A pressure gradient zone will develop between areas of high and low pressure on the earth’s surface and this pressure gradient results in a net force that is directed from high to low pressure anywhere in the earth’s atmosphere, whether on or above the surface. Conversely, the farther apart they are the lighter the winds. Generally, where the isobars are closest together they will tend to indicate the areas of strongest winds.
These are helpful because they can tell us something of both the direction and strength of the wind in any given location. I n between the two extreme values we can plot isobars (usually at 4mb intervals). Already, knowing that higher pressure generally gives better weather, we have some idea that the best weather is likely to be in the south of the country. The highest pressure appears to be near the Isle of Scilly, in the far south west, at around 1030mb. We can see that the lowest pressure appears to be over the Northern Isles of Scotland, at around 998mb. Can you locate the areas of high and low pressure on it? (Note: 01 = 1001mb etc) The chart below shows a map with pressure readings taken from stations on land in the United Kingdom and some from ships at sea. Pressure values are generally corrected to Mean Sea Level Pressure (MSLP) so that a standardised level can be shown, otherwise the observers height would affect the readings, as pressure decreases with altitude quite substantially. They are helpful because by creating them we can start to identify areas of high and low pressure at both the surface and aloft, that control our weather pattern. We call lines joining points of equal pressure isobars.
These are usually in millibars (mb) or sometimes inches. The first thing a meteorologist will draw up on his or her chart (although nowadays nearly all maps are generated automatically by computers) are lines of equal pressure from observations. One of the most important things anyone wanting to understand the weather can learn is how to recognise the salient features shown on a weather map.
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