The colours fade in late November. The rich kaleidoscope of autumn is dimmed into a dreary world of black and white. The year is waxing cool and the time has come, as Titania put it in A Midsummer Night's Dream, when
The seasons alter; and hoary-headed frosts
Fall in the fresh lap of the crimson rose.
The night air imparts a chill that has been foreign to us now for many, many months.
It is an interesting exercise in meteorology to try to identify a date in autumn before which the first frost of the season seldom comes, and a corresponding date in springtime after which it is unlikely to occur. This can be done by examining the temperature records over a long period and calculating for various parts of the country the average date of the last frost of spring and the first occurrence of its kind in the autumn.
First we have to define what kind of "frost" we mean. 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 - 4 ft above ground level.
The most useful information is gleaned by thinking in terms of the temperature of the "free air", the air well clear of the grass and other vegetation.
By this criterion, 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.
At the other end of winter, 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" 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.
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 earlier in the autumn and later in spring, in sheltered valleys than it is in the more exposed parts of the surrounding countryside.