Pressure

Atmospheric pressure is the weight of all of the air above the height of an observation. The changes in atmospheric pressure are a commonly used way to forecast short-term changes in weather. A rule of thumb to remember when observing atmospheric pressure is:

• High Pressure - stable conditions, sinking air with cloudless skies and fair weather.
• Low Pressure - unstable conditions and rising air, that can produce clouds and precipitation.

The surfing community commonly refers to low-pressure systems as “onions.” This term is due to the appearance of the isobars (lines of equal pressure) drawn around areas of low pressure. The tighter these lines are drawn, resembling the layers of an onion, the stronger the storm. The spacing between the isobars also gives a good indication of the strength of the winds, and the orientation of the isobars generally indicates the wind direction associated with a storm.

Source: Ocean Prediction Center | NWS