B737-700 CAPTAIN REPORTS THAT COMPANY DEICING REGS FAIL TO ADDRESS SITUATIONS WHERE MORE THAN TWO FORMS OF REPORTED PRECIPITATION MUST BE ACCOUNTED FOR IN THE DETERMINATION OF TYPES OF FLUID REQUIRED AND HOLDOVER TIMES.

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NUMEROUS COMPANY ACFT ARE DEICING AND TAXIING OUT WITH CURRENT WX AT ABOUT 1400 FT BROKEN; 4 MI VISIBILITY; LIGHT WINDS; TEMP 1 DEG C AND LIGHT ICE PELLETS; LIGHT DRIZZLE; AND LIGHT SNOW. MY FLT IS BEING DEICED WHEN WE RECEIVE A DISPATCH MESSAGE WITH THIS WX (BASICALLY NO CHANGE). THERE IS NO MENTION OF ANY ILLEGALITY FOR OUR FLT TO CONTINUE. SEVERAL PLANES HAVE TAXIED ALREADY AND MANY OF THESE ONLY USED TYPE 1 AND NOT TYPE 4. THE ACTUAL WX IS JUST NOT THAT BAD! THE FO AND I AGREE THAT TYPE 4 IS NEEDED; SO THE DEICE TRUCK AGREES TO DO IT ONLY AFTER SOME HESITATION. ANOTHER PLT SAYS OVER THE RADIO TO CHK THE CHART; WHICH WE DO AND CAN SEE NOTHING TO PREVENT US FROM LEAVING. BECAUSE I NOW HAVE A BAD FEELING; I CONTACT DISPATCH BY VOICE. WE ARE INFORMED THAT OUR DEICE CHARTS ONLY ACCOUNT FOR 2 TYPES OF PRECIP -- MDW HAS 3; SO WE ARE GROUNDED. I ASK THEN IF THIS IS SOMETHING I HAVE MISSED OVER THE LAST 30 YRS OF FLYING. 'OH NO; THIS HAD HAPPENED A COUPLE OF YRS AGO ALSO;' DISPATCH SAID. NEWS TO ME! ANOTHER PLT NOW SAYS OVER THE RADIO THAT HIS DISPATCHER HAS TOLD HIM NOT TO GO BY ACARS. I TRY TO SOLVE SOME OF THE CONFUSION BY CONTACTING OPS TO EXPLAIN DISPATCH'S '3 STRIKES YOU ARE OUT' INTERP. OPS WANTS NOTHING TO DO WITH THIS AND TELLS ME I MUST HAVE DISPATCH DO ALL THE TALKING. I EXPECTED THIS BUT AM HOPING TO CONVEY MY NEWLY ACQUIRED INFO TO ANYONE LISTENING ON OPS FREQ. WE TAXI BACK TO THE GATE TO WAIT FOR THE WX TO CHANGE AS DO ALL OUR OTHER PLANES. WHILE WE WAIT; NUMEROUS ACFT LAND AND TAKE OFF -- INCLUDING; I BELIEVE; OTHER AIRLINES. ABOUT 10 MINS PRIOR TO OUR SECOND PUSH; ANOTHER COMPANY FLT HAS PUSHED; DEICED; AND REQUESTED TAXI FOR TKOF WITHOUT HAVING THE '3 STRIKES' REMOVED FROM THE ATIS. WX GOES TO 2 ITEMS AND WE PUSH AND DEICE AGAIN. ONCE AGAIN THERE IS A LOT OF CHATTER ON THE RADIO ABOUT IF TYPE 4 IS REQUIRED. I THINK WE ALL GET TYPE 4 AND DEPART. OUR DEICE CHARTS DO NOT CLEARLY DEFINE THIS '3 STRIKES' SITUATION AS NO-GO. MAYBE; BECAUSE IT NEVER HAS BEEN BEFORE. WE ALWAYS DEICED TO THE WORST CONDITION AND WENT FROM THERE. IF THIS IS A 'LAW' -- THEN SPELL IT OUT. IF NOT; THEN FIX 'OUR' RULES TO ALLOW FOR TRADITIONAL WORST CASE DEICE AND GO.