Download Chapter 17 Moisture, Clouds, and Precipitation * Phase changes of

Chapter 17
Moisture, Clouds, and Precipitation
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Phase changes of water
* _________________ – liquid to gas
* _________________– gas to liquid
* _________________ – solid to liquid
* _________________ – liquid to solid
* _________________ – solid to gas
* _________________ – gas to solid
During phase changes heat is absorbed or released (_________________)
Mixing Ratio – mass of water vapor relative to mass of dry air
* Does not vary with air volume changes
* Does not change with temperature
* Saturation mixing ratio
___________________ – amount of water vapor present related to the amount air can hold
* Given as a percentage
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• Highest RH occurs during the coolest times
• Lowest RH occurs during time of greatest air temperature
• A hot humid day results in a low relative humidity value, making this a poor
indicator of actual humidity
* Cannot be used to compare moisture content at different locations with different
temperatures
Dew Point • High dew point equals high moisture content
• _____________________ – difference between temperature and dew point
– Smaller difference equals more saturated air therefore more likely
clouds and precipitation are occurring
• Good estimate of moisture content
• Used to forecast the minimum temperature
Frost Point – temperature where saturation point is below freezing
Distribution of Water Vapor
* Moisture is increased by
• Evaporation
• Advection
* ____________ = major source of precipitation for the eastern half of the United States
Measuring Humidity
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• Two thermometers to measure wet and dry bulb temperatures
• Wet bulb depression – difference between dry and wet bulb
* Aspirated psychrometer
• Uses a fan for air flow instead of slinging the instrument
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Uses hair expansion and contraction to measure humidity
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No net exchange of energy
• Expansion of air causes cooling and compressing air causes warming
• Rising air will expand and cool adiabatically
• Sinking air will compress and warm adiabatically
* Dry adiabatic lapse rate (__________________)
• Lapse rate in dry air
* Moist adiabatic lapse rate (-3.3oF/1000ft)
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The Environmental Lapse Rate (ELR)
* Overall decrease in air temperature with height
* Changes diurnally from place to place
* When temperatures decrease rapidly with height this establishes a steep ELR
• Creates unstable air and can assist with storm development
Stability of the air
* Static stability – determines if air rise, falls, or remains at a certain level
• Related to temperature and controlled by buoyancy
– Positive and negative buoyancy
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• Statically neutral
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Absolutely unstable air
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* Positive buoyancy
* Parcel of air cools slower than that of surrounding air
• Parcel of rising air is always warmer than surrounding air
• DAR and WAR less than ELR
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Absolutely stable air
* If air is forced to rise and it sinks back to its original position
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* Parcel of air cools faster than surrounding air
• Parcel of air is always cooler than surrounding air
• DAR and WAR greater than ELR
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* Associated with high pressure
Absolutely Neutral air
* If air is forced to rise and parcel remains in the location where lifting ceased
* Parcel of air temperature equal to surrounding air
Conditionally unstable air
* Parcel of air forced to rise is initially stable but eventually becomes unstable after
saturation is reached
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* ELR is between the DAR and the WAR
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Creating clouds needs air to rise and cool
4 Mechanisms that lift air
* ________________ – lifting caused by a mountain
• Air hits a mountain and is forced upwards
• Windward side – upwind side of the mountain rising air causes cooling
– Abundant precipitation
• ______________– downwind side of the mountain sinking air caused warming
– Lack of precipitation (_______________________)
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• Boundaries between different air masses
• Warm/moist air is forced to rise and form clouds
• Cold (a) and warm (b) fronts occur
* Convergence
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• Convergence causes air to rise and cool
* Localized convection
• Air heated at the surface becomes warmer and less dense and rises freely into
updrafts
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Cloud Classifications
* Based on appearance and height
* High clouds (______________________)
• Bases above 6000m (19,000 ft)
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• Cirrus (Ci) – thin white wispy clouds
• Cirrostratus (Cs) – more extensive often produces a halo around Sun or Moon
• Cirrocumulus (Cc) – layered clouds with billows or rolls. Often look like fish
scales.
* Middle clouds (_______________)
• Bases between 2000 and 6000m (6-19,000 ft)
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• Altostratus (As) – more extensive than cirrostratus. Sun and Moon are slightly
visible.
