With the current atrocious weather happening at the moment we thought that it would be a good time to talk about all things pressure related.
Predicting the weather is now achieved by high tech satellites and computer based models that produce weather maps that can take many atmospheric factors into account predicting conditions in any given location to a reasonable accuracy. For the instruments at the Clock Work Shop any weather predictions rely on recording barometric pressure in Inches (inches of mercury) or for more modern barometers or barographs millibars.
It is important to set up your barometer for its location “above sea level”. On mercury barometers this is a little tricky as the hand has to be removed and replaced in the correct position. We insure that these are set up correctly at the shop. Aneroid barometers and barographs always have a method of adjusting the reading hand which is simply done with a screwdriver, knob or key. Then the set hand is moved over the reading hand you can then see if pressure is rising or falling this is more obvious on a barograph with paper chart.
Here are some rules for predicting the weather, please note the words on the dial “stormy, rain, change, fair, dry etc” are a throw back to when barometers were being developed and are not entirely accurate. Changes in the position of the indicating hand can be quite small i.e. .02 to.10 in 12 to24 hours. Changes in pressure readings can be greater in winter months than summer months.
A Rising Barometer
A rapid rise indicates unsettled weather
A gradual rise indicates settled weather
A rise with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.
A rise with moist air and a low temperature, indicates wind and rain from the Northward
A rise with southerly wind indicates fine weather.
A Steady Barometer
With dry air and seasonable temperature, indicates a continuance of very fine weather.
A Falling Barometer
A rapid fall indicates stormy weather
A rapid fall with westerly wind indicates stormy weather from Northward.
A fall with a northerly wind indicates storm, with rain and hail in summer and snow in winter.
A fall with increased moisture in the air and the heat increasing indicates wind and rain from southward.
A fall with dry air and cold increasing (in winter) indicates snow.
A fall after very calm and warm weather indicates rain with squally weather.
A thermometer and hydrometer can be useful to make these forecasts and both should be set up in a position sufficiently exposed to the external air for better accuracy.
Barometers are not always used to predict the weather, there are such things as mountaineering or baloonist barometers often the size of a pocket watch. They work by using the moveable bezel to zero with the indicating hand. The bezel is calibrated in feet so as you climb higher barometric pressure drops, so you can then read off your height in feet on the bezel.