Question: Why do my tomatoes crack as they ripen?
Tomatoes tend to crack when they receive irregular water. If your tomatoes have gone through a dry spell and you try to make up for it with frequent waterings, the inside of the tomato will plump up faster than the outside can stretch and grow. As a result the outer skin of the tomato splits open or cracks.
You can prevent future tomatoes cracking by making sure that they receive water regularly, whether or not it rains. Tomatoes do appreciate a good drenching from mother nature, but it is best to irrigate with drip hoses and water at root level when watering is necessary. Mulching the area around the tomatoes will also help maintain ground moisture levels. Look for varieties that are resistant to cracking.
If your garden has experienced a hot, dry, sunny period and rain is expected, you can go ahead and pick any tomatoes that have begun to turn color. The fruits are more likely to crack when it rains after a dry period. Tomatoes that have begun to ripen on the vine will quickly finish up on a sunny windowsill. However, if you harvest already cracked tomatoes before they are completely ripe, they will most likely go bad before they have time to ripen.
What's Troubling my Tomatoes? Utah State University Website