Meteorology: Understanding the Atmosphere            Ackerman and Knox

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If you have a backyard garden in a region where frost occurs, you know the importance of trying to protect your plants and the difficulty of predicting frost. There are a series of questions you can ask yourself to aid in predicting the formation of frost for your own backyard
What is the temperature? If at sunset the temperature is close to freezing then there is a better chance for the formation of frost. Remember the temperature reported by your local weather station is measured 1.5 meters above the ground, much higher than your plants. So if the forecast is for a temperature of 4C (39F), the temperature of your plants may be close to freezing.
How fast is the wind blowing? If the wind is blowing strong then there is vertical mixing that inhibits the formation of a radiation inversion and the cooling of the surface, and thus frost.
What is the general weather pattern? Be aware of the meteorological condition causing the wind! Is   a cold mass of air   heading your way? If so, you had better protect your plants or harvest.
What sort of cloud cover is there? Clouds are good emitters of terrestrial energy so they reduce the energy losses at the ground during the night. If it is cloudy, and will stay cloudy, then the likelihood of frost is reduced. If it is to be a clear night then frost is more likely to develop.
What is the condition of the ground? If the soil is warm and moist, then the plants may be O.K. Conduction will transfer heat upwards from below the surface and inhibit the development of the temperature inversion. Also know your backyard. Is your garden located in a local low spot which will allow cold air to drain into it?
How long will the night be? Energy losses exceed energy gains during night. The longer the night, the more time the ground has to cool and so the colder it will get.
What is the dew point? This is a very important question to ask yourself. A rule of thumb--if the dew point is above 7C (45F) at sunset then you are probably OK. Below 4C (40F) you better cover your plants.
Are you in doubt? If you're not sure if frost will form, you might as well spend a few minutes and cover the plants to enjoy them a little longer.