Frost data is invaluable for knowing when to plant new seeds and bulbs, when a plant should move from inside to outside, and when your garden will come into flower. Frost dates will vary, depending on where you live, so it is always a good idea to keep an eye on them, as they are correct six years out of every seven, although, of course, local variations do apply.
The general rule is that the further north you are, the later your frosts end and the earlier they begin. The more south or west you are, or closer to the coast, the less likely you are to have frost. Urban gardeners can also have lighter frosts than those in rural areas, and if you know your garden well, then you will begin to see a pattern.
The maps above show a breakdown of the length and frequency of frosts to be expected in different regions. Keep this in mind when planting, especially tender plants that are sensitive to low temperatures.