• Altocumulus (Ac) – a series of puffy clouds arranged in rows. Allows less
light through than cirrocumulus and the scales are larger in size.
* Low clouds (__________________)
• Bases below 2000m (6,000 ft)
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• Stratus (St) – uniform layer of low clouds often a darker gray in color. Covers
a wide region.
• Nimbostratus (Ns) – low layered clouds that produce light precipitation
• Stratocumulus (Sc) – low layered clouds with some vertical development
* Clouds with vertical development
• Occur with unstable air
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Cumulus humulus (Cu) - local updrafts create individual clouds. “Fair
weather” clouds
• Cumulus congestus - contains numerous updrafts and form towering cumulus
• _____________________ – towering cumulus clouds otherwise known as
“thunderheads”.
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– Characteristic anvil shape made of ice
– Extreme updrafts and downdrafts
* Unusual clouds
• ___________ - form from mountains disturbing the wind flow. Flying saucer
shape.
• ____________ – large globular clouds hanging down from the anvil of severe
thunderstorms
• ________________ – large horizontal roll clouds leading a strong downdraft
from thunderstorms
• Nacreous (Mother of Pearl) Clouds – pearl like clouds forming at extremely
high altitudes (50,000-80,000 feet)
Forms of Condensation
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Dew
* Liquid condensation on surface objects
* Occurs on clear windless nights
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* Directly from gaseous water vapor to solid ice crystals
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Frozen Dew
* Dew formation followed by a temperature drop
* Creates a tight surface bond making it difficult to remove
* Often responsible for “black ice”
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Fog – essentially a cloud at the ground
* Precipitation fog
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• Water evaporates off a hot road
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• Warm and cold air mixing
• Cold air over warmer water
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* Radiation Fog
• At night, ground cools radiatively and cold air sinks to the lowest levels
(“________________”)
• Clear cool nights with light winds
• Burns off when sun rises
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• Warm, moist air moving over cool surface
• Warm spring air over cold snow surface
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* Upslope Fog
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• Adiabatic process from upslope advection
* Warm Cloud Process (_______________________)
• Entire cloud is above freezing
• Produces rain in tropical regions
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• Largest drops fall the fastest
• Large (collector) drops fall and collide with smaller drops
– Smaller drops usually are pushed aside
• Colliding drops can stick (coalesce), or can bounce off
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* Cold cloud process (_________________)
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• Vapor pressure is greater around water drops than ice crystal
• Pressure gradient exists between vapor and crystal causing water vapor
molecules to collect on the crystal
• Ice crystal grows larger, while liquid drops evaporate
• Falling ice crystals collide with liquid drops and freeze them on contact
(riming)
• Falling ice crystals collide with other ice crystals and can join (aggregation)
– Works best when cloud is slightly below freezing
– Helps to explain why larger snowflakes occur during warmer
temperatures
• Only takes about 30 minutes for precipitation to fall after initial ice formation
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Forms of Precipitation
* Snow –
* Measured in inches
* Moisture within the snow depends on air temperature
* Crystal types often determine accumulation rate
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• Needles and plates are favored by colder temperatures
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Rain – precipitation reaches ground as liquid drops
• Measured in inches
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• Extensively rimed ice crystals
– Sometimes becomes the nucleus for hail
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• Forms multiple layers as ice rises and falls within the cloud
• Terminal velocities can reach 100 mph with largest hailstones
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• Largest hailstone =
• Hail Video
* Sleet
• Falls as ice pellets (often mistaken for hail)
• Snow falling melts, then re-freezes as it enter a shallow cold layer near the
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• Creates extremely dangerous driving conditions
• Damages power lines and trees from the extra weight of ice
Measuring Precipitation
* __________________ – measures amount of rainfall
• Tipping bucket
Raingage Measurement Errors
* Measurements are taken only at points
• Wide variations across small spaces
• Over and Under estimates
– Wind, evaporating water, non-level surfaces, splashing water, drifting
snow
Precipitation Measurement by Weather Radar
* Precipitation estimates
* Real-time information
Snow Measurement
* Accumulated snow measured
• Water equivalent of snow = 10:1 ratio
• Ratio can vary between 4:1 and 50:1 based on the temperature
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Dry ice and Silver iodide act as CCNs
Can be used to remove fogs on airport runways, but only in cold air fogs which is more
uncommon
End of Chapter 17
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