MOISTURE, CLOUDS & PRECIPITATION CONDENSATION AND CLOUD FORMATION

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1 MOISTURE, CLOUDS & PRECIPITATION WHEN IT COMES TO UNDERSTANDING ATMOSPHERIC PROCESSES, WATER VAPOR IS THE MOST IMPORTANT GAS IN THE ATMOSPHERE IMPORTANT HEAT ABSORBING GAS CRITICAL TO THE HEATING OF THE ATMOSPHERE WATER S CHANGES OF STATE SOURCE OF ALL CONDENSATION AND PRECIPITATION WATER VAPOR IS AN ODORLESS, COLORLESS GAS THAT MIXES FREELY WITH THE OTHER GASES LATENT HEAT CAN CHANGE STATES AT TEMPERATURES AND PRESSURES EXPERIENCED ON EARTH PRODUCES HYDROLOGIC CYCLE THE PROCESS OF CHANGING STATE REQUIRES THAT HEAT BE ABSORBED OR RELEASED MEASURED IN CALORIES ONE CALORIE IS THE HEAT NECESSARY TO RAISE THE TEMPERATURE OF ONE GRAM OF WATER 1 C (1.8 F) THUS, WHEN 10 CALORIES OF HEAT ARE ADDED TO 1 GRAM OF WATER, A 10 C (18 F) TEMPERATURE RISE OCCURS EVAPORATION TAKES APPROXIMATELY 600 CALORIES OF ENERGY TO CONVERT 1 GRAM OF WATER TO WATER VAPOR LATENT HEAT OF VAPORIZATION HIGHER TEMP (FAST) MOLECULES ESCAPE THE SURFACE AVERAGE MOLECULAR MOTION (TEMP) OF THE REMAINING WATER IS REDUCED EVAPORATION IS A COOLING PROCESS CONDENSATION PROCESS WHEREBY WATER VAPOR CHANGES TO THE LIQUID STATE CREATES CLOUDS AND FOG WATER MOLECULES MUST RELEASE THEIR STORED HEAT ENERGY LATENT HEAT OF CONDENSATION MELTING REQUIRES THE ABSORPTION OF 80 CALORIES OF HEAT PER GRAM OF WATER LATENT HEAT OF MELTING LATENT HEAT OF FUSION (FREEZING) RELEASES 80 CALORIES SUBLIMATION CONVERSION OF A SOLID DIRECTLY TO A GAS DEPOSITION VAPOR TO A SOLID (E.G. FROST) HUMIDITY: WATER VAPOR IN THE ATMOSPHERE WATER VAPOR IS THE MOST IMPORTANT GAS IN THE ATMOSPHERE HUMIDITY IS THE GENERAL TERM FOR THE AMOUNT OF WATER VAPOR IN THE AIR Shultz revisin f Schtt 1

2 SATURATION SATURATED AIR IS AIR THAT IS FILLED WITH WATER VAPOR TO CAPACITY CAPACITY IS TEMPERATURE DEPENDENT WARM AIR HAS A MUCH GREATER CAPACITY. RELATIVE HUMIDITY MORE MOISTURE IS REQUIRED FOR SATURATION WATER VAPOR ADDS PRESSURE (CALLED VAPOR PRESSURE) TO THE AIR PART OF THE TOTAL ATMOSPHERIC PRESSURE THAT CAN BE ATTRIBUTED TO THE WATER VAPOR CONTENT. FAMILIAR YET MISUNDERSTOOD RELATIVE HUMIDITY IS A RATIO FOR THE AIR S ACTUAL WATER VAPOR CONTENT COMPARED WITH THE AMOUNT OF WATER VAPOR REQUIRED FOR SATURATION AT THAT TEMPERATURE (AND PRESSURE) INDICATES HOW NEAR THE AIR IS TO SATURATION, RATHER THAN THE ACTUAL QUANTITY OF WATER VAPOR IN THE AIR RELATIVE HUMIDITY CAN BE CHANGED IN TWO WAYS ADD OR SUBTRACT MOISTURE CHANGE THE AIR TEMPERATURE DEW POINT MEASURE OF HUMIDITY TEMPERATURE TO WHICH A PARCEL OF AIR WOULD NEED TO BE COOLED TO REACH SATURATION MEASURE OF ACTUAL MOISTURE CONTENT FOR EVERY 10 C (18 