WEAX-201
Chapter 6--Condensation, Dew, Fog, and Clouds

Condensation……WITH STYLE!!

A little less eye-catching, but the same principle:
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A cold drink “sweats” because warm, moist air comes in contact with the cold surface
 The air cools to below it’s dew point temperature
 Condensation occurs

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Dew forms when the temperature cools to the dewpoint temperature
 If T = Td < 32 ° F, 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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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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Total mass of CCN put into atmosphere each year is about
2x10 12 kg
Two types of CCN:
 Hydroscopic (water seeking)
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Water vapor readily condenses on these
Ocean salt is a good example (sticky salt shaker when humid)
Hydrophobic ( water repelling)
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Water vapor does not readily condense on these (wax on car)
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Note: 1cm3 is about the size of your thumb
Type of Particle
Small CCN
Large CCN
Giant CCN
Fog and cloud droplets
Approximate Radius (
< 0.2
0.2 - 1.0
> 1.0
> 10.0
m m)
# of Particles per cm 3 range (typical)
1000
100
1
300

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Two types of haze:
 Dry haze - large/giant particles in the air
(smoke, smog, dust)
 Wet haze - H
2
O condenses onto hydroscopic CCN – can
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Can occur at RH as low as 75%
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Wet haze has a dull gray, white color

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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 < 5 ° F
Fog in heavily polluted areas can be a health problem since it becomes acidic

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Radiation Fog
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Advection Fog
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Upslope Fog
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Steam Fog

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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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Once the T reaches T d
, 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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Common off the west coast of the U.S.
 Cold current along coastline
 Warm water further to the west

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Westerly winds advect warm moist air over colder water
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Warm, moist air to the cold water via conduction
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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 can be an important source of moisture for plant life along the California-it rarely rains there during the summer months
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Why are advection fogs rare in the tropics?

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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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As the parcel ascends, it expands and cools to the dew point ( lapse rate =10 o C 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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Common here in late fall and winter
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Seen over lakes or heated pools in winter
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Need cold air over a warm body of water

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Heat and moisture are transferred from the warm water to the cooler, drier air
 This occurs in a shallow layer near the lake’s 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 air—steam fog is formed
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On a cold morning, you can see your breath. Why?

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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

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

High Clouds:
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Cirrus (Ci)
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Cirrostratus (Cs)
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Cirrocumulus
(Cc )
Middle Clouds
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Altostratus (As)
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Altocumulus (Ac)
Low Clouds:
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Stratus (St)
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Stratocumulus (Sc)
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Nimbostratus (Ns)
Clouds with vertical development:
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Cumulus (Cu)
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Cumulonimbus
(Cb)
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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:
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High clouds
Middle clouds
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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 are comprised largely of ice
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Cloud-base heights for high clouds:
 Tropical Region// Middle Latitudes //Polar Regions
 6-18 km 5-13 km 3-8 km

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

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

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

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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

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

Ac
Cc

Special type of Ac cloud that forms in high speed wind conditions
Usually downwind of mountain ranges

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

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

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

As
St

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

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

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Cumulus Clouds
 Look like cotton balls/cauliflower in the sky
 Whiter than Sc, and often more verticallydeveloped
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Sub-categories of cumulus:
 cumulus humilis - slightly developed Cu
 cumulus congestus (or “moderate Cu”) moderately developed

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Cumulus humilis
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Cumulus congestus

Developing Cumulus

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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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Scud are ragged low clouds drifting beneath the actual cloud base
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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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Form as air flows over mountains
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Look like pancakes, UFOs
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Appear to stay stationary

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

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

Unusual Clouds—”Hole Punch
Clouds”
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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 snow—this cleared a hole in the Ac deck
 From the WKRG-TV page
 Occurred in December in southern Alabama

Contrails
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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??

Description
Clear (CLR or
SKC)
Few
ASOS (NWS) Human
0 to 5%
Scattered (SCT)
Broken (BKN)
Overcast (OVC)
> 25 to
<50%
> 50 to
<87%
> 87 to
100%
0
Meaning
No Clouds
Sky obscured NA NA
Few clouds visible
3/8 to
4/8
Partly cloudy
5/8 to
7/8
Mostly cloudy
8/8 Sky is covered by clouds
Sky is hidden by surface-based phenomena, such as fog, blowing snow, smoke and so forth, rather than by cloud cover