EMB135 DEPARTED WITHOUT DEICING WHILE LIGHT FREEZING RAIN WAS BEING REPORTED ON ATIS.

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I WAS ACTING AS FO ON A SCHEDULED PART 121 OPERATED FLT FROM STL ARPT. WHILE THE CAPT WAS TENDING TO HIS PREFLT DUTIES; I TENDED TO GETTING ATIS; CLRNC; AND CALCULATING THE WT AND BAL. AFTERWARDS WE COMPLETED A BRIEFING OF CURRENT CONDITIONS AT DEP AND PROPOSED ARR ARPTS. AFTER DOORS HAD BEEN CLOSED AND THE NECESSARY CHKLISTS HAD BEEN COMPLETED; I CALLED FOR PUSH AND TAXI FROM THEIR RESPECTIVE FREQS. WHILE TAXIING OUT OF THE NON MOVEMENT AREA; GND-METERING ADVISED US TO PICK UP THE MOST RECENT ATIS. WHILE LISTENING; THE CAPT AND I NEGLECTED TO HEAR THE LIGHT FREEZING RAIN EXISTENCE ON THE FIELD DUE TO FREQ CONGESTION AND CONTINUED TOWARD THE DEP RWY AS USUAL. AFTER DEPARTING THE TERMINAL AREA; THE CAPT TURNED ON ATIS AGAIN QUESTIONING WHAT HAD BEEN PREVIOUSLY NOTED; AND REALIZED THE FREEZING RAIN AND OUR SITUATION OF NOT BEING DEICED AND ANTI-ICED AS PER OUR GENERAL OPS MANUAL. PRIOR TO OUR DEP; OUR SEPARATE DUTIES TO CHK THE ACFT CTL SURFACES AND VERIFY THE PROPER SETTINGS FOR ENG AND AIRFRAME ANTI-ICE SELECTORS; WHICH WERE BOTH ON DUE TO CURRENT ARPT CONDITIONS AS OBSERVED FROM THE COCKPIT HAD BEEN COMPLETED. I DO REALIZE WHY THE SITUATION ESCALATED INTO WHAT IT DID; THIS FLT WAS MY SECOND FLT EVER EXPERIENCING TEMPS BELOW FREEZING BEFORE TKOF. ALL MY FLT TRAINING WAS CONDUCTED IN TEMPS ABOVE FREEZING. THE PREVIOUS DAY WAS MY FIRST DAY EXPERIENCING FREEZING CONDITIONS. AT THE TIME I HAD READ ALL THE COMPANY MEMOS; HOWEVER; HAD NOT HAD ANY EXPERIENCE; SO IT DID NOT REGISTER. I HAVE THEN SINCE READ OVER THE MEMOS AGAIN AND WITH MY EXPERIENCE HAVE UNDERSTOOD THEM BETTER. THE CAPT AND I DISCUSSED THE SITUATION AND I FULLY UNDERSTAND THE SEVERITY OF THE SITUATION AND CAN ASSURE THAT THIS MISTAKE WILL NEVER RECUR. THE TKOF AND FLT WERE COMPLETED WITH NO FURTHER OCCURRENCES; HOWEVER; WE REALIZED THE SITUATION AND LEARNED FROM OUR MISTAKES AND FULLY INTEND TO PAY BETTER ATTN IN THE FUTURE.