C210 PLT FLIES INTO CONTINUOUSLY DETERIORATING WX AND BECOMES CAUGHT BTWN INCREASING TIGHTER AND TIGHTER LAYERS. DECLARES EMER AND DSNDS IFR.

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FLT COMMENCED IN DURANGO; CO; VFR (CAVOK) AND PROGRESSED WITH NO FLT PLAN BUT MONITORED ZDV AND RESPECTIVE ALBUQUERQUE FREQS AND ALSO OCCASIONALLY LISTENED TO FLT WATCH. FLT PROGRESSED TO FARMINGTON THEN GALLUP. SCATTERED LAYER BELOW APPROX 2000 FT AGL STARTED AT GALLUP; ALSO SOLID OVCST AT FL190. SCATTERED LAYER BECAME 9/10 OVCST AROUND SHOW LOW; AZ. PIC HAD VISUAL OR OPTICAL ILLUSION THAT HE COULD SEE THE BOUNDARIES OR THE EDGE OF OVCST OR SOLID LOWER CLOUD STRATA. AGAIN; PIC THOUGHT THAT CONTINUED FLT WOULD MEAN FLYING OVER LOW SOLID STRATA INTO CLR VFR WX. FLT CONTINUED AND ACFT WAS ACTUALLY BEING SANDWICHED TIGHTER AND TIGHTER BTWN LAYERS. LOW CEILING WAS ACTUALLY BUILDING THICKER. TALK TO FLT WATCH WAS THAT MODERATE TURB WAS RPTED NEAR CASA GRANDE. FLT WENT INTO IMC SAN CARLOS NDB AND ACFT WAS IN LIGHT RAIN AND LIGHT RIME ICING STARTED. PLT WAS ON 132.9 WHICH WAS VERY BUSY. OTHER SECTORS 125.2 ARE NOT BUSY. FINALLY; PIC ELECTED TO DECLARE AN EMER TO GET IFR CLRNC TO CASA GRANDE. ACFT BROKE OUT FROM OVCST APPROX 7000 FT 5 MI NE OF CGZ (CASA GRANDE ARPT). PLT HAD RECENTLY PURCHASED HALF OF TURBO C210 CENTURION AND WAS 'NEW' TO HIGH ALT FLT. RECOMMEND THAT OPTICAL ILLUSION BE INCORPORATED INTO TEXTBOOKS OR INSTRUCTION CURRICULUM. PLT HAD READ ABOUT NIGHT LNDG VISUAL PHENOMENA; BUT HAD NEVER 'EXPERIENCED' VISUALLY ANYTHING LIKE THIS. ALSO; BREAKING UP HVY SECTORS OF ATC TFC AND FREQ SATURATION (E AND W) OF PHOENIX SKYHARBOR MIGHT HELP. PIC HAD IMPRESSION THAT ATC WOULD NOT RESPOND UNLESS PRIORITY WAS ASKED FOR.