Is there a seasonal difference in the average cloud floor?
Cloud base varies as response to several factors, normally, cloud bases are lower in the wintertime than during the summer, particularly for stratus clouds. 

Cloud development is mostly dependent on temperature and the amount of water vapor in the air. The temperature has to be low enough to cause the air to become saturated. If the air is relatively dry, a colder temperature is required for water vapor to condense into water droplets. During winter, the temperature of the air in the lower part of the atmosphere is lower than during the summer, so stratus clouds can usually form at somewhat lower heights during the winter than during the summer. 

This is a generalization that does not hold true under all conditions and at all times. 

Also some more regional patterns can be present since the differentiation between Summer and Winter can be difficult to establish, for example in the tropics.

Taken from: https://stab-iitb.org/newton-mirror/askasci/wea00/wea00274.htm