Water vapor

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Water vapor

• Water vapor
• Odorless, colorless gas
• Invisible
• Mixes freely with other gases in atmosphere
• Steam
• Tiny water droplets
• Which can you see in this picture?

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Phase changes
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Latent Heat
Water vapor

• Temp. doesnt rise as the ice melts
• Where does the heat go?
• Used to disrupt the internal crystalline
  structure of the ice
• Latent (hidden) heat the heat absorbed or
  released during a change of phase

4
Latent heat and storms

• Latent heat will be released when water condenses
  and returns to liquid.
• An important source of energy for T-storms,
  tornadoes, hurricanes

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Humidity amount of water vapor in air

• Saturation The air holds all the water vapor it
  can at a given temperature
• The higher the temperature, the more water vapor
  air can hold
• Relative humidity - as a percentage
  water vapor in air
  water vapor that air
  can hold
• If more water vapor, relative humidity______
• If less water vapor, relative humidity_______
• If the air cools, relative humidity___________
• If the air warms, relative humidity__________

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Dew Point

• Dew point is the temperature to which air must be
  cooled to reach saturation.
• Dew is condensation that is formed when air that
  is in contact with a cool surface loses heat
  until it reaches saturation.
• If rising air cools to the dew point, what forms?

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Frost vs. frozen dew

• Frost forms from water vapor deposited as ice,
  skipping the liquid phase
• Frozen dew forms when water vapor condenses onto
  a surface as a liquid and then freezes.

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Condensation

• Water vapor to water, latent heat released
• Occurs on either
• an object
• a condensation nucleus, usually dust

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• Convective cooling is due to air rising and
  moving away from the heat source (vertical).
• Advective cooling occurs when wind carries warm
  air across a colder area (horizontal).

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• Adiabatic cooling is air cooling that results
  only from changes in pressure
• Rising expansion cooling
• Sinking compression heating
• Air cools 1C for every 100m the air rises
• Dewpoint drops 0.2C for every 100m the air rises
• As air rises, will its temperature reach its dew
  point?

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Convective and adiabatic cooling, forming clouds

• Air cools to dew point
• Water vapor condenses on condensation nuclei
• Cloud forms

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Stratus clouds

• Sheet-like and layered.
• Low base in the sky
• Layer of warm, moist air lies above a layer of
  cool air.
• Little rain.
• Made of water droplets
• Most extensive in sky - overcast

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Cumulus clouds

• Puffy, vertical growing clouds.
• piled or heaped
• Made of water droplets of the bottom, ice
  crystals at the top
• Cumulonimbus thunderheads

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Cirrus clouds

• Highest clouds, above 6000 m.
• Wispy and feathery.
• Low temperatures at high altitudes, made of ice
  crystals.
• No precipitation

15

• Fog forms near the surface when air close to the
  ground is cooled by any of these
• Heat radiates away overnight
• Warmer air blows over colder land
• Air is pushed up in mountains

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Precipitation

• Water droplets in clouds are kept up by wind
• Coalescence is the combination of different-sized
  cloud droplets to form larger droplets.
• When a drop is large enough it falls as rain

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Winter storm how does each kind of
precipitation form?
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Hail

• A water droplet is pushed by updrafts to the top
  of the cumulonimbus cloud, where it freezes
• As the hailstone falls more water sticks to it
• Hailstone is blown back to top of cloud where
  outer water layer freezes
• What conditions would produce the largest
  hailstones?

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• An air mass is a large body of air with uniform
  temperature, wind direction, and moisture
  content.
• PPolarcold
• TTropicalwarm
• mmaritimeformed over the ocean moist
• ccontinentalformed over land dry

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25.2 Fronts

• A front is a boundary between air masses.
• The type of front depends upon how the air masses
  are moving.
• There are four major fronts.
• 1.Cold front
• 2. Warm front
• 3. Stationary front
• 4. Occluded front

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Cold Front

• A cold air mass overtakes a warm air mass.
• The moving cold air lifts the warm air.
• Forms cumulus cumulonimbus
• Short-lived violent storms just behind the front

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Warm front

• A warm air mass overtakes a cooler air mass.
• The less dense warm air rises over the cooler
  air.
• Moderate precipitation over large area, ahead of
  the front

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Stationary front

• Neither air mass is displaced.
• The two air masses move parallel to the front
  between them.
• Moderate to light precipitation

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Occluded front

• A fast-moving cold front overtakes a warm front
• Warm air is lifted off the ground.
• The advancing cold front hits more cool air and
  loses energy
• Minimal precipitation

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Wave Cyclone Low pressure

• Typical in mid-latitudes (here)
• These are large storms up to 2,500 Km in
  diameter.
• Winds spiral up around the low-pressure region at
  the center.
• Brings cloudy, stormy weather.

Counter clockwise in N. Hemisphere
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Anticyclone High pressure

• The air sinks and flows outward from a center of
  high pressure.
• Clockwise in the Northern hemisphere.
• Brings in dry weather.

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• Hurricanea severe tropical storm with winds over
  120km/hr. They develop over warm, tropical
  oceans. The winds spiral in toward a
  low-pressure storm center.
• Typhoonhurricanes in the western Pacific Ocean.

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• Thunderstorma storm with thunder, lightning, and
  strong winds. Warm, moist air is heated and
  rises.
• Tornadoa whirling, funnel-shaped cyclone. When
  a thunderstorm meets high, horizontal winds and
  causes it to rotate.
• Water Spout Tornadoes over the ocean. They are
  smaller and less powerful.

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25.4 Forecasting weather

• A station model shows the weather conditions at a
  point on a map using symbols

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Isobars

• Isobars are lines on a weather map that connect
  points of equal atmospheric pressure.
• If they are close, it shows a rapid change in
  pressure and high wind speeds.
• If they are widely spaced apart, then there is a
  slow change in pressure and low wind speeds.
• Isobars that form circles show centers of high or
  low air pressure.