The face of the barometer in your hallway, if there is one there, is probably inscribed with helpful advice about the weather on the way.
Dire warnings of "rain" and "storms" give way to cheerful intimations of "bright" and "sunny" weather, as the needle moves backwards and forwards through a range roughly defined in "clock" terms by the region between "10 to" and "10 past" the hour.
If you look more closely you will also see it also provides a reading of the atmospheric pressure. The movements of the needle reflect, as we know, the irregular rise and fall of pressure from day to day, encompassing a range of as much as 40 or 50 millibars, depending on how intense the depressions are that pass nearby.
But superimposed on these larger pressure changes caused by weather systems, is a regular daily rhythm, a cyclical twice-daily rise and fall of pressure known as the "diurnal variation".
Everywhere in the world peaks occur at about 10 a.m. and 10 p.m. local time, with corresponding troughs 12 hours later in the afternoon and very early in the morning.
This atmospheric tide is undetectable in volatile conditions, but in quiet weather, when the background pressure remains relatively steady for a day or two, the oscillation can be clearly seen from the record of a barograph.
It is greatest near the equator, where the twice-daily rise and fall may be 3 mbs or more; its intensity decreases gradually towards the poles, so that over Ireland the change is only a fraction of a millibar, but visible nonetheless.
The twice-daily rise and fall is the result of a double pressure wave travelling around the Earth in the same direction as the Sun. In some ways it resembles the ocean's lunar tides, and indeed there was a time when it was thought the moon might be responsible. Now we know that the gravitational effect of the moon upon the atmosphere is quite minute, and that the pressure anomaly is caused by solar heating of the atmosphere as the Earth rotates beneath the Sun.
But why a "double" daily cycle, rather than a 24-hour one? The complex answer is related to the fact that the atmosphere has a natural "resonance" with a period of about 12 hours. In other words, if you were to give it a slight push, like a wobbly jelly, and leave it to its own devices, it would sway back and forth at that frequency - a predilection that causes the daily thermal stimulus originating from the Sun to manifest itself as a double 12-hour wave.