Transcript meteo_1_lecture_3

Chapter 4
Atmospheric Moisture,
Condensation, and Clouds.
Chapter 2-3 review
The sun’s electromagnetic spectrum and some of the
descriptive names of each region. The numbers
underneath the curve approximate the percent of energy
the sun radiates in various regions.
0.4 μm = 400 nm
0.7 μm = 700 nm
Chapter 2-3 review
The hotter sun not only radiates more energy than that of the cooler earth (the area
under the curve), but it also radiates the majority of its energy at much shorter
wavelengths. (The area under the curves is equal to the total energy emitted, and the
scales for the two curves differ by a factor of 100,000.)
Chapter 2-3 review
The daily variation
in air temperature
is controlled by
incoming energy
(primarily from the
sun) and outgoing
energy from the
earth’s surface.
Where incoming
energy exceeds
outgoing energy
(orange shade), the
air temperature
rises. Where
outgoing energy
exceeds incoming
energy (gray
shade), the air
temperature falls.
Chapter 2-3 review
Chapter
2-3 review
The average annual incoming solar radiation (yellow line) absorbed by
the earth and the atmosphere along with the average annual infrared
radiation (red line) emitted by the earth and the atmosphere.
Water can exist in 3 phases, depending
upon pressure and temperature.
http://www.sci.uidaho.edu/scripter/geog100/l
ect/05-atmos-water-wx/ch5-part-2-waterphases.htm
http://chemwiki.ucdavis.edu/Physical_Che
mistry/Physical_Properties_of_Matter/Phas
e_Transitions/Phase_Diagrams_1
Evaporation, Condensation,
& Saturation
• Evaporation is the change of liquid into
a gas and requires heat.
• Condensation is the change of a gas
into a liquid and releases heat.
• Condensation nuclei
• Sublimation: solid to gaseous state
without becoming a liquid.
• Saturation is an equilibrium condition
in which for each molecule that
evaporates, one condenses.
http://clasfaculty.ucdenver.edu/callen/1202/Intro/EarthPlaceSpace/UniqueEarth.html
Latent Heat of Vaporization = 600 calories / 1g
Latent Heat of Condensation = 600 calories / 1g
Latent Heat of Fusion= 80 calories / 1g
Latent Heat of Vaporization = 600 calories / 1g
Latent Heat of Condensation = 600 calories / 1g
Latent Heat of Fusion= 80 calories / 1g
How much energy to sublimate?
http://www2.chemistry.msu.edu/courses/cem152/snl_cem152_SS12/pracprob/practiceexam1.html
(a) Water molecules at the
surface of the water are
evaporating (changing
from liquid into vapor)
and condensing (changing
from vapor into liquid).
Since more molecules are
evaporating than
condensing, net
evaporation is occurring.
(b) When the number
of water molecules
escaping from the liquid
(evaporating) balances
those returning
(condensing), the air
above the liquid is
saturated with water
vapor. (For clarity, only
water molecules are
illustrated.)
Condensation is
more likely to
occur as the air
cools. (a) In the
warm air, fastmoving H2O
vapor molecules
tend to bounce
away after
colliding with
nuclei.
(b) In the cool air,
slow-moving vapor
molecules are more
likely to join
together on nuclei.
The condensing of
many billions of
water molecules
produces tiny liquid
water droplets.
http://www.srh.noaa.gov/jetstream/atmos/hydrocycle_max.htm
Humidity
• Any of a number of ways of specifying the
amount of water vapor in the air
• Absolute humidity: mass of water
vapor/volume of air
– Water vapor density
– Not commonly used due to frequent change of
volume
AH = mass of water vapor (g) / Volume of Air (m^3)
Humidity
• Vapor pressure: the pressure exerted by water
vapor molecules in an air parcel
– Fraction of total vapor pressure (1% or so)
– More water molecules = high vapor pressure
• Saturation vapor pressure: the vapor pressure
at which an air parcel will be saturated,
changes with temperature
Fig. 4-4, p. 87
Saturation vapor
pressure increases with
increasing
temperature. At a
temperature of 10°C,
the saturation vapor
pressure is about 12
mb, whereas at 30°C it
is about 42 mb. The
insert illustrates that
the saturation vapor
pressure over water is
greater than the
saturation vapor
pressure over ice.
Humidity
• Specific Humidity: mass of water vapor/mass
of air
• Mixing ratio: mass of water vapor/mass of dry
air
• Neither measurement changes with volume,
must add or subtract water vapor.
Mixing Ratio = mass of water vapor (g) / Mass of
dry Air (kg)
Humidity
• Relative Humidity: (actual water
vapor/saturation water vapor)*100
– RH can be changed two ways:
• Change vapor content
• Change saturation
– Decrease temperature causes an increase in
relative humidity (inverse relation).
(a) At the same air temperature, an increase in the water
vapor content of the air increases the relative humidity as
the air approaches saturation.
(b) With the same water vapor content, an increase in air
temperature causes a decrease in relative humidity as the
air moves farther away from being saturated.
When the air is cool (morning), the relative humidity is high.
When the air is warm (afternoon), the relative humidity is
low. These conditions exist in clear weather when the air is
calm or of constant wind speed.
Humidity
• Relative Humidity and Dew Point
– Dew point is the temperature at which saturation
occurs
– Cool air parcel to dew point and liquid water
condenses
– A good measure of actual water vapor content
– Relative humidity indicates how close to
saturation, dew point indicates the temperature
to which air must be cooled for saturation to
occur.
