My tomatoes keep splitting!

For some reason, a LOT of my small tomatoes split like the picture.  My plants are on an automatic timer, watered every day.  Do I need to water more often?  Water longer?  Add some fertilizer?  Any good ideas?

<Edit> I forgot to mention that the temps around here have been over 100 degrees for a couple of months (except one day) and up to as high as 110 degrees.  Also, I found many places that say to water evenly, water about every week, etc.. and most of them also say that Mulch should be placed around the plants... I will add mulch to see if it cures my problem.