Question

Why does the number and intensity of storms increase during the summer seasons ?

Answer 1

The two factors necessary or contributing to the formation of a thunderstorm are: - A lot of moisture
       - Rapidly rising warm air
In summers, most of the time these two conditions will be met due to the high temperatures. This is why the number of storms increase during the summer seasons.  

Answer 2

For a storm to occur there should be a lot of precipitation, moisture in the air and also there should be a lot of wind.  Wind  with a large speed arises due to large pressure differences.  Pressure differences arise due to temperature differences between regions.  In summers there is  lot of moisture in air due to evaporation of water from water bodies.

Also there is a lot of wind movement , due to unequal temperatures between sea to land.