F) INCREASE IN THE DEW-POINT TEMPERATURE, THE AIR CONTAINS ABOUT TWICE AS MUCH WATER VAPOR COOLING THE AIR BELOW THE DEW-POINT CAUSES CONDENSATION DEW, FOG, OR CLOUD FORMATION WATER VAPOR REQUIRES A SURFACE TO CONDENSE ON MEASURING HUMIDITY COMMONLY MEASURED USING A HYGROMETER HYGRO = MOISTURE, METION = MEASURING PSYCHROMETER- A DEVICE CONSISTING OF TWO THERMOMETERS (WET BULB AND DRY BULB) THAT IS RAPIDLY WHIRLED AND, WITH THE USE OF TABLES, YIELDS THE RELATIVE HUMIDITY AND DEW POINT DRY BULB- GIVES AIR TEMPERATURE WET BULB- HAS A THIN MUSLIN WICK TIED AROUND THE END THE CLOTH SLEEVE IS SATURATED WITH WATER AND A CONTINUOUS CURRENT OF AIR IS PASSED OVER THE WICK Shultz revisin f Schtt 2

3 ASPIRATED PSYCHROMETER SMALL FAN IS USED TO VENTILATE SLING PSYCHROMETER HAS AN ATTACHED HANDLE THAT ALLOWS IT TO BE WHIRLED WATER EVAPORATES FROM THE WICK, AND THE HEAT ABSORBED BY THE EVAPORATING WATER MAKES THE TEMPERATURE OF THE WET BULB DROP THE DRIER THE AIR, THE GREATER THE EVAPORATION AND THE LOWER THE WET-BULB TEMPERATURE COMPARED TO THE DRY-BULB LARGER DIFFERENCE = LOWER RELATIVE HUMIDITY SMALLER DIFFERENCE = HIGHER RELATIVE HUMIDITY THE DIFFERENCE BETWEEN DRY BULB AND WET BULB IS THE WET-BULB DEPRESSION HAIR HYGROMETER OPERATES ON THE PRINCIPLE THAT HAIR OR CERTAIN SYNTHETIC FIBERS CHANGE THEIR LENGTH IN PROPORTION TO CHANGES IN THE RELATIVE HUMIDITY LENGTHENS AS RELATIVE HUMIDITY INCREASES SHRINKS AS RELATIVE HUMIDITY DROPS ELECTRIC HYGROMETER CONTAINS AN ELECTRICAL CONDUCTOR COATED WITH A MOISTURE-ABSORBING CHEMICAL WORKS ON THE PRINCIPLE THAT THE PASSAGE OF CURRENT VARIES AS THE RELATIVE HUMIDITY VARIES Q: WHY IS THE AIR IN BUILDINGS SO DRY IN WINTER? A: IF H20 (G) REMAINS CONSTANT, AN INCREASE IN TEMP CAUSES A DECREASE IN RELATIVE HUMIDITY. DURING WINTER, OUTSIDE AIR IS COOL WITH LOWER DEW POINTS. THIS AIR IS DRAWN INTO THE HOME AND HEATED CAUSES HUMIDITY TO PLUNGE! EXAMPLE: FIND THE RELATIVE HUMIDITY IF THE ACTUAL AIR TEMP. IS 15 C (59 F) AND THE WET BULB TEMP. IS 11 C (51.8 F) STEP #1: FIND THE WET-BULB DEPRESSION: USE THE APPROPRIATE TABLE. FIND THE DEW POINT IF THE ACTUAL AIR TEMPERATURE IS 25 C AND THE WET-BULB TEMPERATURE IS 20 C. Shultz revisin f Schtt 3

4 THE WET-BULB TEMPERATURE IS ALWAYS HIGHER THAN THE DEWPOINT UNLESS THE AIR IS SATURATED. THE DEWPOINT, WET-BULB TEMPERATURE, AND ACTUAL AIR TEMPERATURE ARE THE SAME ONLY WHEN THE RELATIVE HUMIDITY IS THE PROCESS OF CLOUD FORMATION: ADIABATIC COOLING SATURATION OCCURS EITHER WHEN SUFFICIENT WATER VAPOR IS ADDED TO THE AIR OR, MORE COMMONLY, WHEN AIR IS COOLED TO ITS DEW POINT NEAR EARTH S SURFACE, HEAT IS READILY EXCHANGED BETWEEN THE GROUND AND THE AIR ABOVE DURING THE