Average surface dew-point temperatures (°F) across the
United States and Canada for January.
Average surface dew-point temperature across the
United States and Canada (°F) for July.
Inside the cloud the air temperature (T) and dew point (Td) are the
same, the air is saturated, and the relative humidity (RH) is 100 percent.
However, at the surface where the air temperature and dew point are
not the same, the air is not saturated (even though it is raining), and the
relative humidity is considerably less than 100 percent.
The polar air has the higher relative humidity, whereas the
desert air, with the higher dew point, contains more water
vapor.
Humidity
• Relative humidity & human comfort
– “It’s not the heat, it’s the humidity.”
– High relative humidity equates to less evaporative
cooling.
– Sweat cannot evaporate and cool the body.
– Wet bulb temperature
– Heat Index
Air temperature (°F) and relative humidity are combined to determine
an apparent temperature or heat index (HI). An air temperature of 95°F
with a relative humidity of 55 percent produces an apparent
temperature (HI) of 110°F.
Humidity
• Measuring humidity
– Sling psychrometer
– Hygrometer
The hair hygrometer measures relative humidity by
amplifying and measuring changes in the length of human
(or horse) hair.
Dew and Frost
• Dew forms on objects near the ground surface
when they cool below the dew point
temperature.
– More likely on clear nights due to increased
radiative cooling
• White frost forms when temperature cools
below the dew point and the dew point is
below 0°C.
Dew and Frost
• Particles suspended in the air around which
water condenses or freezes
– Hydrophobic/hygroscopic
• Dry condensation nuclei (above dew point)
reflect and scatter sunlight creating blueish
haze.
• Wet condensation nuclei (75% relative
humidity) reflect and scatter sunlight creating
greyish or white haze.
The high relative humidity of the cold air above the lake is
causing a layer of haze to form on a still winter morning.
Fog
• Saturation reached condensation forms a
cloud near the ground
• Radiation fog: forms when the ground cools
through conduction and radiation; ground fog
• Advection fog: forms when the wind moves
moist air over a cold surface and the moist air
cools to its dew point.
• Upslope fog: forms as moist air slowly rises,
cools, and condenses over elevated terrain.
Radiation fog tends to form on clear, relatively calm nights
when cool, moist surface air is overlain by drier air and rapid
radiational cooling occurs.
Radiation fog nestled in a valley in central Oregon.
• Advection Fog: warm, moist fog moves
horizontally (advects) over a cool surface.
– Summer fog on the Pacific coast
Advection fog rolling in past the Golden Gate Bridge in San Francisco.
As fog moves inland, the air warms and the fog lifts above the surface.
Eventually, the air becomes warm enough to totally evaporate the fog.
Upslope fog forms as moist air slowly rises, cools, and
condenses over elevated terrain.
Fig. 4-20, p. 100
Even in summer, warm air rising above thermal pools in
Yellowstone National Park condenses into a type of steam fog.
Average
annual
number of
days with
dense fog
(visibility less
than 0.25
miles) across
North
America.
(Dense fog
observed in
small
mountain
valleys and
on mountain
tops is not
shown.)
Clouds
• Classification of clouds: use Latin words to
describe height and appearance
• Factors described
– Height: low, mid, high, vertical
– Appearance: shape, density, color
Cirrus clouds.
Cirrocumulus clouds.
Cirrostratus
clouds with a
faint halo
encircling the
sun. The sun
is the bright
white area in
the center of
the circle.
Altocumulus clouds.
Altostratus
clouds. The
appearance of
a dimly visible
“watery sun”
through a deck
of gray clouds
is usually a
good indication
that the clouds
are altostratus.
The nimbostratus is the sheetlike cloud from which light rain
is falling. The ragged-appearing cloud beneath the
nimbostratus is stratus fractus, or scud.
Stratocumulus clouds forming along the south coast of
Florida. Notice that the rounded masses are larger than
those of the altocumulus.
A layer of low-lying stratus clouds hides the mountains in
Iceland.
Cumulus clouds. Small cumulus clouds such as these are
sometimes called fair weather cumulus, or cumulus humilis.
Cumulus congestus. This line of cumulus congestus clouds is
building along Maryland’s eastern shore.
A cumulonimbus cloud. Strong upper-level winds blowing
from right to left produce a well-defined anvil. Sunlight
scattered by falling ice crystals produces the white (bright)
area beneath the anvil. Notice the heavy rain shower falling
from the base of the cloud.
A generalized illustration of basic cloud types based on
height above the surface and vertical development.
Some Unusual Clouds
• Not all clouds can be placed into the ten basic
cloud forms.
• Unique atmospheric processes and
environmental conditions create dramatic and
exotic clouds.
• Unusual clouds and weather balloons often
cause of UFO reports.
A lenticular
cloud
forming
over Mt.
Rainier in
Washington
State.
A pileus cloud forming above a developing cumulus cloud.
Mammatus clouds forming beneath a thunderstorm.
A contrail forming behind a jet aircraft.
The clouds in this photograph are nacreous clouds. They
form in the stratosphere and are most easily seen at high
latitudes.
The wavy clouds in this photograph are noctilucent clouds.
They are usually observed at high latitudes, at altitudes
between 75 and 90 km above the earth’s surface.