Hooke's grasp of pressure changes


Tradition has it that it was Robert Hooke who first tried to quantify the relationship between the height of the mercury column in a barometer and variations in the weather.

Although best known, for "Hooke's Law" of elasticity, he is remembered by meteorologists as the originator of the standard inscriptions on the domestic "weather-glass". In 1670, Hooke equipped his mercury barometer with a float, connected by a chain to operate a pointer on a clock-like dial. The corresponding height of the mercury column was engraved on the dial, and the word "Change" inscribed at 29.5ins; "Rain", "Much Rain", and "Stormy" were inserted at half-inch intervals on the low side, and "Fair", "Set Fair" and "Very Dry" on the high side.

The idea - and the terms themselves - became very popular throughout Europe, where, in due course, people began to speak of the barometer monte a la maniere d'Angleterre - "set up in the English manner". The descriptions are still with us in only slight variations of their original form.

But is it, as Bassanio said, that "the world is still deceived with ornament?" Are these engraved legends any good for telling us what the weather has in store for us?

It must first be said that the barometer merely measures the atmospheric pressure at a single point, so it is asking rather a lot of the instrument to expect consistently accurate predictions based on this information alone. However, the behaviour of the barometer does offer clues to the future.

It is well to realise that the pressure tendency, whether pressure is rising or falling, is of more importance than its absolute value - how high or low it is.

It is true, in a very general way, that high pressure is associated with quiet, settled weather, and low values often bring changeable and windy conditions. But there are many exceptions to this rule. You will get more guidance from the instrument if you also note the direction of movement of the needle over several hours.

Rising pressure usually means an improvement in the weather. But the meteorologist's rule of thumb is: "rapid rise, rapid fall".

A very rapid rise in pressure seldom lasts long and is soon followed by a sharp drop which heralds the approach of the next depression.

A slow steady rise, on the other hand - between a quarter and half a millibar per hour - is a good omen; it often means that an anticyclone is becoming established over your area, which may result in a longish spell of calm, sunny weather.