Tomatoes are affected by two types of cracking. Radial cracks run from the stem end toward the blossom end; concentric cracks form circular patterns around the stem. Cracking is usually caused by changes in the growth rate of the fruit. Rapidly growing tomatoes are more susceptible. Wide fluctuations in temperature and rain promote cracking too. Maintain uniform soil moisture to help prevent cracking. (Tomatoes often split radially when watered by irrigation or rainfall after a prolonged dry spell.) Avoid excess nitrogen fertilizer, which promotes rapid growth. Some varieties are more crack-resistant than others. Try some different cultivars if you continue to experience cracking fruits.