As a former Navy pilot who studied how to predict the weather and an experienced gardener, I've learned that a few easily observed atmospheric conditions and the lay of your land will tell you exactly when the first frost will hit your garden. With this information you can plan to protect some plants and get others ready for the compost pile.
If the sky is clear, the air is dry, and the temperature is falling, chances are frost will settle on your garden. But if the sky is cloudy, frost is less likely, because low, thick clouds act like a blanket. They prevent the heat that Earth radiates at night from escaping into the atmosphere, keeping frost from settling on your plants.
During the day, Earth absorbs heat from the sun. At night, it radiates that heat back into the atmosphere, and cold air gradually settles around your plants. But a slight breeze mixes the somewhat warmer air from above with the cold air near the soil. So frost is less likely on a night with a gentle breeze (but not strong wind) blowing.
Cold air is dense. The molecules are packed tightly together, and the air is heavy. It is so heavy that it flows downhill and, like water, pools in low places. The temperature in a valley may be as much as 18° lower than the temperature on the adjoining slope. Likewise, the temperature in your garden varies from a point on a sunny slope near a wall to the hollow in the bottom of your garden. However, trees on a slope will warm the draining air as it passes through because trees transpire, or give off moisture, reducing the likelihood of frost even in the hollow.
A gentle slope facing south receives solar radiation—heat and light—longer than other sites. And the radiation is more intense. That's why a southern slope is the best location for a garden. Also, cold air drains down slopes (as described in a "Frost Pocket"), so gardens on top of slopes will get frost later than those at the bottom or on level spots.
Trees act like a blanket that prevents ground heat from escaping into the atmosphere. They also exude moisture, raising the dew point. A vegetable garden surrounded on three sides by mature oaks will survive the first two or three frosts untouched—at least my vegetable garden does.
Cold air rushing down a slope collects not only in hollows (see "Frost Pocket") but also behind stone walls, fences, and rows of dense vegetation, such as hedges. Frost occurs sooner at the base of these barriers. To delay frost on plants growing beside a barrier, provide an opening in the wall or hedge through which the cold air can drain. On the other hand, a south-facing stone wall is a "heat sink." During the day the sun warms the stones, which release the heat at night, making the plants on the south side of the wall less prone to frost.
As the evening temperature falls, the air holds less and less moisture, until it condenses and dew forms. The temperature at which this happens is called the dew point. When dew forms, heat is released. That heat helps to keep the air temperature at or slightly below the dew point. So, the more moisture in the air at sunset, the less the likelihood that frost will occur during the night. (This is why commercial growers turn on sprinklers when frost is predicted. The added moisture in the air raises the dew point.)
Some forecasters give the dew point in their evening reports. If not, call the local National Weather Service office or visit the NWS Web site at www.nws.noaa.gov.