Docstoc

Precipitation

Document Sample
Precipitation Powered By Docstoc
					       Precipitation
•   Precipitation   Formation
•   Precipitation   Characterization
•   Precipitation   Measurement
•   Precipitation   Representation
•   Precipitation   Analysis
            Precipitation
•Any aqueous deposit, in liquid or solid form,
that develops in a saturated atmosphere
(Relative Humidity equals 100 %) and falls to
the ground generally from clouds.

•Most clouds, however, do not produce
precipitation. In many clouds, water droplets
and ice crystals are too small to overcome
natural updrafts found in the atmosphere. As a
result, the tiny water droplets and ice crystals
remain suspended in the atmosphere as clouds.
 The Atmosphere and Clouds
• Lifting mechanisms are required for moist air
  masses to cool and approach saturation
  conditions.
• Horizontal variations in atmospheric pressure
  cause air to move from higher pressure to
  lower pressure, resulting in the generation of
  WIND.
• Vertical displacement also causes air to move
  but at a slower rate than the horizontal
  movement.
• The vertical movement and lifting of air
  results in the formation of CLOUDS.
         General principle of
       precipitation formation
• Warm moist air rises (warm air is lighter than
  cold air) by one of three lifting mechanisms
  (convective, cylonic, or orographic)
• The moist air is cooled adiabatically, i.e. no
  heat exchange with environment.
• The relative humidity increases (saturation
  vapor pressure decreases with temperature)
• When the dew point temperature is reached,
  condensation begins.
  Formation of Precipitation
• Condensation
  – Dynamic Cooling: Adiabatic reduction of
    temperature associated with upward movement of
    air.
  – Cooling of moist air to a temperature below the
    saturation point for water vapor.
  – Mixing of air masses having different
    temperatures.
• Coalescence
  – Growth of droplets.
  – Collisions: large drops run into smaller drops, large
    drops decrease the drag on drops above.
Convergence Lifting
  Precipitation Measurement

• Non Recording Station: rain gage
  measures inches of rainfall
• Recording Gage: tipping bucket –
  measures time distribution of rainfall 1
  or 5 min intervals - .01”
• RADAR:NEXRAD
Recording Gage:
 tipping bucket
      gage
Rainfall Hyetograph
         Spatial Variability

• In addition to the precipitation at a particular
  point varying with time, precipitation also
  varies with respect to area
• For most hydrologic analyses, it is important
  to know the areal distribution of
  precipitation.
• Usually average depths for representative
  portions of the watershed are determined
  and used for this purpose (ADP)

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:6/4/2012
language:Latin
pages:37