• Precipitation Formation
• Precipitation Characterization
• Precipitation Measurement
• Precipitation Representation
• Precipitation Analysis
•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
• Horizontal variations in atmospheric pressure
cause air to move from higher pressure to
lower pressure, resulting in the generation of
• Vertical displacement also causes air to move
but at a slower rate than the horizontal
• The vertical movement and lifting of air
results in the formation of CLOUDS.
General principle of
• 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,
Formation of Precipitation
– Dynamic Cooling: Adiabatic reduction of
temperature associated with upward movement of
– Cooling of moist air to a temperature below the
saturation point for water vapor.
– Mixing of air masses having different
– Growth of droplets.
– Collisions: large drops run into smaller drops, large
drops decrease the drag on drops above.
• Non Recording Station: rain gage
measures inches of rainfall
• Recording Gage: tipping bucket –
measures time distribution of rainfall 1
or 5 min intervals - .01”
• 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
• Usually average depths for representative
portions of the watershed are determined
and used for this purpose (ADP)