GPSMAP 640 Owner’s Manual Marine Mode: Using XM WX Weather and AudioUnderstanding Surface WindsWind barbs indicate from which direction the wind is coming. Thewind barb is a circle with a tail. Wind speed is indicated by thedifferent combinations of flags that are attached to the tail.Calm5 knots10 knots15 knots20 knots50 knotsWind DirectionUnderstanding Wave HeightThe Wave Height feature shows wave heights. The wave heightsfor an area are shown by color intensities.Understanding Wave PeriodThe Wave Period feature provides the time (in seconds) betweensuccessive waves.Understanding Wave DirectionThe Wave Direction feature shows the direction in which a wave ismoving, as indicated by the direction of red arrows.Viewing Fishing InformationThe Fishing view contains current water temperatures, surfacepressure conditions, and fishing forecasts.Fish ForecastingUnderstanding Surface PressureThis feature shows pressure isobars and pressure centers. Theisobars connect points of equal pressure. Pressure readings can helpdetermine weather and wind conditions in an area. High-pressureareas are generally associated with fair weather. Low-pressureareas are generally associated with clouds and the chance ofprecipitation. Isobars packed closely together show a strong-pressure gradient. Strong-pressure gradients are associated withareas of stronger winds.Pressure units are shown in millibars (mbar), inches of mercury(inHg), and hectopascals (hPa).