A hard, or killing, frost is based on movements of large air masses. During the day, the sun's radiant heat warms the earth. After sunset, the heat radiates upward, lowering temperatures near the ground. However, if the night is overcast, the clouds act like a blanket, trapping heat and keeping air temperatures warm enough to prevent frost. Feel the breeze.
Wind also influences the likelihood of frost.
In the absence of wind, the coldest air settles to the ground. The temperature at plant level may be freezing, even though at eye level it is above freezing. A gentle breeze, however, will prevent this settling, keep temperatures higher, and save your plants. Of course, if the wind is below freezing, you'll probably have fried green tomatoes for tomorrow's supper. Check the moisture.