An essential element of understanding weather is an understanding of atmospheric pressure and the changes that occur. Atmospheric pressure is the weight of the air pressing down on us. A column of air measuring one square meter (approximately 11 square feet) weighs 10 tons at sea level. However, that pressure can vary for any number of reasons.
Atmospheric pressure is also known as barometric pressure and is measured by a barometer. A competent sailor keeps a weather eye on the barometer and is aware of changes in pressure. Changes in pressure are measure in either inches (Imperial measure used in the United States) or millibars (metric used in Canada).
Air pressure is continually rising and falling as weather systems move through. We already know that when air is warmed, it rises. This creates a zone of low pressure. Cold air on the other hand is denser and sinks to the earth's surface, increasing the air pressure. These changes are measurable and have well-recognized qualities.
Points of similar pressure are connected, creating isobars – those familiar lines we see on weather maps. Nature abhors a vacuum and any time there is an imbalance of forces, nature attempts to re-balance them With air pressure, this occurs by air moving from a High pressure zone to a Low pressure zone.
Air would normally flow directly from a High to a Low. But ...
remember the Coriolis Effect? It causes the direction of air flow (wind)
to 'bend' – in the Northern Hemisphere, to the right – as it spirals
out and away from the centre of the High.
Buys Ballot's Law
This gives rise to what is known as “Buys Ballot's Law”. Named after Christophorus Buys Ballot, a Dutch meteorologist in the 19th century, Buys Ballot's Law says that, in the Northern Hemisphere, if you stand with your back to the wind and extend your left arm directly out from your side, you will be pointing at the centre of the Low pressure zone. Knowing where the Low is centred and what your position is relative to its centre can be very useful in your planning. In the Northern Hemisphere, winds blow counter-clockwise around a Low. Winds will blow generally parallel to isobars, so seeing where you are located relative to the movement of the Low will give you some insight into how the winds are going to change as it passes. The closer the isobars, the stronger the wind.
Here are some useful things, courtesy of NOAA, to keep in mind about barometric pressure.
The FALL of the barometer (decreasing pressure)
The RISE of the barometer (increasing pressure)
The barometer rises highest of all for north and east winds; for all other winds it sinks.
The barometer UNSETTLED (unsteady pressure)
Motion upwards, indicates the approach of fine weather
Motion downwards, indicates the approach of foul weather.