Weather plays a major part of a Model Rocket flight. Winds can force the rocket to curve sideways, known as windcocking. Higher moisture increases the density of the atmosphere, which can slow the rocket down. Temperature also can alter the flight of the rocket by:
1) Cold days - the air is denser, the rocket will be slightly slower, but more stable.
2) Warm/hot days - The air is less dense, the rocket will be slightly faster, but slightly less stable until it reaches a higher speed.
There is plenty of things a model rocketeer can do to adjust to the weather. Tilt the rocket slightly into the wind to compensate for the recovery (winds will drag the rocket when the parachute is deployed), Finding a rocket with a lower Stability Factor to counteract the windcocking effect somewhat (slightly smaller guide fins). As for the temperature, there isn't much you can do to deal with that.