A LONG-STANDING SCIENTIFIC QUESTION PRODUCING GREAT UNCERTAINTY IN THE CLIMATIC ROLE OF CIRRUS CLOUDS IS WHETHER THEY ARE FORMED THROUGH HOMO- OR HETEROGENEOUS ICE NUCLEATION (HENCEFORTH HOM AND HET). SINCE ICE PARTICLE CONCENTRATIONS IN HOM CIRRUS TYPICALLY RANGE FROM 500 TO 50 000 LITER-1 AND IN HET CIRRUS TYPICALLY RANGE FROM 0.5 TO 500 LITER-1 THE RADIATIVE PROPERTIES OF THE TWO CLOUD TYPES DIFFER MARKEDLY. WE PROVIDE A MEANS OF ANSWERING THIS QUESTION USING CALIPSO (CLOUD-AEROSOL LIDAR AND INFRARED PATHFINDER SATELLITE OBSERVATION) SATELLITE RETRIEVALS OF THE EFFECTIVE ABSORPTION OPTICAL DEPTH RATIO OR EFF BASED ON IMAGING INFRARED RADIOMETER (IIR) CHANNELS AT 12.05 M AND 10.6 M. A NEW PHYSICAL UNDERSTANDING OF EFF REVEALS THAT IT IS A RELATIVE MEASURE OF THE CONCENTRATION OF SMALL ICE CRYSTALS (LENGTH D<60 M) IN AN ICE PARTICLE SIZE DISTRIBUTION (PSD) AND THAT IT IS ALSO TIGHTLY RELATED TO N/IWC WHERE N = ICE PARTICLE NUMBER CONCENTRATION AND IWC = CLOUD ICE WATER CONTENT. COMBINING OUR THEORETICAL KNOWLEDGE OF HOM HET AND EFF WITH IN SITU PSD MEASUREMENTS IT IS DEMONSTRATED THAT EFF CAN BE USED TO DIFFERENTIATE BETWEEN HOM AND HET CONDITIONS. QUALITATIVELY OUR RESULTS SHOW THAT THE OCCURRENCE OF HOM CIRRUS DEPENDS ON LATITUDE SEASON OROGRAPHY AND WHETHER THE CIRRUS ARE OVER LAND OR OCEAN. ONE OF THE GOALS OF THIS PROJECT IS TO QUANTIFY THIS UNDERSTANDING BY EVALUATING WHICH MECHANISM PREVAILS NEAR CLOUD TOP WHERE ICE NUCLEATION GOVERNS THE CIRRUS CLOUD MICROPHYSICAL PROPERTIES. THIS CAN BE DONE BY MATCHING EXISTING AIRCRAFT MEASUREMENTS OF PSDS NEAR THE TOPS OF CIRRUS CLOUDS WITH CORRESPONDING EFF RETRIEVALS AND THEN CALCULATING EFF FROM THESE PSDS. RELATING THE CALCULATED AND RETRIEVED EFF VALUES THE RESULTING EFF RELATIONSHIPS CAN BE COMBINED WITH CALIOP (CLOUD-AEROSOL LIDAR WITH ORTHOGONAL POLARIZATION) RETRIEVALS OF GEOMETRICAL CLOUD THICKNESS TO CHARACTERIZE CIRRUS GLOBALLY AND SEASONALLY IN TERMS OF THE CONTRIBUTIONS OF HOM AND HET NEAR CLOUD TOP. THIS MECHANISTIC UNDERSTANDING OF CIRRUS SHOULD GREATLY IMPROVE EFFORTS TO MODEL THESE CLOUDS IN CLIMATE MODELS. A FINDING FROM OUR PRELIMINARY RESEARCH IS THAT HOM CIRRUS DOMINATES IN THE POLAR REGIONS DURING WINTER WHEN CIRRUS CLOUD COVERAGE IS MAXIMUM AND CONTINUES TO PREVAIL OVER ANTARCTICA DURING SPRING AND FALL. THIS EXERTS A STRONG GREENHOUSE EFFECT ON POLAR SURFACE TEMPERATURES PERHAPS RAISING THEM SEVERAL DEGREES RELATIVE TO HET CIRRUS CONDITIONS. THE OBSERVATION THAT HET CONDITIONS MANIFEST DURING SUMMER (PERHAPS DUE TO INCREASED MINERAL DUST CONCENTRATIONS) INDICATES THAT A DELICATE SEASONAL BALANCE MAY EXIST BETWEEN HOM AND HET CONDITIONS. SINCE THE POLAR REGIONS ARE MOST SENSITIVE TO THE EFFECTS OF CLIMATE CHANGE AND ARE WHERE A CLIMATE TIPPING POINT IS MOST LIKELY TO MANIFEST IT IS IMPORTANT TO UNDERSTAND ALL THE FACTORS DETERMINING POLAR SURFACE TEMPERATURES. A SECOND OBJECTIVE OF THIS PROJECT IS TO EVALUATE WHETHER ANY TREND EXISTS IN THE FRACTION OF HOM CIRRUS IN EACH POLAR REGION OVER AT LEAST A NINE YEAR PERIOD. THE THIRD OBJECTIVE OF THIS PROJECT IS TO ARGUABLY FOR THE FIRST TIME REALISTICALLY REPRESENT THE CIRRUS CLOUD EFFECTIVE DIAMETER DE AND MASS-WEIGHTED ICE FALL SPEED VM IN TWO GLOBAL CLIMATE MODELS (GCMS) AND COMPARE THESE NEW SIMULATIONS WITH THOSE PRODUCED BY THE STANDARD GCM CONFIGURATIONS. REALISTIC SIMULATIONS WOULD BE ACHIEVED BY CONVERTING THE RETRIEVED EFF CLOUD TEMPERATURE (T) RELATIONSHIPS INTO DE T RELATIONSHIPS AND PREDICTING VM FROM DE. THUS THE LATITUDE AND SEASONAL DEPENDENCE OF THESE CIRRUS PROPERTIES WOULD BE ACCOUNTED FOR AS WELL AS THEIR OBSERVED DEPENDENCE ON LAND AND OCEAN SURFACES. COMPARISONS WITH STANDARD SIMULATIONS WOULD REVEAL CURRENT GCM WEAKNESSES AND THE CONSEQUENCES OF THESE WEAKNESSES IN REGARDS TO CIRRUS CLOUDS. THE IMPACT OF ASSUMING ALL CIRRUS ARE HET CIRRUS WOULD ALSO BE EVALUATED.