Air Masses

What you can expect when the barometer rises or falls
All forms of boating are directly affected by the weather—but what exactly is weather? It’s simply the reaction of the atmosphere—a relatively thin layer of air that surrounds the Earth—to changes in temperature and pressure.In simple terms, the atmosphere consists of huge invisible bubbles of air, often hundreds of miles in diameter, rising and falling, warming and cooling, like the colored wax in a lava lamp.The restless motion of the air masses is caused by simple physical laws, but the results are so complicated that even professional meteorologists armed with the latest computer technology can rarely issue a forecast that is valid more than three days ahead. Nevertheless, boaters who know a few basic principles concerning the movement of air masses can often forecast the weather in their immediate vicinity with reasonable accuracy.Warm bubbles of air are low in pressure and will rise; cold bubbles of air are higher in pressure and will fall. Air in a bubble of higher pressure will try to balance the pressures by moving toward a bubble of lower pressure; the action starts when two bubbles meet. These boundary zones are called fronts.The larger the difference in pressure between the two air masses, the faster the air will flow from high to low pressure. That’s the air movement we call wind.If your barometer drops 6 millibars (0.177 inch) or more in 12 hours, thus indicating that the pressure is changing rapidly, you may safely assume that a major low-pressure storm system is fast approaching—especially if the cloud cover has been increasing and the winds have either changed direction or increased in speed.Air that flows into a low-pressure area is forced to ascend; as it rises, it cools, causing the water vapor it contains to condense and form clouds—hence, the increasing cloud cover in the vanguard of a storm. But air descending from a high-pressure area and moving outward from its center tends to be warm and dry, so fewer clouds form. When your barometer is high, therefore, expect fine weather.See also Barometer; Buys Ballot’s Law; Clouds; Weather Fronts; Weather Maps.