The air around us, like ourselves, is the victim of its history. Its character, determined mainly by temperature and moisture content, has its origins in the type of surface over which it has passed in the few days before it reaches us.
The air, being almost "transparent" to short-wave solar radiation, is not heated directly by the sun. The sun's energy, by and large, passes directly through the atmosphere without affecting it. It is absorbed by the ground or by the oceans; the air then takes its temperature and humidity by contact with the surface over which it flows.
Thus, it is, as Shakespeare says, "what's past is prologue".
Regardless of any global warming or influence of El Nino, if the breezes blowing over us have originated, as they have in recent times, in balmy, southern regions in the vicinity of the Sahara, then it follows as the day follows night that the temperature will be unusually high.
If, on the other hand, the air approaches our shores at this time of the year from the north-east, it will have originated in the icy regions of the Arctic and will be cold and raw. Both scenarios are independent of whether the sun happens to be shining on Ireland at the time or not.
The approaching air may also be either maritime or continental, depending on whether it comes to us after spending a long time over the ocean or over a large area of land.
Maritime air masses are humid, with plenty of moisture available for the formation of fog or rain, provided other necessary conditions are met. Continental air is dry and may well result in cloud-free skies.
A glance at the weather map is often sufficient to identify the broad character of the air reaching our shores and, indeed, this is how meteorologists compile their forecasts in such circumstances.
Since the wind blows along the isobars, with low pressure to the left, the orientation of these lines on the chart indicates the general direction from which the air approaches us.
When the isobars show the air to have come from a southerly direction, temperatures higher than usual can be expected. Conversely, air flowing down from the north will be relatively cold. And so the seeds of the future, as it were, are buried in the recent past. The forecaster proceeds, as Warwick put it to dejected Henry, by: Figuring the nature of the times deceased,
The which observed, a man may prophesy,
With a near aim, of the main chance of things
As yet not come to life.