EVENING, THE SURFACE RADIATES HEAT AWAY, CAUSING THE SURFACE AND ADJACENT AIR TO COOL RAPIDLY CREATES DEW AND SOME TYPES OF FOG CLOUD FORMATION OFTEN TAKES PLACE DURING THE WARMEST PART OF THE DAY ADIABATIC TEMPERATURE CHANGES WHEN ENERGY IS USED TO COMPRESS AIR, THE MOTION OF THE GAS MOLECULES INCREASES AND THEREFORE THE AIR TEMPERATURE RISES DESCENDING AIR IS COMPRESSED DUE TO INCREASING PRESSURE AS AIR EXPANDS IT COOLS BECAUSE THE AIR PUSHES (DOES WORK ON) THE SURROUNDING AIR AND MUST COOL BY AN AMOUNT EQUIVALENT TO THE ENERGY EXPENDED RISING AIR WILL EXPAND DUE TO DECREASING AIR TEMPERATURE ADIABATIC TEMPERATURE CHANGE- COOLING OR WARMING OF AIR CAUSED WHEN AIR IS ALLOWED TO EXPAND OR IS COMPRESSED, NOT BECAUSE HEAT IS ADDED OR SUBTRACTED EX) HAIRSPRAY, MOUSSE, ETC. ADIABATIC COOLING AND CONDENSATION METEOROLOGISTS OFTEN CONSIDER AN IMAGINARY VOLUME OF AIR ENCLOSED IN A THIN ELASTIC COVER, CALLED A PARCEL TYPICALLY CONSIDERED TO BE A FEW HUNDRED CUBIC METERS IN VOLUME AND IS ASSUMED TO ACT INDEPENDENTLY OF THE SURROUNDING AIR NO HEAT TRANSFER INTO OR OUT OF THE PARCEL DRY ABDIABATIC RATE ASCENDING AIR EXPANDS AND COOLS ADIABATICALLY UNSATURATED (DRY) AIR COOLS AT THE CONSTANT RATE OF 10 C PER 1000 M THIS IS THE DRY ADIABATIC RATE 1 C/100 M OR 5.5 F PER 1000 FT WET ADIABATIC RATE Shultz revisin f Schtt 4

5 IF A PARCEL OF AIR RISES HIGH ENOUGH, IT WILL EVENTUALLY COOL TO ITS DEW POINT PROCESS OF CONDENSATION BEGINS AIR HAS REACHED THE DEW POINT CONDENSATION IS OCCURRING AND LATENT HEAT IS BEING LIBERATED HEAT RELEASED BY THE CONDENSING WATER REDUCES THE RATE OF COOLING WET ADIABATIC RATE VARIES FROM 5 C AND 9 C PER 1000 M AIR WITH HIGH MOISTURE CONTENT 5 C (WARM AIR) AIR WITH LOW MOISTURE CONTENT 9 C (USUALLY COLD AIR) FOR CONVENIENCE 6 C (33 F) THE WEATHERMAKER: ATMOSPHERIC STABILITY STABILITY CAN BE DETERMINED BY COMPARING THE PARCEL S TEMPERATURE TO THAT OF THE SURROUNDING AIR A COOLER PARCEL IS MORE DENSE AND WOULD SINK AIR OF THIS TYPE, CALLED STABLE AIR, RESISTS VERTICAL MOVEMENT IF A RISING PARCEL WAS WARMER AND LESS DENSE THAN THE SURROUNDING AIR, IT WOULD CONTINUE TO RISE UNTIL IT REACHED AN ALTITUDE WHERE IT S TEMP. EQUALED THAT OF ITS SURROUNDINGS CALLED UNSTABLE AIR AN ASCENDING PARCEL BECOMES COOLER (DENSER) THAN AMBIENT AIR AND A DESCENDING PARCEL BECOMES WARMER TYPES OF STABILITY STABILITY OF AIR IS DETERMINED BY MEASURING THE TEMPERATURE OF THE ATMOSPHERE AT VARIOUS HEIGHTS ENVIRONMENTAL LAPSE RATE ABSOLUTE STABILITY PREVAILS WHEN THE ENVIRONMENTAL LAPSE RATE IS LESS THAN THE WET ADIABATIC RATE MOST STABLE CONDITIONS OCCUR WHEN THE TEMP. IN A LAYER OF AIR ACTUALLY INCREASES WITH ALTITUDE. WHEN SUCH A REVERSAL OCCURS, A TEMPERATURE INVERSION IS SAID TO EXIST. FREQUENTLY OCCURS ON CLEAR NIGHTS AS A RESULT OF RADIATIONAL COOLING OF EARTH S SURFACE CREATED BECAUSE THE GROUND AND THE AIR IMMEDIATELY ABOVE WILL COOL MORE RAPIDLY THAN AIR ALOFT Shultz revisin f Schtt 5

