FROST, as we all know, is a phenomenon very common in winter but a freak occurrence in July and August - one so unusual, in fact, that we could almost say it does not happen. It ought to be possible, therefore, to identify a date in springtime after which frost is unlikely to occur, and a corresponding date in autumn before which the first frost of the following season seldom comes. One way of getting a measure on the situation is to examine the temperature records over a long period and calculate for various parts of the country the average date of the last frost of spring and its first occurrence in the autumn.
First we have to define what kind of frost we mean. The term simply refers to a temperature of zero degrees Celsius or below, but meteorologists are careful to distinguish between air frost and ground frost. There is often a marked change in temperature with height in the first few feet above the ground, and on a cool clear night the temperature on the grass may be several degrees lower than that at the height at which the air temperature is normally measured - four feet above ground level. Since ground frost is not uncommon in any season of the year, it makes more sense when analysing the dates in the way described above to think in terms of the temperature of the free air, dwell clear of the grass and other vegetation.
By this criterion, we find that in the extreme southwest of the country, near the tips of the peninsulas of Cork and Kerry, the average date of the last "winter" frost is March 1st. Elsewhere around the coastal strip it typically occurs in late March or early April, and in the midlands the last air frost of the season occurs, on average, during the first half of May. The relative mildness of coastal regions is also in evidence in the autumn. In the midlands of Ireland, the first air frost of the winter occurs in October but areas adjacent to the sea are generally frost free until December. At the extreme tip of west Cork, the first air frost of the winter does not occur, on average, until early January.
Obviously all these dates vary from year to year, and are also affected by the lie of the local land. Altitude, for example, is an important factor; moreover, frost occurs more readily, and is therefore likely later in spring and earlier in, the autumn in sheltered valleys than in the more exposed parts of the surrounding countryside.