Download AS 120 Principles of Aeronautical Science

WEAX-201
Chapter 6--Condensation, Dew, Fog, and Clouds
Condensation……WITH STYLE!!
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 it’s
dew point temperature
 Condensation occurs
Formation
of
Dew
and
Frost
► 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.
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
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
Note:
vapor does not readily condense on these (wax on car)
1cm3 is about the size of your thumb
Type of Particle
# of Particles per cm3
Approximate Radius (mm)
range (typical)
Small CCN
< 0.2
1000
Large CCN
0.2 - 1.0
100
Giant CCN
> 1.0
1
Fog and cloud droplets
> 10.0
300
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
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 < 5°F
► Fog in heavily polluted areas can be a health
problem since it becomes acidic
Types of Fog:
► Radiation
Fog
► Advection Fog
► Upslope Fog
► Steam Fog
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.
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
Valley Fog
Advection Fog
► Common
off the west
coast of the U.S.
 Cold current along
coastline
 Warm water further
to the west
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
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?
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
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
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
Steam Fog
►
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
►
On a cold morning, you can see
your breath. Why?
Foggy Weather
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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
Introduction to Cloud Types
►
High Clouds:
►Cirrus (Ci)
►Cirrostratus (Cs)
►Cirrocumulus
(Cc)
Low Clouds:
►Stratus (St)
►Stratocumulus (Sc)
►Nimbostratus (Ns)
Middle Clouds
►Altostratus (As)
►Altocumulus (Ac)
Clouds with vertical
development:
►Cumulus (Cu)
►Cumulonimbus
(Cb)
NOTE: cloud names come from Latin words:
• cirrus - curl
• stratus - layer
• cumulus - heap
• nimbus - violent rain
►
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
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
High Clouds - Cirrus (Ci)
Cirrus Clouds (Ci)- high, thin wispy
clouds at jet stream level in the
upper troposphere
Associated with fair weather
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)
Cirrocumulus Clouds (Cc)
► High
clouds
► Resemble
fish scales or
small
rounded
white puffs
► About the
size of your
thumbnail
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
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
Cc
Ac
Special type of Ac cloud that forms in high speed wind conditions
Usually downwind of mountain ranges
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
Low Clouds
► 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
St
As
Nimbostratus Clouds (Ns)
► Darker
gray, "wet" looking low clouds
► Produce light/moderate precipitation over
a large region
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
Vertically Developed Clouds Cumulus (Cu)
► Cumulus
Clouds
 Look like cotton balls/cauliflower in the sky
 Whiter than Sc, and often more verticallydeveloped
► Sub-categories
of cumulus:
 cumulus humilis - slightly developed Cu
 cumulus congestus (or “moderate Cu”)
moderately developed
Cumulus
► Cumulus
humilis
congestus
Developing Cumulus
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
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
Other Unusual clouds - Lenticular
Clouds
► Form
as air flows over mountains
► Look like pancakes, UFOs
► Appear to stay stationary
Lenticular Cloud Time Lapse
Other Unusual Clouds - Pileus
► 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
Other Unusual Clouds - Pollution
Induced
► Hot
air from a smoke
stack can rise high
enough to produce a
cloud
Unusual Clouds—”Hole 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 snow—this cleared a
hole in the Ac deck
 From the WKRG-TV page
 Occurred in December in
southern Alabama
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
►
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.
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??
-
Sky Conditions
Description
ASOS (NWS) Human
Meaning
Clear (CLR or
SKC)
0 to 5%
0
No Clouds
Few
>5 to <25%
>0 to
2/8
Few clouds visible
Scattered (SCT)
> 25 to
<50%
3/8 to
4/8
Partly cloudy
Broken (BKN)
> 50 to
<87%
5/8 to
7/8
Mostly cloudy
Overcast (OVC)
> 87 to
100%
8/8
Sky is covered by clouds
NA
Sky is hidden by surface-based phenomena,
such as fog, blowing snow, smoke and
so forth, rather than by cloud cover
Sky obscured
NA