It is hot! Maybe not enough water? I am watering twice a day.
Paul, this is common in tomatoes that do not get enough drainage. You did not say if you have them in the ground or in pots or planters. Be sure your soil has vermiculite , organic fiberous matter like moss added around the soil, feed them with epsom salts (2 TBS in gallon of water) and water them often, especially in the heat. If they are in pots, be sure they have holes in the bottom of the pots and the water can drain away. If you want to save water, let it drain into another container and use that water on other plants too. Lots of water, drainage of water, and food like calcium and epsom salts help much. Also I use egg shells pureed in the blender (not just crushed, it isn't broken down enough) in the water. Feed them about every ten days all summer. Some people feed them once a month but mine stay greener and grow healthier feeding them more than once a month in their growing season.
Are your tomatoes in the ground or in pots? When you water tomatoes in the ground make sure you are watering at the roots and give them enough water so that ground is saturated a few inches down into the ground from the plants-then you should not have to water twice a day. Giving them only a little bit of water twice a day but not enough water will stress the plants. If they are in pot-- depending in the size of the pots--they may very well need to be watered twice a day in the hot summer sun. I have 2 huge cherry tomato plants on my deck that need water every day. If you can move the potted plants into a partially shaded area during the hottest weeks of the summer they will stay moist longer. I position mine so they don't get the hot afternoon sun--but get at least 6 hours of sun.