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Altocumulus Clouds (Ac)- Shallow, puffy or
wave-like in appearance Appear to be larger than
your thumb, but smaller than your fist when
holding your arm up to the sky
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Cc
Ac
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Special type of Ac cloud that forms in high speed
wind conditions Usually downwind of mountain
ranges
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Altostratus Clouds (As)

• Grayish/blue-gray appearance
• Thin layer covering entire sky uniformly
• Found ahead of approaching storms
• Can see the sun through altostratus, but NO halo
  will be observed

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Low Clouds

• Cloud-base heights for low clouds
• Tropical Region//Middle Latitudes//Polar
• 0-2 km 0-2 km 0-2
  km

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Stratus Clouds (St) -Uniform grayish cloud
covering the entire sky -Fairly common here in
the winter -Light, continuous drizzle
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St
As
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Nimbostratus Clouds (Ns)

• Darker gray, "wet" looking low clouds
• Produce light/moderate precipitation over a large
  region

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Stratocumulus Clouds (Sc)

• Low, lumpy, puffy clouds in patches or rounded
  masses
• Fair weather clouds (usually)
• Appear the size of your fist when holding your
  arm up to the sky

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Vertically Developed Clouds - Cumulus (Cu)

• Cumulus Clouds
• Look like cotton balls/cauliflower in the sky
• Whiter than Sc, and often more vertically-develope
  d
• Sub-categories of cumulus
• cumulus humilis - slightly developed Cu
• cumulus congestus (or moderate Cu) moderately
  developed

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• Cumulus humilis

• Cumulus congestus

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Developing Cumulus
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Cumulonimbus Clouds (Cb)

• Thunderstorms
• Develop from growing Cu
• Can extend up to the troposphere
• Can contain both water and ice
• Produce precipitation (rain, snow, hail, etc)
• Produce lightning and severe weather
• Form a distinctive "anvil" cloud at the top of
  the storm

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Other unusual clouds - Scud

• Scud are ragged low clouds drifting beneath the
  actual cloud base
• Often form due to turbulent mixing of air
• Warm air from the updraft
• Cool air from the downdraft
• Cause huge problems for general aviation

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Other Unusual clouds - Lenticular Clouds

• Form as air flows over mountains
• Look like pancakes, UFOs
• Appear to stay stationary

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Lenticular Cloud Time Lapse
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Other Unusual Clouds - Pileus

• Forms as a growing thunderstorm deflects moist
  air up and over the top of the building cumulus
  congestus or cumulonimbus

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• Mammatus clouds
• Found in the anvil portion of intense
  thunderstorms
• Indicate strong downdrafts in upper portions of
  a thunderstorm
• Indicate hail or tornadoes

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Other Unusual Clouds - Pollution Induced

• Hot air from a smoke stack can rise high enough
  to produce a cloud

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Unusual CloudsHole Punch Clouds

• Occurs in Ac clouds
• Ci above dropped light snow that fell into the
  lower Ac deck
• Acted much like cloud seeding (chap 8)
• Cloud droplets in the Ac cloud coalesced on the
  falling snowthis cleared a hole in the Ac deck
• From the WKRG-TV page
• Occurred in December in southern Alabama

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Effects of Aircraft on Clouds
Contrails

• Jets passing through thick Ci clouds can raise RH
  to the point where precipitation begins, and
  clouds begin to clear

Courtesy WKRG-TV
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• If a jet is flying through air with low humidity,
  the moist air might produce a short-lived
  contrail. Thus, the forecast would be for the
  weather will remain fair. However, if the
  contrail is a thick, long lasting trail it
  indicates that high humidity is in the atmosphere
  and it could be a sign of a storm approaching.

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• - Cloudiness has increased by about 20 over some
  portions of the US
• These areas are along the major air traffic
  routes
• The cloud increases are due to contrails
  produced by aircraft
• Implication for climate??

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Sky Conditions