Ground frost, where the temperature on the ground is at zero or below, is not uncommon in any season of the year. Air frost, however, in the context of the temperature of the "free air" well clear of grass and other vegetation, although very common in winter, becomes less frequent as the year matures.
In the extreme south-west 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.
Obviously all these dates vary from year to year, and they are also affected by the lie of the local land. Altitude, for example, is an important factor; also, frost occurs more readily, and is therefore likely later in the spring, in sheltered valleys than it is in the more exposed parts of the surrounding countryside.
Late frosts are anathema to gardeners. For this reason, frames and cloches often used to cover low-growing plants and seedlings. They not only provide protection from untimely frosts in spring, but also artificially make the summers warmer, thus promoting growth.
In promoting growth, cloches work exactly like a greenhouse. Firstly the glass is more or less transparent to short-wave radiation from the sun and lets in the energy. But it is almost opaque to the long-wave radiation emanating from the ground and so traps the heat inside.
Secondly, since a relatively small volume of air is confined within the cloche and is not continually replaced by cooler air wafted by the wind from somewhere else, any heat that may be trapped inside accumulates efficiently.
By protecting delicate plants from extreme cold, cloches effectively bring forward the date of the last damaging frost of spring. Again two processes are at work. Firstly, as we have seen, loss of energy by longwave radiation from the ground is significantly reduced by a covering of glass, and so the tendency for the temperature of the soil to drop dramatically on cold clear nights is much diminished.
Secondly, the fact that it becomes warmer under glass by day than it is outside means that the soil within, acting rather like a storage heater, would need to cool that much more at night to reach freezing point. For this very reason, the protection against low night-time temperatures is significantly enhanced if the glass has been in position throughout the day, rather than being merely put in place at sunset.