Chapter 18: Water in the Atmosphere

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Chapter 18 Water in the Atmosphere

• 18.1 Humidity and Condensation
• 18.2 Clouds
• 18.3 Precipitation

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18.1 Humidity and Condensation

• Water vapor is everywhere and it is always on the
  
• move.
• Characteristics of Water
• It is unique because it exists in all three
  states of matter, depending on its temperature.
• Solid at 0oC or below,
• Liquid between 0oC-100oC, and a
• Gas (water vapor) at 100oC or above.

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18.1 Humidity and Condensation

• 1. Condensation-the change from water vapor to
  liquid water. Ex. dew, fog and clouds.
• 2. Condensation releases heat and evaporation
  absorbs heat.

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18.1 Humidity and Condensation

• Humidity
• Specific Humidity- the actual amount of water
  vapor in the air at a given time and place.
• Air is saturated when the rate of condensation
  equals the rate of evaporation.

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18.1 Humidity and Condensation

• 3. Relative Humidity- How near the air is to its
  maximum capacity for holding water vapor. See
  example on page 392.
• 4. Humidity is measured using a psychrometer.

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18.1 Humidity and Condensation

• Condensation
• Dew Point- The temperature at which saturation
  occurs and condensation begins.
• Two conditions are needed for water vapor to
  condense 1) there must be materials for the
  water vapor to condense on and
• 2) air must cool to or below its dew point.

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18.1 Humidity and Condensation

• If temperatures are 0oC or less dew turns to
  frost.
• Fog forms when a cold surface cools the warmer
  moist air above it.

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18.2 Clouds

• Clouds form when air cools to its dew point.
• Types of Clouds
• Four main types- low, middle, high and clouds of
  vertical development.
• If air movement is horizontal then the clouds
  form in layers called stratiform. If the air
  movement is mainly vertical, clouds grow upward
  in great puffs called cumuliform.

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18.2 Clouds

• 3. Stratus and Strato-describe clouds that form
  in layers and are low.
• 4. Cumulus and Cumulo- describe clouds that grow
  upward. They are puffy with flat bases.
• 5. Cirrus and Cirro- describe high, feathery ice
  clouds.

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18.2 Clouds

• 6. Alto-describes clouds located between 2000 and
  7000 meters.
• 7. Nimbus and Nimbo- refer to dark rain clouds.

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18.2 Clouds

• Cloud Formation
• Condensation Level- atmospheric level at which
  condensation occurs.
• Adiabatic Lapse Rate- the rate at which air cools
  as it rises.

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18.2 Clouds

• 3. Cumulus clouds- Form when moist air rises and
  cools to its dew point.
• 4. Stratiform clouds- Form when the air is stable
  and the cloud can't move up or down.

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18.3 Precipitation

• Precipitation-is any form of water that falls
  from a cloud to the earth's surface.
• How Precipitation Forms
• Condensation droplets collide and combine to form
  bigger droplets.
• Ice crystals form in clouds that are below
  freezing.

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18.3 Precipitation

• 3. Sleet- Frozen raindrop
• 4. Freezing Rain- is rain that freezes when it
  hits the ground.
• 5. Hail-Is a frozen raindrop that grows by
  colliding with other drops.

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18.3 Precipitation

• B. Measuring Precipitation
• The National Weather Service uses a rain gauge to
  measure rain in hundredths of an inch.
• 2. The measurement represents what the depth of
  water would be if it did not soak into the
  ground, flow away or evaporate.

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18.3 Precipitation

• Where does this occur?
• The main cause of precipitation is the rising and
  cooling of moist air.
• 2. Near the equator and across mountain ranges
  are great areas to find precipitation.

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18.3 Precipitation

• 3. There is little or no precipitation where air
  sinks and warms.
• 4. Also air that sinks on the downwind side of
  mountain is very dry. See pg. 404.

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18.3 Precipitation

• D. Weather Modification
• Implementing methods of changing the weather.
• 2. Rain Seeding- Droplets of frozen carbon
  dioxide or silver dioxide are dropped into a
  supercooled cloud.
• Hail forming.