6 WHEN WARM AIR OVERLIES COOLER AIR, IT ACTS AS A LID AND PREVENTS APPRECIABLE VERTICAL MIXING TRAPS POLLUTANTS NEAR THE SURFACE SURFACE INVERSION ABSOLUTE INSTABILITY OCCURS WHEN THE ENVIRONMENTAL LAPSE RATE IS GREATER THAN THE DRY ADIABATIC RATE ASCENDING PARCEL OF AIR IS ALWAYS WARMER THAN ITS ENVIRONMENT AND WILL CONTINUE TO RISE BECAUSE OF ITS OWN BUOYANCY PRODUCES SMALL FLUFFY CLOUDS FAIR WEATHER CONDITIONAL INSTABILITY MOIST AIR WITH A LAPSE RATE BETWEEN THE DRY AND WET ADIABATIC RATES BETWEEN 5 & 10 C PER 1000 METERS THE ATMOSPHERE IS SAID TO BE CONDITIONALLY UNSTABLE WHEN IT IS STABLE FOR AN UNSATURATED PARCEL OF AIR, BUT UNSTABLE FOR A SATURATED PARCEL QUICK SUMMARY A COLUMN OF AIR IS DEEMED UNSTABLE WHEN THE AIR NEAR THE BOTTOM OF THIS LAYER IS SIGNIFICANTLY WARMER (LESS DENSE) THAT THE AIR ALOFT INDICATES A STEEP ENVIRONMENTAL LAPSE RATE UNDER THESE CONDITIONS THE AIR ACTUALLY TURNS OVER, AS THE WARM AIR BELOW RISES AND DISPLACES THE COLDER AIR ALOFT AIR IS CONSIDERED TO BE STABLE WHEN THE TEMP DROPS GRADUALLY WITH INCREASING ALTITUDE VERY LITTLE VERTICAL MOVEMENT STABILITY & DAILY WEATHER STABLE AIR RESISTS UPWARD MOVEMENT AND WITHOUT OUTSIDE PROCESSES FORCING AIR ALOFT THERE WOULD NOT BE ANY CLOUDS WHEN STABLE AIR IS FORCED ALOFT, THE CLOUDS THAT FORM ARE WIDESPREAD AND HAVE LITTLE VERTICAL THICKNESS WHEN COMPARED TO THEIR HORIZONTAL DIMENSION PRECIPITATION, IF ANY, IS LIGHT TO MODERATE CLOUDS ASSOCIATED WITH THE LIFTING OF UNSTABLE AIR ARE TOWERING AND OFTEN GENERATE THUNDERSTORMS AND POSSIBLY TORNADOES UNSTABLE AIR CREATES CAULIFLOWER SHAPED CLOUDS THAT APPEAR TO BE GROWING AS IF BUBBLES OF HOT AIR ARE SURGING UPWARD Shultz revisin f Schtt 6

7 SATURATION OCCURS MOST COMMONLY WHEN AIR IS COOLED TO ITS DEW POINT, OR LESS OFTEN WHEN WATER VAPOR IS ADDED TO AIR THERE MUST BE A SURFACE ON WHICH THE WATER VAPOR CAN CONDENSE WHEN DEW OCCURS, OBJECTS AT OR NEAR THE GROUND SERVES THIS PURPOSE IN THE AIR, TINY BITS OF PARTICULATE MATTER CALLED CONDENSATION NUCLEI SERVE AS THE SURFACE IN THEIR ABSENCE A RELATIVE HUMIDITY >100% IS NEEDED TO PRODUCE CLOUDS CONDENSATION NUCLEI SUCH AS MICROSCOPIC DUST, SMOKE AND SALT PARTICLES ARE PROFUSE IN THE AIR IN THE LOWER ATMOSPHERE THE ABUNDANCE OF CONDENSATION NUCLEI CAUSES RELATIVE HUMIDITY TO RARELY EXCEED 101% SALT IS A PARTICULARLY GOOD NUCLEI BECAUSE IT ABSORBS WATER HYGROSCOPIC NUCLEI HYGRO = MOISTURE, SCOPIC = TO SEEK CLOUDS CONSIST