That s when the wind starts to howl. Air travels from the high pressure area to the low pressure area. The warmer air cooling quickly losses it s moisture, heavy rain and lot s of wind blowing from high to low develops quickly. The lower the pressure, the more wind. Pressure on the weather map is measured in millibars, with equal points of pressure connected forming ridge lines, that we call isobars.
Pressure is determined with the aid of a barometer, measuring of course the atmospheric pressure.
So lets put it altogether and see how to forecast marine weather using the aforementioned tools of analysis along with a prayer to St. Brendan. You're six hundred miles east of Ambrose Light on a course of 270 degrees. Wind is coming out of the northwest 15 miles per hour, seas five to seven feet, north to north west. You get a weather map print out on your weather fax. You observe you're sailing on a high pressure area out of the northeast.