Transcript Atmospheric Moisture PPT File

Atmospheric
Moisture Notes
page 575-580
Water exists in the
atmosphere in three phases
Solid--ice
 Liquid--water
 Gas—water vapor


When water
changes from one
phase to another
energy is absorbed
or released

Latent heat is the heat energy that is
absorbed or released during a phase change
 When liquid water evaporates the water
absorbs energy from the environment
 When the water vapor changes back into a
liquid (condenses) the energy is released
to the atmosphere
Moisture in the air
Humidity-the amount of moisture in the air
 Relative humidity- the amount of

moisture in the air compared to the amount it
could hold at a given temperature

Dew point-the temperature at which
moisture in the air condenses

Saturated-when air is holding all the
moisture it can
Relative humidity

Measured in percents
Rising Air

As air rises, it expands
because it moves in to a
region of lower air
pressure.
When air expands it
cools.
 When water
molecules cool they
slow down and tend
to stick together
more easily

Hot air holds
more moisture than cold air

Fast moving
molecules rebound
on contact

Slow moving
molecules
condense upon
collision
Condensation
Occurs on a
surface
 If the temperature is
above freezing we
call it dew
 If the temperature is
below freezing we
see frost

Condensation Nuclei

If there are particles like dust
in the air the water condenses
on these and falls to the
ground as precipitation. The
particles are called
condensation nuclei.
small particles in air created
from/by:
 dust
 volcanoes
 factory smoke
 forest fires
 ocean salt
Dew Point

When air cools below a
certain temperature called
the dew point the air’s
ability to hold water vapor
decreases and water
condenses on any
available surface.
 This can happen on
grass or a windshield and
we have early morning
dew
Dew Point Lab






Fill a beaker half full of
water
Allow to reach room
temperature
Record temp
Add a few ice cubes
and slowly stir
Record the temperature
at which moisture starts
condensing on the
outside of the beaker
This is the dew point
Frost
 If
the dew
point is at or
below freezing
we have frost
Hoar Frost

Very soft ice crystals
form on objects
whose temps are
well below freezing
Whose woods these are I think I know. His house is in the
village though;
He will not see me stopping here To watch his woods fill up
with snow. (Robert Frost)
Both frost and condensation
occur on surfaces
Relative Humidity-the amount of water in the
air compared to the amount
it can hold at a given
temperature
Fog

When a large
mass of air cools
and reaches its
saturation point
(dew point) the
relative humidity
approaches 100%
producing a
cloud near the
ground called
fog.
Fog
FOG
The fog comes
on little cat feet.
It sits looking
over harbor and city
on silent haunches
and then moves on.
Carl Sandburg
Measuring
Relative Humidity
A sling psychrometer is used to
measure relative humidity
Other instruments

A thin polymer film

measures the polymer’s ability to store
electricity
Dew cell
 Hair hygrometer


Hair becomes longer as relative humidity
increases
Sling Psychrometer lab
We will use the SP
an instrument with
two thermometers
 A dry bulb
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

that measures the
temperature
And a wet bulb

the bulb with water
soaked gauze
So how does it work?
What happens
to your skin
when you
sweat and
then stand in a
breeze?
As the wet-bulb thermometer
goes through the air
1. Water
evaporates
2.Energy
is used,
water
changes
from
liquid to
vapor
3. Wet-bulb temperature goes down
What influences how
much the wet-bulb
temperature goes
down?
Temperature
Humidity
Relative Humidity Chart
Difference between wet and dry bulb
D
r
y
b
u
l
b
t
e
m
p
0
1 2
81 64
3 4 5 6
46 29 13
2
4
6
8
10
12
84
85
86
87
88
89
52
57
60
63
66
68
68
71
73
75
77
78
37
43
48
51
55
58
22
29
35
40
44
48
7
16
24
29
34
39
7
8
9
11
19 8
24 15 6
29 21 12
10
After readings are taken, a chart is used
to find the relative humidity
1.
2.
3.
Find the dry-bulb
temperature reading
on the chart.
Find the temperature
difference between
the dry bulb and wet
bulb thermometers on
the chart ( 1-5)
Follow both numbers
into the chart. Where
the two numbers meet
is the percent relative
humidity.
Some programs
calculate this info for you
Relative Humidity-
the amount of water in the air compared
to the amount it can hold at a given
temperature
Relative Humidity (RH) Lab
Wet bulb temp__________
 Dry bulb temp_______________

Dry ____________
 Minus Wet ___________

equals difference ______________
 Relative humidity _____

Clouds
Cloud development

As air rises it cools when it reaches the
dew point it condenses
Adiabatic cooling

It needs something to condense on. This is
called a condensation nuclei. This can be a
speck of dust etc. Sometimes the relative
humidity can be above 100%. If the air is too
clean there is nothing for the moisture to
condense on. This is when cloud seeding may
be effective. Silver iodide is shot into the air
to give the moisture the needed nucleus.

