Boating Weather: Atmospheric Pressure

Pressure changes in our atmosphere are due to uneven heating of air. Wind flows from high pressure toward low pressure.
Pressure changes in our atmosphere are due to uneven heating of air. Wind flows from high pressure toward low pressure.

“Man lives at the bottom of an ocean of air,” reads the opening of the chapter on the atmosphere and its circulation in Knight’s Modern Seamanship. It goes on to say that sailors should be able to supplement official forecasts with personal observations in order to make intelligent decisions.

There are numerous sources of weather information these days, starting with the immediate convenience of weather apps on smartphones, Internet weather sites, and television weather reports. For deeper background, US Coast Pilots and the Sailing Directions, published by the Coast and Geodetic Survey of the Navy Department, along with Pilot Charts to name a few, are all helpful in providing useful information on prevailing winds, fog, ocean currents, and average weather conditions. These references, along with a broad understanding of how our atmosphere produces weather, will help you interpret local conditions and make those intelligent decisions about boating.

Gravity binds the gases surrounding our planet in layers. The lowest layer of the atmosphere is called the troposphere. Generally speaking it extends 6.5 miles up from the surface, has both vertical and horizontal air circulation, and is the area to which storms are confined. Our air is comprised of a mixture of gases, 77 percent  nitrogen, 21 percent oxygen, and one percent each of argon, carbon dioxide, and water vapor.  It is the water vapor in our air that is the most important in creating weather. That, and the heating of our atmosphere from convection, conduction, and radiation — and the fact that water has more capacity to absorb and hold heat than any solid or liquid except ammonia. These are the factors that help create thermal land and sea-breezes. The higher level of our atmosphere, the stratosphere, which is 10 to 20 miles from the surface, has virtually zero water vapor and hence, no storms.

Air is, well, as light as air, but the total weight of the atmosphere is enormous. If you were to weigh a one-square-inch cross section of our atmosphere from the surface to its outermost limits almost 70 miles up, it would exert 14.7 pounds of pressure at sea level. We typically measure atmospheric pressure with barometers, in inches of mercury or in millibar units. Fourteen point seven pounds per square inch is equivalent to 29.92 inches of mercury or 1013.25 millibars; this is the average, but atmospheric pressure at any location is constantly changing and it varies from place to place. Variations in pressure are due to temperature changes. Warm air expands, becomes less dense and has a lower atmospheric pressure. Conversely, cold air is denser, heavier, and has higher atmospheric pressure readings.

If we draw lines on a map to connect spots that have the same or equal pressure, we call these isobars. Pressure gradient refers to how close isobars are together. A “strong” gradient means isobars are closer together. Wind speed depends primarily on the pressure gradient. Wind direction depends on the direction of the pressure gradient from high to low, and is influenced or deflected by the rotation of the earth. This deflection, right in the northern hemisphere and left in the southern hemisphere, is known as the Coriolis Effect. The final determination of wind speed and direction is friction; land causes more friction, water less, so you might surmise that winds blow harder over water.

Prevailing winds on a global scale. Illustration courtesy of WikiMedia Commons.
Prevailing winds on a global scale. Illustration courtesy of WikiMedia Commons.

Since uneven heating of the earth causes differences in atmospheric pressure, which generates winds, and the equator is subject to more heat than the poles, it stands to reason that the excess heat at the equator is the basis for our worldwide wind pattern known as the prevailing winds. I won’t go into specifics of the prevailing winds except to say that sailors usually try to take advantage of them, and understanding them will enhance your weather observations and boating acumen.

Understanding barometric pressure, and knowing that winds runs parallel to isobars, has helped mariners for many years. One example is expressed by Buys Ballot’s Law: At sea, in the northern hemisphere, when the barometer needle falls and a rotating (cyclonic) storm is expected, if you face the wind, the storm center (low pressure) will be  to your right. Armed with this knowledge, ship captains have long been able to steer courses to avoid the worst parts of storms.

Being familiar with the causes of prevailing winds and knowing how to interpret barometric changes are valuable tools in any boater’s skill set.

For more about wind and weather, read these articles:

Written by: Peter d'Anjou

A USCG licensed captain and former merchant mariner, Peter d'Anjou is now a freelance writer and editor. A one-time executive editor at Sailing World magazine, he writes about his passion for racing and boating. Having managed a large yacht repair facility in the NE U.S. his background in boat construction and repair translate to the practical side of boat ownership.


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