Precipitation

1
Precipitation

• Hydrology (Spring 2011)
• Illinois State University
• Instructor Eric Peterson

2
Precipitation - General

• Deposition of liquid water and ice particles from
  the atmosphere to Earths surface
• Gravity pulls to Earths surface
• Dominant deposition mechanism for moisture from
  the atmosphere

3
Precipitation Formation

• The formation of precipitation requires
• Moisture convergence (the source of moisture)
• Vertical motion (which cools the moist air)
• Condensation of water vapor onto nuclei
• If atmospheric conditions are favorable,
  condensed water particles (such as liquid drops
  or ice) may grow to sizes large enough to produce
  precipitation at the ground surface.

4
Vapor Pressure (e)

• Measure of how much water vapor is in the air
• Saturated vapor pressure (esat) e for saturated
  conditions
• Which type of air mass has the capacity to hold
  more moisture a warm air mass or a cold air
  mass?
• WHY?

5
Hornsberger et al., 1998
6
Which is denser, moist air or dry air

• Dry Air
• Effective molecular weight 21 O2 78 N2
• (21 x 32) (78 x 28) 7 22 29
• Pure Water Vapor
• Molecular Weight H2 O 2 16 18
• Moist Air
• Effective Molecular weight 96 Dry Air 4
  Water Vapor
• (96 x 29) (4 x 18) 28.5
• So moist air is less dense (less mass / volume)
  than dry air.
• What would happen if the moist air begins to rise?

7
Conditions for Precipitation

• Cooling of the air mass
• How does an air mass cool?

8
(No Transcript)
9
Mechanisms of Lift
10
Precipitation Systems

• The major types of precipitating systems observed
  over the globe include
• Thunderstorms (including squall lines and
  tornadoes)
• Tropical systems (including hurricanes, typhoons,
  and tropical cloud clusters)
• Frontal Storms (also known as extratropical
  cyclones)
• Orographic storms (produced by forced lifting of
  air over mountain barriers)

11
Convective Precipitation

• Air expands when heated by solar energydecreases
  density
• Creates instability and low pressure at the
  surface
• Common during summer days in humid areas
• Most thunderstorms are convective

12
Severe Thunderstorms in Midwest
13
Tropical Cyclones

• Larger than thunderstorms
• Associated with Low latitudes
• Impt. Facet of Atmospheric/Ocean interaction
• Terminology
• HurricanesGulf of MX and Atlantic
• TyphoonsPacific
• CyclonesIndian Ocean

14
Hurricane Fran
15
Frontal/Extratropical Cyclones

• Interaction of air masses along a frontal
  boundary
• Dominant precipitation in continental areas
• Strongly associated with High and Low pressure
  centers

16
Snow Storm of the Century
17
Orographic Lift

• Air mass is forced over a geographic high
• Precipitation caused by cooling associated with
  expansion of air mass
• Impt. Source of precip in mountainous areas

18
(No Transcript)
19
Adiabatic Temperature Change

• ADIABATIC COOLING
• When a parcel of air expands it pushes outward
  against the pressure exerted by the environment
• The energy required to expand the parcel comes
  from the heat in the air parcel
• Thus, there is less heat energy in the air and
  the temperature is lowered.
• The term "adiabatic" refers to the idealized
  situation that no energy enters or leaves the
  parcel except through expansion
• Conduction, radiation and mixing of surrounding
  air can be ignored.

20
Adiabatic Temperature Change

• ADIABATIC WARMING When air is compressed, energy
  is added to the gas and the temperature
  increases.
• Adiabatic warming or cooling depends only on the
  net change in the pressure of the air parcel, not
  on how fast it occurs

21
Dry Adiabatic Lapse Rate

• Based on the rate that pressure decreases with
  altitude, the rate of decrease of temperature can
  be calculated to be 10C per kilometer ( 5.5F
  per 1000 ft).
• If the air parcel descends its temperature
  increase at the same rate.
• The dry adiabatic lapse rate applies to air that
  is not saturated.The actual lapse is usually
  less than the dry adiabatic lapse rate due to
  radiation, entrainment, and condensation.
• Mixing of surrounding air into an air parcel is
  called entrainment.

22
(No Transcript)
23
Wet Adiabatic Lapse Rate

• When saturated air rises it expands and cools but
  this causes moisture to condense and release
  latent heat so the cooling with altitude is less
  than the dry adiabatic lapse rate.The wet
  adiabatic lapse rate depends on temperature and
  varies between 5 and 9C per kilometer.
• The wet adiabatic lapse rate is smallest in warm
  moist air since lots of latent heat is released.
  It is only slightly less than the dry adiabatic
  lapse rate for cold dry (but still saturated) air
  since little latent heat is released

24
Conditions for Precipitation

• Cooling of the air mass
• Condensation phase change from gas to liquid
• Requires condensation nuclei small particle of
  dust, previously formed ice or water drop, salt
  from ocean, clays, nitrogen oxides, etc.
• Coalescence of water particles to form drops
• Growth of drops until gravity is able to bring to
  Earths surface

25
Mean Annual Precipitation

• The National Weather Service (NWS) computes the
  "normal" precipitation to determine the
  climatology of a region. The normal is defined as
  the average over the most recent 30-year period
  (currently, from 1971-2000).The following maps
  were produced using an expert system called PRISM
  (Paratermeter-elevation Regressions of
  Independent Slopes Model), developed by Chris
  Daly and others Daly et al., 1994 at Oregon
  State University. Additional climatological maps
  are available from the PRISM Mapping Project.

26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
Include Temporal Variation
32
(No Transcript)