Clouds & precipitation by 0QnAc6

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									Clouds & precipitation
              Water vapor
 Source of all precipitation (rain, snow, etc.)
 Mixes well with other gases in the
  atmosphere
 Colorless, odorless gas
 Water can easily change from one state to
  another freely at the temperature exhibited
  by the atmosphere
     Water’s changes of state
 Processes that involve changes of state
  require that heat be absorbed or released
 Under certain conditions, heat may be
  added to a substance without an
  accompanying temperature change
 Heat that does not involve a temperature
  change is called latent heat
    Liquid/gas change of state
 Evaporation: process of changing a liquid
  to a gas
 About 600 calories is required to convert
  one gram of liquid water to water vapor
 During evaporation, the higher
  temperature (fast-moving) molecules
  escape the surface
     Liquid/Gas changes of state
               (cont.)
   The energy absorbed by water molecules
    in changing a liquid to a gas is called the
    latent heat of vaporization
             Condensation
 Condensation: Process in which water
  vapor changes to a liquid
 Condensation is an energy-releasing
  process
 Energy that is required to release this
  stored energy is called the latent heat of
  condensation
 In order for condensation to occur, water
  molecules must release their stored heat
  energy equal to what was absorbed during
  evaporation
 This released energy plays an important
  roles in violent weather (such as
  hurricanes, tornadoes, etc.)
    Solid/liquid changes of state
 Melting: process in which a solid is
  changed into a liquid
 One gram of ice (solid water) requires
  about 80 calories of heat to change water
  to the liquid state
 Freezing: process in which a liquid is
  changed into a solid
 Freezing one gram of water releases 80
  calories of heat
    Solid/gas changes of state
 Sublimation: process of changing a solid
  to a gas
 Deposition: process of changing a gas to
  a solid
 (example: water vapor deposits as ice
  crystals on cold objects)
               Humidity
 Humidity: The amount of water vapor
  present in the air
 Several methods in which water-vapor
  content can be expressed:
   1) mixing ratio
   2) relative humidity
   3) dew point temperature
               Saturation
 Saturation: The maximum quantity of
  water vapor that the air can hold at any
  given temperature and pressure
 During saturation, the number of water
  vapor molecules returning to the surface
  will equal to the number of water
  molecules leaving the surface
              Mixing ratio
 Mixing ratio: The mass of water vapor in a
  unit of air compared to the remaining mass
  of dry air
 Mixing ratio is not affected by changes in
  temperature and pressure
 Mixing ratio is not easy to measure by
  direct sampling
           Relative humidity
 Relative humidity is the ratio of the air’s
  actual water-vapor content to its potential
  water vapor capacity at a given
  temperature
 If relative humidity is 100%, the air is
  saturated
       Relative humidity (cont.)
   Relative humidity can be changed one of
    two ways:
     1) Add or remove water vapor
        from the air
     2) change the temperature of
        the air
      Dew-point temperature
 Dew point temperature (also called the
  dew point): The temperature in which air
  has to be cooled to reach saturation
 When air is cooled below its dew point,
  some of the water vapor condenses to
  form clouds
   Since dew point is the temperature at
    which saturation occurs, high dew point
    temperatures indicate moist air, whereas
    low dew point temperatures indicate dry
    air
        Measuring humidity
 Humidity can be measured using a
  hygrometer
 A hygrometer consists of two parts: 1)
  dry bulb, which gives the present air
  temperature 2) wet bulb, which aids in
  determining the humidity
 NOTE: If the wet-bulb and the dry bulb
  temperatures are the same, the humidity is
  100%
 When air in the atmosphere is cooled
  below, some of the water vapor
  condenses to form clouds
 When moist air rises, it cools and forms
  clouds
           Cloud formation
 Clouds: visible aggregates of minute
  droplets of water or tiny crystals of ice
 Clouds are classified based on their form
  and height
             Cloud forms
 Cirrus: high, white, and thin. Feathery
  appearance
 Cumulus: Consist of globular, individual
  cloud masses
 Stratus: Described as sheets or layers
  that cover much of the sky
            Cloud heights
 High: Bases that lie above 20,000 feet
 Middle: Bases that lie between 6500 and
  20000 feet
 Low: Clouds that lie below 6000 feet
  above the Earth’s surface
              High clouds
 Consist of cirrus, cirrostratus and
  cirrocumulus
 Cirrus clouds are wispy
 Cirrocumulus consist of fluffy masses
 Cirrostratus consist of layers
              Middle clouds
   Consists of altocumulus and altostratus
              Low clouds
 Consists of stratus, stratocumulus, and
  nimbostratus
 Stratus: Uniform layer of cover much of
  the sky; occasionally brings light
  precipitation
 stratocumulus: clouds appear as long
  parallel rolls or broken globular patches
 Nimbostratus: Low lying clouds that are
  one of the chief rain producers
 Clouds of vertical development
 These clouds are associated with unstable
  air
 Cumulonimbus cloud: A cumulus cloud
  that develops vertically
 Cumulonimbus clouds are associated with
  rain showers and thunderstorms
    Mechanisms which causes air to
                rise
 Orographic lifting
 Frontal wedging
 Convergence
 Localized convective lifting
          Orographic lifting
 Air is forced to rise over a mountainous
  barrier
 Occurs when mountains act like barriers to
  the flow of air
           Frontal wedging
 Warmer, less dense air is forced over
  coooler, denser air
 When masses of warm and cold air
  collide, it produces a front
               Convergence
   This is when air masses in the lower
    atmosphere flow together, and lifting
    results
    The basis of cloud formation :
         Adiabatic cooling
 Adiabatic temperature change:
  Cooling/warming of air caused when air is
  allowed to expand or is compressed.
