What is Graupel?!?

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
358 AM CDT THU OCT 18 2012
...WHAT IS THAT FALLING FROM THE SKY...

FOR THE PAST SEVERAL DAYS THE FORECAST HAS INCLUDED THE POTENTIAL
FOR GRAUPEL THIS AFTERNOON AND FRIDAY. SO WHAT IS GRAUPEL? WELL
GRAUPEL IS ICE PELLETS WHICH TEND TO BE WHITE...SMALL...AND MORE
MUSHY THAN TYPICAL THUNDERSTORM HAIL OR SLEET.

SO HOW IS GRAUPEL DIFFERENT THAN SLEET? TO THE UNTRAINED EYE...AND
EVEN TO TRAINED OBSERVERS...IT CAN BE DIFFICULT TO DISTINGUISH
BETWEEN SLEET AND GRAUPEL JUST BY ITS APPEARANCE. HOWEVER...THE
PROCESSES BY WHICH EACH ONE FORMS IS CONSIDERABLY DIFFERENT.

GRAUPEL TENDS TO FORM IN SITUATIONS WHERE VERY COLD TEMPERATURES
ALOFT RESULT IN A RAPID DROP OFF IN TEMPERATURE AS YOU INCREASE IN
ELEVATION. GRAUPEL IS PRODUCED BY THUNDERSTORMS OR SHOWERS THAT
ARE NOT QUITE INTENSE ENOUGH TO PRODUCE LIGHTNING. GRAUPEL FORMS
AS SNOW FLAKES HIGH IN THE CLOUDS ENCOUNTER SUPERCOOLED WATER
DROPLETS...LIQUID WATER THAT HAS A TEMPERATURE BELOW FREEZING BUT
IS STILL LIQUID...CAUSING THE WATER TO INSTANTLY FREEZE INTO SMALL
ICE PELLETS WHICH AS SOMETIMES REFEREED TO AS SNOW PELLETS.
TYPICALLY...GRAUPEL FORMS WHEN TEMPERATURES AT THE SURFACE ARE
ABOVE FREEZING AND IT GENERALLY DOES NOT CAUSE SIGNIFICANT TRAVEL
PROBLEMS DUE TO MELTING.

SLEET ON THE OTHER HAND FORMS IN AN ENTIRELY DIFFERENT MANNER AND
IN AN ENTIRELY DIFFERENT ATMOSPHERIC SET UP. SLEET FORMS WHEN SNOW
FLAKES FALL INTO A LAYER OF ABOVE FREEZING AIR CAUSING THEM TO
PARTIALLY MELT...THEN REFREEZE AS THEY FALL BACK INTO A SUB
FREEZING LAYER OF AIR CLOSE TO THE GROUND. SO UNLIKE
GRAUPEL...TEMPERATURES NEAR THE GROUND WILL ALMOST ALWAYS BE BELOW
FREEZING WITH AN ABOVE FREEZING LAYER OF AIR ABOVE THE GROUND
NECESSARY TO CAUSE THE SNOW TO PARTIALLY MELT. UNLIKE
GRAUPEL...SLEET HAS A GREATER POTENTIAL TO CAUSE TRAVEL PROBLEMS
SHOULD ENOUGH FALL TO ACCUMULATE.

SO IF YOU SEE ICE PELLETS FALLING FROM THE SKY TODAY OR FRIDAY YOU
NOW KNOW THAT YOU ARE SEEING GRAUPEL AND NOT SLEET.

$$

IZZI


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