I stood there, facing the seated crowd of sailors and asked,  “How many of you have a basic minimum or less understanding of how to read a weather map?” At least 70% of the group raised their hands. 

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“What could it possibly have to do with me or my sailing?” they may have asked themselves at earlier points in their sailing lives. But here they were, committing a day to learn how to decipher the lines and make sense of the squiggles in order to make informed decisions going forward. 

That still doesn’t explain all of those seemingly random swirling lines that surround the highs and the lows. On a surface pressure map, those lines are “isobars”—iso referring to “same” or “equal” and bar referring to barometric pressure. Those lines will tell us how quickly the pressure changes over a horizontal distance. 

Isobars not only imply wind speeds, they also imply wind direction. You may be aware that wind circulates around a Northern Hemisphere high-pressure system in a clockwise direction. 

Wind doesn’t travel concentrically along the isobars, though. The wind will tend to want to get away from the high-pressure area and be drawn in toward the low-pressure area. Hence, at the surface of the planet over water, wind around a high-pressure area will toe out from the isobars at about 15 to 20 degrees and toe in towards the low-pressure system by the same amount. 

Of course there are other lines on that weather map. Some of them, indicating frontal boundaries, have little icons located along their length. The ones with triangles represent cold fronts. The ones with semicircles represent warm fronts.

Cold air is denser than warm air. When it tries to move towards a warm air mass it’s like a bus trying to move a stack of pillows out of the way. It can do it pretty quickly. So, when a cold front comes through an area, the temperature in that area can change pretty quickly. 

Occluded fronts result from cold fronts overrunning warm fronts and the air masses mixing with each other. As those air masses mix, there tends to be less temperature differential and less wind along an occluded front than even along a warm front.

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