Warm air can hold
more moisture than
cold air
Adiabatic COOLING

a change in
temperature due
solely to a change in
altitude


dry adiabatic lapse
rate= 10c/km
Moist adiabatic lapse
rate= 6C/km
The reason for the
difference is that latent
heat is released when
water condenses
 water's high heat of
vaporization creates a
significant release of
the energy when it
condenses (and is an
important source of
energy in the
development of
thunderstorms)

Atmospheric Lifting
For clouds to form air must lift
 3 lifting mechanisms
Convectional
 Orographic
 Frontal

Convectional Lifting

Some areas of
Earth's surface pick
up heat better than
others.
 As air warms it
becomes less dense
and rises. As it rises
it cools and sinks
Circulatory motion is called
convectional lifting

If cooling occurs
close to the air's
saturation
temperature,
condensing
moisture forms a
cumulus cloud
 Form and dissipate
over the same area
Orographic Lifting

An air mass lifts
when it is pushed
upward over an
obstacle such as a
mountain range
As rising air cools, if it is
humid, it forms a cloud.

If stable air stratus
 If unstable air
cumulus
As air moves down the other slope it warms
The descending air is dry because the
moisture was removed on the other side
This is called the
Rain Shadow Effect.

Example: Eastern
Colorado and
western Kansas
 See figure 27.10
Frontal Lifting
 When
fronts collide warm less
dense air is forced up over cold
dense air
Fronts—dryline

The source region for cT air is the desert Southwest, the high
plains and Mexico with relation to the United States. The air has
low dewpoints and warm to hot afternoon temperatures but with
mild nighttime temperature. Skies are generally clear in cT air.
This allows daytime heating during the day and radiational cooling
at night. The cT air mass is most prevalent in summer; in the cool
season it is not as discernible. Due to the buoyancy and elevation
of cT air across North America, this air will advect into the midlevels of the atmosphere once it moves out of its source region.
This creates a cap of mild dry air. If this air advects over PBL mT
air, the severe thunderstorm threat increases significantly. The
boundary of cT is most noticeable with the creation of a dryline. A
dryline separates mT air from cT air. Depending on the strength of
the dryline, convergence along the dryline and the dynamics
above the dryline, severe thunderstorms can form near a dryline
boundary.
Clouds
FOUR MAJOR CLOUD
TYPES
High
 Middle
 Low
 Clouds with Vertical Development
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Cloud terms
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Stratus
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Cumulus
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
layered
slow change in temperature
Light precip
Piles up-look like cotton balls
Form where air temp changes quickly with
height
Nimbus—means rain
HIGH--above 6000 m
(cirro-high)
Cirrus
 thin and wispy
 Ice crystals

Cirrus
The most common form of high-level clouds
are thin and often wispy cirrus clouds.
 heights greater than 20,000 feet (6,000
meters)
 composed of ice crystals that originate from
the freezing of super cooled water droplets.
 Cirrus generally occur in fair weather and
point in the direction of air movement at their
elevation.

Cirrostratus

sheet-like and nearly transparent
Cirrostratus are sheet-like, high-level
clouds composed of ice crystals
cirrostratus can cover the entire sky and be up
to several thousand feet thick
they are relatively transparent, as the sun or
the moon can easily be seen through them
These high-level clouds typically form when a
broad layer of air is lifted by large-scale
convergence
Halo
Sometimes the only indication of their presence
is given by an observed halo around the sun
or moon.
Halos result from the refraction of light by the
cloud's ice crystals.
Cirrostratus clouds, however, tend to thicken as
a warm front approaches, signifying an
increased production of ice crystals.
As a result, the halo gradually disappears and
the sun (or moon) becomes less visible.
Cirrocumulus
Cirrocumulus
A patch or layer of cloud consisting of tiny
individual cloudlets at high-level is called
cirrocumulus
 The cloudlets may make a regular dappled
or rippled pattern
 Sometimes they look like the scales on a
fish - a "mackerel" sky that may mean that
unsettled weather is on its way
 Like all high-level clouds, cirrocumulus is
made of ice crystals

