Geography 103
Weather
Lecture 4: Clouds and Precipitation (Quiz 1)
1. Moisture Indices
(1). Saturation
Vapor pressure (SVP)
A. The
pressure produced by the maximum amount of water vapor an air parcel ( 1 kg)
can hold at a given air temperature.
B. SVP is determined by air temperature
A warm air parcel can hold more water vapor (moisture)
than a cold air parcel of the same mass.
C. At 0 oC, a 1-kg air parcel can hold at
most 3.5 g water vapor which exerts about 6.11 mb vapor pressure.
D. SVP increases almost twice as air temperature increases by 10 oC within a normal atmosphere surface
air temperature range (0 oC to 30
oC).
(2). Existing vapor pressure
(EVP)
A. The pressure produced by the existing or actual amount of water vapor in a
given air parcel of 1 kg.
B. EVP is determined by dew point temperature (dew point).
EVP is SVP at dew point temperature.
(3). Dew point temperature
(A). Air temperature at which saturation or condensation
occurs.
(B). Dew point temperature is < or = air temperature.
(4). Saturation
(A). The water vapor in an air parcel already reaches the maximum capacity that
air parcel can hold at a given
air temperature.
a. SVP =EVP
b. Air temperature = dew point temperature.
(B). Condensation follows if air temperature cools or excess water vapor is
added into the air parcel.
(5). Condensation
Water vapor becomes liquid water.
(6). Relative humidity
A. The ratio of the amount of water vapor held by an air parcel (EVP) to the
maximum amount of water vapor
the air parcel can hold at a given air temperature.
B. RH = EVP/SVP =SVP at dew point/SVP at air temperature.
(7). Psychrometer
A. Dry-bulb temperature: air temperature.
B. Wet-bulb temperature
2. Phase change
(1). Effective diameter: mean molecular diameter (2
Angstrom).
(2). Mean free path (MFP):
The distance an air molecule can travel before colliding with others.
(3). Phase: State of matter
(solid, liquid, gas): ice, water, and vapor.
(4). Latent heat: Amount of
heat required for phase change.
A. Freezing(crystallization) or melting at 0 oC: 80 cal/g.
B. Evaporation or condensation
(A). is dependent on air temperature
600 - 0.54 T, where T is air temperature in oC.
(B). At 0 oC: 600
calories/g; At 100 oC: 546 calories/g.
(D). Evaporation happens even air temperature is below freezing temperature.
C. Sublimation or deposition: 680 cal/g
3. Evaporation
(1).
Definition: Higher-speed water molecules with a sufficient large kinetic energy
overcomes the surface
tension and fly off water surface, carrying with them latent heat.
(2). Surface tension: The
mutual molecular attractive force among water surface molecules
.
(3). Evaporation is a
cooling process: Higher-speed molecules carry kinetic energy (latent heat) away
with
the remaining water molecules having lower average speed.
(4). Factors affecting
evaporation rate
A. Water temperature
B. Air temperature (SVP): warm air can hold more water vapor.
C. Wind speed: high evaporation with high wind speed.
4. Condensation
(1). Conditions for
condensation
A. Saturation: EVP = SVP or air temperature = dew point or RH = 100%.
B. Condensation nuclei
(A). Hygroscopic nuclei: Water soluble such as salt particles, dry ice or
Silver iodide.
(B). Hydrophobic nuclei: Insoluble such as dust
particles, oils.
(2). Equilibrium relative
humidity (ERH)
The relative humidity at which saturation or
condensation occurs: number of water molecules evaporates
equal number of vapor molecules condenses.
A. Nature of nuclei
(A). Normal ERH = 100%.
(B). Hygroscopic nuclei (salt): ERH = 80%.
(C). Hydrophobic nuclei: ERH > 100%
B.
Curvature or size of water droplet
When air is saturated with respect to a flat surface, it is unsaturated with
respect to a curved droplet
of pure water.
(A). Flat water surface: smaller equilibrium saturation vapor pressure.
ERH = 100%.
(B). Larger curvature (smaller water droplet): Larger equilibrium saturation
vapor pressure:
ERH > 100% (supersaturation)
C.
Solute effect
(A). Solution containing ions of hygroscopic nuclei reduces equilibrium
relative humidity and
condensation occurs at a relative humidity lower than 100%.
(B). Ions of hygroscopic nuclei (salt) in solution bind closely with water
molecules and make
evaporation difficult, reducing ERH.
(3). Condensation forms:
fog, cloud, dew, and frost.
5. Fog
(1). Steam
fog (the Arctic sea smog or evaporative fog)
A. Cold air
advection (horizontal movement) over relatively warm ocean surface.
