Reading the highs and lows
Old seadogs often still refer metaphorically to “the glass is falling”. This is not a reference to a tumbler of rum but to the mercury falling on a sailing ship ’ s barometer.
This was the most reliable warning available to sailors that gales and storms were approaching.
Atmospheric pressure is defined as “the force exerted on a surface by the air above it as gravity pulls it to earth”.
In the field of meteorology, it is measured in millibars (mb) or hectoPascals (hPa).
Standard sea-level pressure is 1,013.2mb.
As a general rule of thumb, pressure readings above this are regarded as High Pressures (HP) and below this as Low Pressures (LP). These pressure systems are easily identified on a synoptic (weather) chart.
Along our coastline and adjacent interior, pressures of 1,020mb or higher are usually associated with fine, settled weather, whereas pressures of below 1,000mb signal approaching foul weather.
To put these readings into a broader perspective, the lowest pressure ever recorded was 870mb on October 12 1979, during Typhoon Tip in the western Pacific Ocean.
The measurement was based on an instrumental observation made from a reconnaissance aircraft.
In comparison, the highest pressure ever recorded was 1,083.8mb at Agata, Siberia, Russia on December 31 1968.
Typically, in our part of the country, a strong easterly to north-easterly wind is associated with a rapidly falling pressure, while a gusting westerly to south westerly follows a cold front, with a rising pressure and general clearing of clouds and rain from the west.
Interestingly, knowledgeable fisherman put their rods away grumpily when the pressure is falling rapidly, and the easterly wind is picking up.
The adage “when the wind is east, fishing’s least, when the wind is west, fishing’s best” is very accurate along the Ndlambe coastline.
The warmer surface water layer is stripped off by a strong easterly and cold water wells up from the deep and fish go off the bite for a few days.
To keep track of pressure changes it is a very good idea to purchase a highquality aneroid wall barometer, preferably of German origin.
These come in all shapes and sizes with attractive wood surroundings such as teak, meranti and oak. It is advisable to purchase a larger barometer as the pressure indicators on smaller types are often very small and hard to read. The positioning of a barometer in your house is critical too.
Try to find a room where it is sheltered and not close to open doors or windows as wind drafts can skew the readings quite significantly.
Check the reading (with a sharp tap on the glass) and reset your barometer at a fixed time once a day, in the early morning or evening.
As an aside, the first mercury barometer was invented by an Italian, Evangelista Torricelli, in about 1644. The aneroid barometer was invented much later by a Frenchman, Lucien Vidie, which registers the change in the shape of an evacuated metal cell to measure variations in the atmospheric pressure.
“Aneroid” means fluidless, where no liquids are used such as alcohol or mercury, and the metal cell is usually made of phosphor bronze or beryllium copper.
If you wish to give yourself an early Christmas present, you may want to look at purchasing a fully Automatic Weather Station (AWS) which will provide you with a continuous readout of pressure, wind, temperature, and humidity.
A remote version will allow you to monitor the weather both inside and outside your house and can also be linked to your PC.
An AWS with all bells and whistles will set you back from R7,000-R10,000. Reliable makes include Davis and Oregon Scientific.
If you try to combine your knowledge of atmospheric pressure, cloud types and local wind patterns you will soon learn to discern the vagaries of our fascinating coastal weather.