OF MILLIONS UPON MILLIONS OF TINY WATER DROPLETS, ALL SO FINE THEY REMAIN SUSPENDED IN AIR TYPES OF CLOUDS MAY CONSIST OF WATER DROPLETS, ICE CRYSTALS, OR BOTH VISIBLE AGGREGATES OF MINUTE DROPLETS OF WATER OR TINY CRYSTALS OF ICE PROVIDE A VISIBLE INDICATION OF WHAT IS GOING ON IN THE ATMOSPHERE CLASSIFIED ON THE BASIS OF THEIR FORM AND HEIGHT 1) CIRRUS (= A CURL OF HAIR) HIGH, WHITE AND THIN. OCCUR AS PATCHES OR AS DELICATE VEIL-LIKE SHEETS OR EXTENDED WISPY FIBERS THAT OFTEN HAVE A FEATHERY APPEARANCE 2) CUMULUS (= A PILE) CONSIST OF GLOBULAR INDIVIDUAL CLOUD MASSES. NORMALLY EXHIBIT A FLAT BASE AND HAVE THE APPEARANCE OF RISING DOMES OR TOWERS. CAULIFLOWER STRUCTURE 3) STRATUS (= A LAYER) SHEETS OR LAYERS THAT COVER MUCH OR ALL OF THE SKY. NO DISTINCT INDIVIDUAL CLOUD UNITS 3 LEVELS OF CLOUD HEIGHTS 1) HIGH BASES > 6000 M (20,000 FT) 2) MIDDLE HEIGHTS BETWEEN M ( FT) 3) LOW FORM BELOW 2000 M HIGH CLOUDS 3 CLOUD TYPES MAKE UP THE FAMILY OF HIGH CLOUDS 1) CIRRUS THIN AND DELICATE AND SOMETIMES APPEAR AS HOOK FILAMENT MARE S TAILS Shultz revisin f Schtt 7

8 2) CIRROCUMULUS FLUFFY MASSES 3) CIRROSTRATUS FLAT LAYERS BECAUSE OF LOW TEMPERATURES AND SMALL QUANTITIES OF WATER VAPOR AT HIGH ALTITUDES, ALL HIGH CLOUDS ARE THIN AND WHITE AND ARE MADE UP OF ICE CRYSTALS NOT CONSIDERED PRECIPITATION MAKERS MIDDLE CLOUDS HAVE THE PREFIX ALTO 1) ALTOCUMULUS COMPOSED OF LARGE, DENSE GLOBULAR MASSES 2) ALTOSTRATUS CREATE A UNIFORM WHITE TO GRAYISH SHEET CAUSING THE SKY WITH THE SUN OR MOON VISIBLE AS A BRIGHT SPOT A. INFREQUENT LIGHT SNOW OR DRIZZLE MAY OCCUR LOW CLOUDS 3 MEMBERS 1) STRATUS UNIFORM FOGLIKE LAYER OF CLOUDS THAT COVER MUCH OF THE SKY, MAY PRODUCE LIGHT PRECIPITATION 2) WHEN THEY DEVELOP A SCALLOPED BOTTOM THAT APPEARS AS LONG PARALLEL ROLLS OR BROKEN GLOBULAR PATCHES = STRATOCUMULUS 3) NIMBOSTRATUS NIMBUS = RAINY CLOUD, STRATUS = TO COVER WITH A LAYER A. CHIEF PRECIPITATION MAKER B. FORM IN ASSOCIATION WITH STABLE CONDITIONS (FORCED TO RISE) CLOUDS OF VERTICAL DEVELOPMENT SOME CLOUDS DO NOT FIT INTO ANY ONE OF THE THREE HEIGHT CATEGORIES HAVE BASES IN THE LOW HEIGHT RANGE BUT OFTEN EXTEND UPWARD INTO THE MIDDLE OR HIGH ALTITUDES CLOUDS OF VERTICAL DEVELOPMENT ASSOCIATED WITH UNSTABLE AIR CUMULONIMBUS FORMED WHEN CUMULUS CLOUDS GROW DRAMATICALLY DUE TO POWERFUL UPWARD MOVEMENT AND MAY PRODUCE RAIN OR THUNDERSTORM FOG A CLOUD WITH ITS BASE AT OR VERY NEAR THE GROUND FORM AS A CONSEQUENCE OF RADIATION COOLING OR THE MOVEMENT OF AIR OVER A COLD SURFACE OR WHEN ENOUGH WATER VAPOR IS ADDED TO THE AIR TO BRING ABOUT SATURATION FOG CAUSED BY COOLING Shultz revisin f Schtt 8

