Ceri Sweeney kicked a penalty midway through the second half for Cardiff but it was nothing more than a whimper as teenager Paddy Jackson kicked a late effort to notch up his first senior points for Ulster and put the result beyond any doubt.
The wind around Ravenhill caused home fly-half Humphreys trouble in the opening exchanges, the No.10 missing an early penalty only to make amends on five minutes for an early lead.
But the scoreline stayed on the slim side until just before the half-hour as Humphreys missed two more kicks at goal, the second well wide of the left upright.
And it was Cave who spared Humphreys' blushes, the centre supporting a superb break by Craig Gilroy to finish off when the wing was ankle-tapped just metres from the Blues' try-line.
But Cave wasn't finished there, going over just minutes later following an Ulster line-out, Nevin Spence showing quick hands in the build up and the 24-year-old using the conditions to his advantage as he slid in from some way out.
And Humphreys put his difficult start to the match behind him by kicking both conversions to give the Ulstermen a 17-0 half-time lead.
Cardiff offered very little in the first half and on 53 minutes their task was made harder when Xavier Rush saw yellow for a high tackle on Pedrie Wannenburg.
But after withstanding heavy Ulster pressure the Blues finally got on the scoreboard with 15 minutes to go, fly-half Sweeney landing a penalty.
However, there was nothing else for the travelling fans to cheer as Ulster's Jared Payne nearly celebrated his debut with a try, only for his fly-hack to go out of touch with nobody between him and the try-line, before Jackson added a late penalty for the hosts.
|Ulster Rugby Score Card|
|Cardiff Blues Score Card|