Why is it important to know the velocity of an object instead of just speed?

Velocity helps measure the rate at which an object changes it's position in a given direction. If you were measuring a tornado, for example, you would want to know both speed and velocity to predict where as well as how fast it will travel. 

If the water vapor content stays the same and the temperature drops, then the relative humidity increases. its vise versa.if the water vapor content stats the same and the temperature rises, the relative humidity decreases. so, yes, i'd say that temperature and humidity have a strong relationship.