9 ADVECTION FOG OCCURS WHEN WARM, MOIST AIR MOVES OVER A COOL SURFACE RADIATION FOG FORMS ON COOL, CLEAR, CALM NIGHTS, WHEN EARTH S SURFACE COOLS RAPIDLY BY RADIATION AS NIGHT PROGRESSES, A THIN LAYER OF AIR IN CONTACT WITH THE GROUND IS COOLED BELOW ITS DEW POINT CREATES POCKETS OF FOG UPSLOPE FOG CREATED WHEN RELATIVELY HUMID AIR MOVES UP A GRADUALLY SLOPING PLAIN OR UP THE STEEP SLOPES OF A MOUNTAIN AIR EXPANDS AND COOLS ADIABATICALLY EVAPORATION FOGS CREATED WHEN WATER VAPOR IS ADDED TO THE AIR STEAM FOG WHEN COOL AIR MOVES OVER WARM WATER, MOISTURE MAY EVAPORATE FROM THE WATER SURFACE TO PRODUCE SATURATION AS THE RISING WATER VAPOR MEETS THE COLD AIR, IT IMMEDIATELY RECONDENSES AND RISES WITH THE AIR THAT IS BEING WARMED FROM BELOW GIVES THE WATER A STEAMY APPEARANCE FRONTAL FOG FORMS DURING FRONTAL WEDGING WHEN WARM AIR IS LIFTED OVER COLDER AIR. HOW PRECIPITATION FORMS CLOUD DROPLETS ARE VERY TINY, AVERAGING UNDER 20 µm (0.02 MM) IN DIAMETER HUMAN HAIR IS ABOUT 75 µm CCN IS ABUNDANT AND AVAILABLE WATER IS DISTRIBUTED AMONG NUMEROUS DROPLETS RATHER THAN CONCENTRATED INTO FEWER LARGE DROPLETS CAUSING SMALL SIZED CLOUD DROPLETS DUE TO THEIR SMALL SIZE, THE RATE AT WHICH CLOUD DROPLETS FALL IS INCREDIBLY SLOW A TYPICAL RAINDROP HAS A DIAMETER OF ABOUT 2000 µm (2 MM) OR 100X THAT OF AN AVERAGE CLOUD DROPLET HAVING A DIAMETER OF 20 µm THE VOLUME OF A TYPICAL RAINDROP IS A MILLION TIMES THAT OF A CLOUD DROPLET FOR PRECIPITATION TO FORM, CLOUD DROPLETS MUST GROW IN VOLUME BY ROUGHLY ONE MILLION TIMES TWO MECHANISMS THAT GIVE RISE TO THESE MASSIVE DROPS: 1. BERGERON PROCESS 2. COLLISION-COALESCENCE PROCESS Shultz revisin f Schtt 9

10 PRECIPITATION FROM COLD CLOUDS: THE BERGERON PROCESS FRIGID CONDITIONS HIGH IN THE TROPOSPHERE PROVIDE AN IDEAL ENVIRONMENT TO INITIATE PRECIPITATION THE PROCESS THAT GENERATES MUCH OF THE PRECIPITATION IN THE MIDDLE LATITUDES IS NAMED THE BERGERON PROCESS DISCOVERED BY A HIGHLY RESPECTED SWEDISH METEOROLOGIST TOR BERGERON THE PROCESS RELIES ON SUPERCOOLING AND SUPERSATURATION SUPERCOOLING PURE WATER SUSPENDED IN AIR DOES NOT FREEZE UNTIL IT REACHES -40 C WATER IN THE LIQUID STATE BELOW 0 C IS REFERRED TO AS SUPERCOOLED CALLED FREEZING NUCLEI VERY SPARSE IN THE ATMOSPHERE DO NOT BECOME ACTIVE UNTIL TEMP REACHES -10 C (-14 F) OR LESS ONLY AT TEMPS WELL BELOW FREEZING WILL ICE CRYSTALS BEGIN TO FORM IN CLOUDS SUPERSATURATION WHEN AIR IS SATURATED (100% RELATIVE HUMIDITY) WITH RESPECT TO WATER, IT IS SUPERSATURATED (RELATIVE HUMIDITY >100%) WITH RESPECT TO ICE ICE CRYSTALS CANNOT COEXIST WITH WATER DROPLETS, BECAUSE THE AIR ALWAYS APPEARS SUPERSATURATED TO THE ICE CRYSTALS ICE CRYSTALS BEGIN TO CONSUME THE EXCESS WATER VAPOR, WHICH LOWERS THE RELATIVE HUMIDITY NEAR