B. Evaporation from warm water surface adds water vapor to the air, leading to
saturation.
C. Fog over water bodies (lakes, ponds, rivers, oceans) in winter.
(2). Radiation fog (Valley fog,
ground fog)
A. Radiation cooling of the ground surface at night.
B. Light winds and clear sky.
(3). Advection fog:
A. Warm air advection (horizontal movement) over relatively cold ocean surface.
B.
Cooling of the relatively warm air to dew point.
C.
(4). Upslope fog:
A.
Cooling of air over sloping lands (air is forced to rise along slopes).
B.
The
6. Clouds (A typical cloud droplet has a
diameter of about 20 microns)
Cloud types
(1). High clouds
Ice crystals only, silver white with a cloud base height: about 15,000 to
20,000 ft high, thin,
nearby a storm.
A. Cirrus (Ci): feather or hooked shaped,
marl’s tail.
B. Cirrostratus (Cs): white sheet, horizontal spreading of clouds, halo (color
ring around the sun
or moon caused by refraction of light by ice crystals), sundog (image of the
sun).
C. Cirrocumulus (Cc): mackerel cloud (fish-scale like), lump shape.
(2). Middle clouds
Ice crystals and water droplets, white gray with a cloud base about 6,000 ft
high, in the storm,
may have light rains.
(A). Altocumulus (Ac): line or roll shape, cotton balls.
(B). Altostratus (As): Sun is barely visible through clouds. Clouds spread
horizontally like a sheet
of blanket.
(3). Low clouds
Liquid water droplets only, dark with a cloud base lower than about 6,000 ft.
A. Stratus (St): Clouds spread horizontally like a sheet of blanket.
B. Nimbostratus (Nb or Ns): Rainy stratus, dark and
flat cloud base.
Scud: dark patch of cloud associated with Ns.
C. Stratocumulus (Sc): Clouds spread horizontally with line or roll shape.
Vesperalis: colorful Sc near sunset and sun rise
(stable air after sunset).
(4). Vertical
clouds (cumulus or Cu)
A. Cumulus humilis (fair weather cumulus): after a
storm.
B. Cumulus congestus: cauliflower shape, precursor of
a cumulonimbus.
C. Castellanous: Castle-like clouds.
D. Cumulonimbus (Cb):
(A). Towering clouds growing from near the ground surface to the tropopause (6 miles) or
slightly higher.
(B). Anvil top: Flat cloud top caused by the inversion layer in the
stratosphere. Inversion limits
clouds from growing taller.
(C). Overshooting (thunderhead): Cb
grows into the lower stratosphere due to the momentum of
updrafts (rising air currents).
(D). Most stormy: thunderstorm, shower rain.
(5). Special
types of clouds
A. Lenticular clouds (leewave
clouds)
(A). A type of Ac in the lee (Mountain slope where air
currents leave) of a mountain.
(B). Causes: Rinsing currents in a lee-wave (not much
moisture which condenses as cumulus
clouds in the windward side).
B. Mammatus clouds
A. Ball- or pouch-shaped clouds.
B. Evaporation from falling raindrops adds water vapor to the air below cloud
base and causes
condensation.
C. May be a sign of tornado strike associated with a Cb.
7. Vertical motions
(1). Topographic uplifting
Air currents are force to rise along the windward slope of a mountain.
(2). Thermal convection
Solar heating of the earth’s surface causes the warm air to rise.
(3). Weather systems
Surface convergence, cyclone (low), and front (warm air overrides the cold air)
8. Precipitation theories
(1). Bergeron’s Theory
A. SVP over the supercooled water surface is greater
than the SVP over the ice surface.
B. Deposition over ice crystals while evaporation over water droplets.
C. Ice crystals grow at the expense of water droplets.
D. Snow or rain
(A). Cloud base height:
a. Snow at mountain (closer to the cloud).
b. Rain on foothill or valley (snow melts into rain due to a sufficient
altitude).
(B). Air temperature below the cloud base
a. Winter snow (Air temperature < 0 oC).
b. Summer rain (Air temperature > 0 oC)
E. Clouds grow beyond the freezing level.
F. Cold cloud: Ice crystals and/or supercooled water droplets in the cloud.
G. Cold rain: Rain originates from a cold cloud.
(2). Coalescence Theory
A.
Different sizes of water droplets with different terminal speeds collide with
each other and
combine together to form larger droplets.
B.
Small droplets fall into the wake of large droplets and attach to them.
C.
Solute effect produce different sizes of water
droplets.
D.
Warm cloud and warm rain: No ice crystals involve in the precipitation process.