Relative humidity is a measure of ratio of actual density of the vapour over its saturation density at any given temperature. The amount of water vapour in the air is dependent of the temperature of the surrounding and the pressure with the altitude of the place.
Generally, relative humidity decreases as with an increasing temperature. As temperature is increase, kinetic energy is increased thus releasing the water molecules in the atmosphere. Now, in the tropical countries, the weather is somehow different. Tropical countries have are more humid than other countries. There are more suspended water molecules at any temperature compared to other countries.