Formation of Cirrocumulus
It forms when cirrus or cirrostratus is
warmed gently from below. This causes air
to rise and sink inside the cloud. Some of
the ice crystals change into water vapor,
and gaps appear. It can be difficult to tell
cirrocumulus from altocumulus
 Cirrocumulus has no shading (which
altocumulus usually has), and because it is
so much higher, the cloudlets of
cirrocumulus are much smaller than those
of altocumulus

Mackerel Sky
MIDDLE-- (2000-6000m)
Altostratus
 Altocumulus
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Altostratus
Altocumulus

parallel bands or rounded masses
Altocumulus
may appear as parallel bands (top
photograph) or rounded masses (bottom
photograph).
 Typically a portion of an altocumulus cloud is
shaded, a characteristic which makes them
distinguishable from the high-level
cirrocumulus.
 Altocumulus clouds usually form by
convection in an unstable layer aloft, which
may result from the gradual lifting of air in
advance of a cold front.

The presence of altocumulus clouds on a
warm and humid summer morning is
commonly followed by thunderstorms later in
the day
LOW

Stratus—
layered uniformly gray,
often cover the entire sky
Stratocumulus

low, lumpy layer of clouds
Stratocumulus clouds
low, lumpy layer of clouds that is
sometimes accompanied by
 weak intensity precipitation
 Stratocumulus vary in color from dark
gray to light gray
 may appear as rounded masses, rolls,
etc., with breaks of clear sky in
between.

Nimbostratus

dark, low-level clouds with precipitation
Nimbus means rain
 Stratus means sheet

Nimbostratus
dark, low-level clouds
 light to moderately falling precipitation
 Low clouds are primarily composed of water
droplets since their bases generally lie below
6,500 feet (2,000 meters)
 However, when temperatures are cold
enough, these clouds may also contain ice
particles and snow

CLOUDS HAVING
VERTICAL DEVELOPMENT

Cumulus
Form where temperatures change quickly
with height
 Cumulus means heap
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Cumulonimbus
Fair Weather Cumulus

puffy cotton balls
floating in the sky
Fair weather cumulus
appearance of floating cotton
 Lifetime of 5-40 minutes
 Known for their flat bases and distinct
outlines, fair weather cumulus exhibit only
slight vertical growth, with the cloud tops
designating the limit of the rising air
 Given suitable conditions, however, harmless
fair weather cumulus can later develop
into towering cumulonimbus clouds
associated with powerful thunderstorms.

Cumulonimbus Clouds

reaching high into the atmosphere
Cumulonimbus clouds (Cb)
much larger and more vertically
developed than fair weather cumulus
 They can exist as individual towers or form a
line of towers called a squall line
 Fueled by vigorous convective updrafts
(sometimes in excess 50 knots)
 the tops of cumulonimbus clouds can easily
reach 39,000 feet (12,000 meters) or higher.

Lower levels of cumulonimbus clouds
consist mostly of water droplets while
at higher elevations, where
temperatures are well below 0 degrees
Celsius, ice crystals dominate
 Under favorable atmospheric conditions,
harmless fair weather cumulus clouds
can quickly develop into large
cumulonimbus clouds associated with
powerful thunderstorms known as
supercells.

Cloud Types at Fronts
Mammatus Clouds
sagging pouch-like structures
 Mammatus are pouch-like cloud structures
and a rare example of clouds in sinking air
 Sometimes very ominous in appearance,
mammatus clouds are harmless and do not
mean that a tornado is about to form; a
commonly held misconception. In fact,
mammatus are usually seen after the worst
of a thunderstorm has passed

More
Mammatus
Mammatus
CONTRAIL

A contrail, also known as a
condensation trail, is a cirrus-like trail of
condensed water vapor often
resembling the tail of a kite. Contrails
are produced at high altitudes where
extremely cold temperatures freeze
water droplets in a matter of seconds
before they can evaporate.