THE SURROUNDING DROPLETS WATER DROPLETS EVAPORATE TO REPLENISH THE DIMINISHING WATER VAPOR, THEREBY PROVIDING A CONTINUAL SOURCE OF VAPOR FOR THE GROWTH OF THE ICE CRYSTALS THE LEVEL OF SUPERSATURATION WITH RESPECT TO ICE CAN BE QUITE GREAT AND THE GROWTH OF ICE CRYSTALS IS GENERALLY RAPID ENOUGH TO GENERATE CRYSTALS LARGE ENOUGH TO FALL SUMMARY OF BERGERON PROCESS BERGERON PROCESS CAN PRODUCE PRECIPITATION THROUGHOUT THE YEAR IN THE MID LATITUDES PROVIDED THE TEMPERATURES IN THE UPPER PORTIONS OF CLOUDS ARE COLD ENOUGH TO GENERATE ICE CRYSTALS ICE CRYSTALS COLLECT WATER VAPOR FORMS LARGE SNOW FLAKES THE TYPE OF PRECIPITATION THAT REACHES THE GROUND DEPENDS ON THE TEMPERATURE PROFILE IN THE LOWER FEW KILOMETERS OF THE ATMOSPHERE Shultz revisin f Schtt 10

11 PRECIPITATION FROM WARM CLOUDS: THE COLLISION-COALESCENCE PROCESS CLOUDS COMPOSED ENTIRELY OF LIQUID DROPLETS MUST CONTAIN SOME DROPLETS LARGER THAN 20 µm (0.02 MM) IF PRECIPITATION IS TO FORM THESE LARGE DROPLETS FORM WHERE GIANT CONDENSATION NUCLEI ARE PRESENT, OR WHEN HYDROSCOPIC PARTICLES SUCH AS SEA SALT EXIST HYDROSCOPIC PARTICLES BEGIN TO REMOVE WATER VAPOR FROM THE AIR AT RELATIVE HUMIDITIES UNDER 100% AND CAN GROW QUITE LARGE AS THESE DROPLETS PLUMMET, THEY COLLIDE WITH SMALLER, SLOWER DROPLETS AND COALESCE AS THEY GROW LARGER, THEY FALL FASTER AND INCREASE THEIR CHANCES OF COLLISION AND RATE OF GROWTH BECAUSE THE NUMBER OF COLLISIONS REQUIRED FOR GROWTH TO RAINDROP SIZE, DROPLETS IN CLOUDS THAT HAVE GREAT VERTICAL THICKNESS AND ABUNDANT MOISTURE HAVE A BETTER CHANCE OF REACHING THE REQUIRED SIZE RAINDROPS CAN GROW TO A MAXIMUM SIZE OF 5 MM, AT WHICH POINT THEY FALL AT THE RATE OF 33 KM/HR (20 MPH) AT THIS SIZE AND SPEED, THE WATER S SURFACE TENSION IS OVERCOME BY THE DRAG IMPOSED BY THE AIR, WHICH PULLS THE DROP APART PRODUCES NUMEROUS SMALLER DROPLETS DROPS THAT ARE LESS THAN 0.5 MM UPON REACHING THE GROUND ARE TERMED DRIZZLE AND REQUIRE ABOUT TEN MINUTES TO FALL FROM A CLOUD 1000 M (3300 FT) OVERHEAD FORMS OF PRECIPITATION MUCH OF THE WORLD S PRECIPITATION BEGINS AS SNOW CRYSTALS OR OTHER SOLID FORMS, SUCH AS HAIL OR GRAUPEL PARTICLES MELT AND REACH THE GROUND AS RAINDROPS RAIN AND DRIZZLE RAIN DROPLETS HAVE AT LEAST A 0.5 MM DIAMETER MOST ORIGINATES IN NIMBOSTRATUS CLOUDS OR TOWERING CUMULONIMBUS CLOUDS THAT ARE CAPABLE OF PRODUCING UNUSUALLY HEAVY RAINFALLS KNOWN AS CLOUDBURSTS DRIZZLE UNIFORM DROPLETS HAVING A DIAMETER LESS THAN 0.5 MM (0.002 INCHES) GENERALLY PRODUCED IN STRATUS OR NIMBOSTRATUS CLOUDS SNOW ICE CRYSTALS, OR AGGREGATES OF ICE CRYSTALS Shultz revisin f Schtt 11

