B737-700 CREW WAS REQUIRED TO DIVERT FROM HOU TO IAH; AFTER HOU ARPT WAS CLOSED.

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OUR FLT WAS TO HOU. THE WX WAS CLR AT HOU AND ENRTE. PLANNED ARR FUEL WAS APPROX 5.6; NO ALTERNATE REQUIRED. UPON CHK-IN WITH HOU APCH; WE WERE ASKED TO SLOW AND GIVEN SPACING VECTORS AND A 360 DEG TURN FOR SEPARATION DUE TO APPARENT ARR CONGESTION INTO HOU. ON DOWNWIND AT HOU; THE ACFT IN FRONT OF US WAS GIVEN HOLDING INSTRUCTIONS. WE ASKED APCH IF WE WERE GOING TO HOLD AND THE NATURE OF THE DELAYS AT HOU. APCH ISSUED US HOLDING INSTRUCTIONS BUT DID NOT GIVE A REASON FOR THE DELAYS. WE HAD FUEL OF APPROX 5.6. WE TOLD APCH WE COULD HOLD FOR 5 MINS THEN WE WOULD HAVE TO DIVERT TO IAH; AND AGAIN ASKED THE REASON FOR THE DELAY. WE CALCULATED A DIVERT BURN OF 1.0 TO IAH. APCH INFORMED US THAT HOU WAS CLOSED FOR AN UNSPECIFIED PROB. WE REQUESTED VECTORS TO IAH. BEGINNING OUR SHORT DIVERT TO IAH; OUR FUEL WAS 5.2; GIVING AN APPROX ARR FUEL OF 4.2. WE DECLARED MINIMUM FUEL TO APCH CTL. WE THEN ATTEMPTED TO CONTACT DISPATCH; UNSUCCESSFULLY. DUE TO THE EXTREMELY SHORT NATURE OF THE DIVERT; WE DID NOT HAVE TIME TO ATTEMPT TO CONTACT DISPATCH A SECOND TIME. WE COMPLETED ALL OF OUR NECESSARY PROCS AND LANDED UNEVENTFULLY AT IAH. WE TOUCHED DOWN WITH A FUEL OF 4.2 AFTER SEVERAL DELAYS ON TAXI; WE SHUT DOWN WITH APPROX 4.0. I CONTACTED DISPATCH TO UPDATE THEM AND COORD OUR FLT BACK TO HOU. LESSONS LEARNED: EVEN IF THE DEST WX IS PERFECT; ALWAYS HAVE A BACKUP PLAN IN CASE OF THE UNEXPECTED. SECONDLY; ALWAYS BE PREPARED TO EXECUTE THAT PLAN RIGHT UP TO TOUCHDOWN AT YOUR DEST.