Humidity is the end result of a huge amount of water vapor in the air. Now water vapor is caused by heat. Since the sun is extremely strong during the summer, then a lot of heat would be produced due to the sun's rays. This in turn would produce water vapor that can be found in the air. A lot of water vapor in the air causes humidity. Now we all know that heat rises so water vapor, being a result of heat, would also rise as well.
