Clouds will help to insulate and slow the loss of the heat, but a cloudless, wind free night will afford no protection whatsoever. The temperature within the soil and in the plant's cells may even drop to a few degrees colder than the air.
As the temperature drops, the moisture in the air condenses into dew, which then freezes when the temperature reaches 32° F. on the plant surfaces.
At 32°, damage to most plants may be minimal, and only affect a leaf or two. However, if the temperature drops far enough for the plant cells to freeze, non hardy plants will die.
Weather conditions can bring about a frost, even in supposedly frost free areas. It is important to heed the warning when your local forecaster announces 'a chance of frost', and take precautions to protect your garden.
Often times it is possible to extend your growing season by several weeks if you are able to keep your plants alive through a single early frost!