Reading pressure charts.

Basics
Reading Pressure charts is similar to reading contour's on a map showing terrain height. Points of equal pressure are joined up by lines called Isobars. The closer together these Isobars are the stronger the wind will be blowing there. Air flows(creating wind) from the high pressure to the low pressure, but it does not do this in a straight line. This is due to the rotation of the earth which makes the air spin into the low like water going down a plug hole. In fact a low is much like a plug hole except it is upside down. The air at the centre of a low is warm and is rising which lowers the pressure as the air gets "stretched out", where as air in a high is cooler and is sinking down on the air below squashing it thus increasing pressure. A really low pressure reading at the centre of low means that air is rising very rapidly within sucking in air from the outside fast, making strong wave generating winds.

The key thing to remember is that in the northern hemisphere air circulates around a low in an anticlockwise direction. Air circulates clockwise around a high. The situation is reversed in the southern hemisphere.

Medium range forecasting
When working with pressure charts from the internet remember:

1. You need to look for SLP (Sea level pressure) charts. These show whats going on where the wind meets the water. Many other pressure charts are out there but they are for different heights in the atmosphere.

2. The main forecast charts range in time from analysis to around 144 hours into the future. Analysis is the current state of the atmosphere. The more distant into the future a chart is the more unreliable it will be.

3. To determine how reliable a forecast is it is best to compare the charts from a number of different forecast models. If all forecasts are similar you can be fairly confident that this is roughly what will happen. If they vary alot then the outcome is not easy to determine. Some of the models available are NGP, AVN, MRF, UKMET(the one you see on the BBC),ECWMF

   4. Winds do not blow exactly along Isobars. They tend to point a little more towards the centre of the low. 5. Knowing when charts are updated can keep you ahead of the game as alot can change in 12/24 hours. At the moment the following models are updated at around the times shown below although these vary quite a bit:

NGP about 9.30am and 9.30pm BST
MRF about 11.30am BST
UKMET before 9am BST and again at about 6pm
The NOAA wave watch 3 WAM charts are updated at around 11.30 am BST which makes sense as I believe they are based on the MRF forecast.

NGP, MRF, AVN here
http://152.80.49.205/PUBLIC/WXMAP/GLOBAL/

WAVE WATCH here:
Height
Period

Different WAVE WATCH visuals here (Has wind as well as height and period)
http://polar.wwb.noaa.gov/waves/main_int.html

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This article has been generously provided for us by Mat Clark.