Precipitation Hydrology (Spring 2011) Illinois State University Instructor: Eric Peterson
Precipitation - General • Deposition of liquid water and ice particles from the atmosphere to Earth’s surface • Gravity pulls to Earth’s surface • Dominant deposition mechanism for moisture from the atmosphere
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.
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?
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?
Conditions for Precipitation • Cooling of the air mass • How does an air mass cool?
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)
Convective Precipitation • Air expands when heated by solar energy—decreases density • Creates instability and low pressure at the surface • Common during summer days in humid areas • Most thunderstorms are convective
Tropical Cyclones • Larger than thunderstorms • Associated with Low latitudes • Impt. Facet of Atmospheric/Ocean interaction • Terminology • Hurricanes—Gulf of MX and Atlantic • Typhoons—Pacific • Cyclones—Indian Ocean
Frontal/Extratropical Cyclones • Interaction of air masses along a frontal boundary • Dominant precipitation in continental areas • Strongly associated with High and Low pressure centers
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
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.
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
Dry Adiabatic Lapse Rate • Based on the rate that pressure decreases with altitude, the rate of decrease of temperature can be calculated to be 10°Cper kilometer ( 5.5°Fper 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.
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 9°Cper 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
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 Earth’s surface
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.