CREW DISAGREEMENT OVER SAFETY OF VFR FLT IN VERY MARGINAL WX CONDITIONS BELOW THE CLASS B AIRSPACE OF CHICAGO.

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ON A FLT FROM MDW TO LOT; THERE WAS AN INDEFINITE DELAY FOR IFR DEPS DUE TO WX. THE CAPT AND I CHKED WX AT BOTH ARPTS. MDW WAS RPTING 1000 FT SCATTERED 2500 FT OVCST WITH 4 MI VISIBILITY. LEWIS WAS RPTING 3900 FT BROKEN AND 6 MI VIA ASOS. THE CAPT DECIDED TO GO VFR. I VOICED MY CONCERN DUE TO THE SCATTERED LAYER OVER MDW. I DIDN'T THINK WE COULD REMAIN CLR OF CLOUDS ON OUR CLBOUT. HOWEVER; IT WAS A SCATTERED LAYER AND WITH THE RPT OF 3900 FT BROKEN LESS THAN 18 MI AWAY; WE BOTH THOUGHT THE SCATTERED LAYER WOULD OPEN UP FAST. THE CAPT RADIOED FOR VFR CLRNC; WE GOT OUR CODE AND WERE CLRED FOR TKOF SHORTLY THEREAFTER. THE CAPT ELECTED TO STAY BELOW THE SCATTERED LAYER UNTIL WE COULD CLB THROUGH A HOLE. OUR ALT VARIED BTWN 500-800 FT AGL FOR THE FIRST 10 MI OR SO. WE WERE BELOW THE BASES AND HAD GOOD VISIBILITY; WE WERE STILL ON DEP (MDW). THE CEILING SUDDENLY LOWERED AND WE WERE IMC. I STATED WE NEEDED TO CLB AND TELL ATC WE COULD NOT REMAIN VFR. INSTEAD; THE CAPT DSNDED TO APPROX 360 FT AGL WHERE GND CONTACT AND VISIBILITY WERE REGAINED. WE WERE ABOUT 5 MI FROM LOT AT THIS TIME. AT ABOUT 2 MI; I CALLED 'ARPT IN SIGHT' AND THE CAPT LANDED ON RWY 27. TAXIING IN; I LISTENED TO ASOS (LOT) WHICH SAID IT WAS 2 1/2 MI VISIBILITY. I DON'T REMEMBER THE CEILING. I THINK IT WAS 800 FT BROKEN. I TOLD THE CAPT WE WERE WRONG TO HAVE DSNDED INSTEAD OF CLBING TO A SAFE ALT AND CONTACTING ATC AND THAT IT WOULD NEVER HAPPEN AGAIN. HE JUST CHUCKLED. I RPTED THE INCIDENT TO HIS SUPVR. I FEEL THESE ARE THE CONTRIBUTING FACTORS: 1) THE CAPT HAS HIGH TOTAL TIME AND APPROX 30+ YRS EXPERIENCE FLYING EXCLUSIVE TO ILLINOIS -- ESPECIALLY CHICAGO AREA. HE IS 65 YRS OLD. HE SAYS HE KNOWS HOW THINGS WORK IN CHICAGO. 2) IT WAS MY SECOND WK OF WORK WITH HIM AND I DIDN'T FEEL I COULD CHALLENGE HIM EFFECTIVELY. 3) THE INDEFINITE HOLD OF DEPS (IFR) GAVE THE SENSE OF URGENCY TO 'SNEAK OUT VFR.' I HAVE SINCE TALKED TO EMPLOYEE AND HAVE BEEN EMPOWERED TO FEEL COMPLETELY FREE TO CHALLENGE UNSAFE CREW DECISIONS WITHOUT RECOURSE. EVEN IF IT MEANS SHUTTING DOWN ON THE GND AND WALKING OFF THE FLT.