Water in the Atmosphere

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Water in the Atmosphere

• Chapter 18

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18.1 Humidity and Condensation

• Kinetic Theory
• Everything is made of particles
• Particles are in constant motion
• Increase energy, particles move faster
• Water Characteristics
• Water can be in any of the three states on the
  surface of the earth
• Solid-
• Commonly called ice
• Liquid-
• Gas-
• Commonly called water vapor (invisible)

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18.1 Humidity and Condensation

• Changes of state (explain these using the kinetic
  theory)
• Condensation-
• Evaporation-
• Causes cooling to occur
• Deposition-
• Sublimation-
• Freezing-
• Melting-

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18.1 Humidity and Condensation

• Specific Humidity- the actual amount of water
  vapor in the air at a given time and place
• About 2 is a typical amount
• The amount is limited by factors like air
  temperature (the amount doubles every time the
  air temperature rises 11 degrees)
• The air becomes saturated when evaporation and
  condensation are occurring at the same rate

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18.1 Humidity and Condensation

• Relative Humidity- how close the air is to
  holding its maximum capacity at a certain
  temperature
• Saturated air has a relative humidity of 100
• You calculate the relative humidity by dividing
  the specific humidity by the maximum capacity
  (see p. 392)
• Measuring Humidity
• Psychrometer two thermometers, one measures air
  temperature, the other has a wet cloth on it and
  measures the temperature inside the wet cloth,
  You take the reading of the air temperature and
  the difference between the two thermometers and
  look at a chart like you see on p. 393
• Why will the wet cloth thermometer read lower?

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18.1 Humidity and Condensation

• Condensation
• On a sunny spring day the air is not saturated
• As the temperature cools in the evening the
  capacity of the air to hold water diminishes
• Eventually the air becomes saturated and
  supersaturated
• At this point, liquid will form on any surface
  the air touches, that liquid we call dew and the
  temperature that the air is at is called the dew
  point
• Dew points differ by how much water the air is
  holding
• Cooling and Condensation
• In order for condensation to occur two conditions
  must be present
• The air must cool below its dew point
• Contact with a colder surface
• Radiation of heat
• Mixing with colder air
• Expanding as it rises
• There must be condensation nuclei for the water
  to form on
• Salt, soot, dust, etc. Must be present for the
  water to form on

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18.1 Humidity and Condensation

• Formation of Dew and Frost
• If the air temperature drops below 0 degrees, the
  water will form a coating of frost instead of a
  coating of dew.
• This type of frost does not kill plants, even
  colder air that makes plant cells burst is the
  cause of a killing frost
• Formation of Fog
• Forms when a cold surface cools the warm, moist
  air above it
• The droplets are so tiny the slightest breeze
  keeps them afloat
• Fog types
• Radiation fog happens on clear nights when the
  ground cools rapidly and warm moist air is
  trapped near the surface (temperature inversion)
• Often occur in humid valleys near rivers or lakes
  in the fall or spring
• These fogs quickly burn away in the morning
  sunlight
• Advection fog forms when warm moist air blows
  over a cooler surface
• Often occur on the coasts of oceans or at the
  mouth of the Mississippi

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18.2 Clouds

• Cloud basics
• Clouds can form at any level
• Clouds can be water droplets, ice crystals, or a
  combination of both
• Cloud vocabulary
• Stratus or strato layered clouds
• Cumulus or cumulo clouds that grow upwards
• Cirrus or cirro feathery clouds
• Alto middle layer clouds
• Nimbus or nimbo dark rain clouds

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18.2 Clouds

• Cloud types
• High
• Cirrus-
• Cirrostratus-
• Cirrocumulus-
• Middle
• Altostratus-
• Altocumulus-
• Low
• Stratus-
• Nimbostratus-
• Stratocumulus-
• Special clouds
• Cumulus-
• Cumulonimbus-

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18.2 Clouds
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18.2 Clouds

• Cloud formation
• The shape of the cloud shows how the air is
  moving through it
• Cumulus- occur on sunny days when air is heated
  on the surface and rises
• The air cools as it rises until it reaches its
  condensation level and forms a cloud
• If there is enough moisture and enough heat the
  cloud will grow and grow until it becomes a
  cumulonimbus cloud (see chart on p. 399)
• Layer clouds
• Form when there is stable air
• The air does not move up so it just spreads out
  to the side until you get an entire front of
  clouds

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18.3 Precipitation

• Precipitation- any form of water that falls from
  a cloud to the earths surface
• How precipitation forms
• Growth of water droplets- small droplets formed
  from condensation bump into other drops and suck
  them in until the drop is heavy enough and the
  rain falls
• The longer the droplet can stay aloft the larger
  the drop will be
• Larger droplets result from strong winds or large
  condensation nuclei
• Growth of Ice crystals- similar process to water
  droplets but occur at higher altitudes
• Depending on the temperature of lower air masses,
  the ice crystal can change to liquid or stay as a
  solid

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18.3 Precipitation

• Kinds of precipitation
• Drizzle- fine water droplets close together
• Rain- larger water droplets farther apart
• Snow- ice crystals that grow enough to fall, when
  they fall into warm moist air they can melt or
  form very large snowflakes
• Sleet- when rain falls through a layer of cold
  air and freezes into pellets
• Freezing rain- when rain falls through cold air
  but doesnt freeze until it comes into contact
  with a surface
• Hail- strong updrafts in large cumulonimbus
  clouds make rain freeze over and over again until
  you have a hail stone, the stronger the updraft
  the larger the hail (see pictures p. 403)

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18.3 Precipitation

• Measuring precipitation
• Rain gauge-
• Measures in hundredths of inches
• Snow is measured in inches, but the amount of
  water in snow makes inches a very Inaccurate
  measure of snowfall

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18.3 Precipitation

• Where does precipitation occur?
• Everywhere, but in different levels
• The most rain falls in warm areas where moist air
  rises regularly
• Equatorial regions
• In storm areas
• Windward sides of coastal mountains (make
  diagram)
• The least air where dry air sinks
• 30 degrees south and north of the equator
• Leeward sides of coastal mountains
• Chinook winds

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18.3 Precipitation

• Weather modification
• Seeding clouds-
• using an airplane to drop dry ice to cool and
  condense a cloud
• Dropping in silver iodide crystals as
  condensation nuclei
• In both methods, clouds must be present