 When air is allowed to expand, it cools,
  when it is compressed, it warms
   When air is allowed to expand, it cools,
    when air is compressed, it warms
Adiabatic Temperature Change
   A temperature change which results when
    air is expanded or compressed without
    addition or removal of heat
          Dry adiabatic rate
 The rate of warming/cooling of
  unsaturated air.
 Unsaturated air cools at a rate of 10oC for
  every 1000 meters of ascent (1oC per 100
  meters or 5.5oF per 1000 feet)
         Wet adiabatic rate
 The rate of adiabatic temperature change
  in saturated air
 The wet adiabatic rate varies based on the
  content of moisture in the air
 Ranges from 5oC per 1000 meters of air
  (high moisture content) to 9oC per 1000
  meters of air (dry air)
         Atmospheric stability
 When air rises, it cools and eventually
  produces clouds
 Clouds vary in size due to the stability of
  the air
 Air is stable if it tends to resist vertical
  displacement (the air sinks)
 Air is unstable if it rises from its original
  position
 Stable air resists vertical movement,
  whereas unstable air ascends freely
  because of its buoyancy.
 Clouds associated with the lifting of
  unstable air are towering and often
  generate thunderstorms or other violent
  weather
                    Fog
 Fog is a cloud with its base at or very near
  the ground
 Clouds result when air rises adiabatically,
  whereas fogs are a consequence of
  radiation cooling
 Fog form when enough water vapor is
  added to bring about saturation
            Types of fogs
 Advection fog
 Radiation fog
 Upslope fog
            Advection fog
 Forms when warm, moist air moves over a
  cool surface
 Example: Cape Disappointment,
  Washington (foggiest place in the world)
             Radiation fog
 Forms on cool, clear nights, when Earth’s
  surface cools rapidly by radiation
 Largest pockets are usually found in river
  valleys
             Upslope fog
 Fog that is prodcued when humid air
  moves up a gradually sloping plain or up
  the steep slopes of a mountain
 Most common in the Great Plains of the
  United States
              Precipitation
 Any form in which atmospheric moisture
  returns the Earth’s surface
 Forms of precipitation: rain, hail, snow,
  and sleet
       How precipitation forms
   Forms based on two mechanisms: 1) ice
    crystal process and 2) collision-
    coalescence process
              Rain/drizzle
 Rain: Drops of water that fall from a cloud
  that have a diameter of 0.002 inch
 Drizzle: Fine, uniform drops of water that
  have a diameter less than 0.002 inch
 Most rain originates from nimbostratus
  clouds or in towering cumulonimbus
  clouds
 Drizzle are produced from stratus or
  nimbostratus clouds
                  Snow
 Snow: Precipitation in the form of ice
  crystals
 Light, fluffy snow is produced when the air
  temperature very low (23oF or lower)
 Heavy snow is produced when air
  temperature are above 23oF. Has a high
  moisture content
                     Sleet
   Sleet: small ice particles which are clear
    and translucent
                    Hail
 Hail: precipitation in the form of hard,
  rounded pellets or irregular clumps of ice
 Hail is produced only in large,
  cumulonimbus clouds, where there is an
  abundant supply of supercooled water
     Measuring precipitation
 Rain: Can be measured by using a
  standard rain gauge
 Snow: Measured based on depth and
  water equivalent

								
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