PHL Tower cleared a commuter jet for takeoff on Runway 8 just as a DHC8 was landing on Runway 35; leading to control problems for the DHC8 crew due to jet blast.

Narrative

The Captain was pilot flying and landing 35 in PHL. We were cleared to land and just crossing the threshold when Tower cleared a commuter jet for takeoff from Runway 8. There was a confusing cautionary statement made by the Tower with the takeoff clearance; something along the lines of: 'Cleared for takeoff Runway 8; Dash 8 landing Runway 35; use caution for prop wash.' Both my Captain and myself were confused why they would get a caution like that. Right after we touched down; we saw grass and dust being blown across our runway from the full-power jet blast of the commuter jet. During our landing roll out; we went through the jet blast and were knocked quite forcefully off the centerline of the Runway. My Captain did a great job of maintaining control of the aircraft and keeping it on the Runway. This event could have ended much worse; possibly running off the Runway. The takeoff clearance was heard and noted by both my Captain and myself; we both saw debris blowing across the Runway; by that point we could only continue the landing; we both confirmed getting hit by jet blast. The event occurred because the proximity of Runway 8 to Runway 35 is close enough to cause jet-blast disruptions for aircraft landing on 35. The Captain maintained control of the aircraft even when it was drifting off center line. I told the Tower that we had been hit by the jet blast; but I never got a response. I believe this would have been avoided if the Tower had waited 5 seconds for us to clear the blast zone of Runway 8. The Tower should not clear takeoffs from Runway 8 when aircraft are landing Runway 35; until the landing aircraft has cleared past Runway 8. I felt this was a reckless move by the Tower; and I am disappointed that this procedure is even considered. If it had been a smaller aircraft; I believe it would have been blown off the Runway.