12 SIZE, SHAPE AND CONCENTRATION OF SNOWFLAKES DEPEND TO A GREAT EXTENT ON THE TEMPERATURE AT WHICH THEY FORM AT VERY LOW TEMPS, MOISTURE CONTENT IS LOW RESULTS IN THE FORMATION OF VERY LIGHT, FLUFFY SNOW MADE UP OF INDIVIDUAL SIX-SIDED ICE CRYSTALS POWDER AT TEMPERATURES WARMER THAN -5 C (23 F), THE ICE CRYSTALS JOIN TOGETHER INTO LARGER CLUMPS CONSISTING OF TANGLED AGGREGATES OF CRYSTALS HEAVY, WET SNOW SLEET AND GLAZE SLEET IS A WINTERTIME PHENOMENA AND REFERS TO THE FALL OF SMALL PARTICLES OF ICE THAT ARE CLEAR OCCURS WHEN WARMER AIR OVERLIES COLDER AIR AND RAIN FREEZES AS IT FALLS GLAZE, OR FREEZING RAIN, CONSISTS OF RAIN DROPS THAT BECOME SUPERCOOLED AND AT LEAST PARTIALLY FREEZE ON CONTACT WITH COLD SURFACES (AT SUBFREEZING TEMPERATURES) FORMING A COATING OF ICE (GLAZE) ON ROADS, TREE BRANCHES, AND OTHER EXPOSED SURFACES DEVELOPS THE SAME WAY AS SLEET EXCEPT THAT THE LAYER OF SUBFREEZING AIR AT THE SURFACE IS SHALLOWER HAIL PRECIPITATION IN THE FORM OF HARD, ROUNDED PELLETS OR IRREGULAR LUMPS OF ICE OFTEN CHARACTERIZED BY CONCENTRIC LAYERING MOST HAVE DIAMETERS FROM 1 CM (PEA SIZED) TO 5 CM (GOLF BALL) HEAVIEST AUTHENTICATED HAILSTONE FELL ON COFFEYVILLE, KANSAS, SEPT. 3, 1970 WITH A 14 CM (5.5 ) DIAMETER AND WEIGHT OF 766 G (1.67 LBS) PRODUCED ONLY IN LARGE CUMULONIMBUS CLOUDS WITH VIOLENT UP AND DOWN DRAFTS WHERE THERE IS A SUPPLY OF SUPERCOOLED WATER PELLETS GROW BY COLLECTING SUPERCOOLED WATER AS THEY FALL THROUGH THE CLOUD IF THEY ENCOUNTER A STRONG UPDRAFT, THEY MAY BE CARRIED UPWARD AGAIN EACH TRIP THROUGH THE SUPERCOOLED PORTION = ADDITIONAL LAYER OF ICE THE NATIONAL WEATHER SERVICE (NWS) DESIGNATES A THUNDERSTORM AS SEVERE IF HAILSTONES HAVE A DIAMETER OF 1.9 CM (0.75 INCHES) OR GREATER Shultz revisin f Schtt 12

13 RIME A DEPOSIT OF ICE CRYSTALS FORMED BY THE FREEZING OF SUPERCOOLED FOG OR CLOUD DROPLETS ON OBJECTS WHOSE SURFACE TEMP. IS BELOW FREEZING MAY LOOK LIKE ICE FEATHERS MEASURING PRECIPITATION RAIN IS THE EASIEST FORM TO MEASURE USE A STANDARD RAIN GAUGE DIAMETER OF ABOUT 20 CM (8 IN) AT THE TOP USES A FUNNEL TO COLLECT AND CONDUCT RAIN CYLINDRICAL MEASURING TUBE MEASURES RAINFALL IN CM OR INCHES NARROW OPENING MINIMIZES EVAPORATION LESS THAN CM = TRACE RECORDING GAUGE RECORD AMOUNT OF RAIN AND ITS TIME OF OCCURRENCE AND INTENSITY WHEN SNOW RECORDS ARE KEPT, TWO MEASUREMENTS ARE NORMALLY TAKEN 1. DEPTH- MEASURED WITH A CALIBRATED STICK 2. WATER EQUIVALENT- SAMPLES ARE MELTED A) GENERAL RATIO IS 10 SNOW UNITS TO 1 WATER UNIT B) VARIES WIDELY- LIGHT AND FLUFFY 30 CM= 1 CM, WET 4 CM = 1 CM WATER PRECIPITATION MEASUREMENT BY WEATHER RADAR RADAR UNITS HAVE A TRANSMITTER THAT SENDS OUT SHORT PULSES OF RADIO WAVES ECHO IS RECEIVED AND DISPLAYED BRIGHTER= INTENSE PRECIPITATI Shultz revisin f Schtt 13