Today we have many computer weather programs, phone apps and satellite communications. Why then do recreational mariners also need a meteorological understanding of wind prediction? Because they still get caught with their pants down.
It helps to know how wind is generated. Wind is simply air in motion between two masses of different atmospheric pressures, which is due to temperature differences and the rotation of the earth.
Land and sea breezes blow on or offshore as the sun heats the land, and it cools off again at night. A change in air temperature equals a change in air density. Wind blows from areas of cooler high pressure toward warmer lower pressure. Warming land causes air to rise, drawing in cooler air from the sea. Then, as the land cools later in the afternoon, and the land and sea air temperatures equalize, the breeze quits. After sunset, as the land air cools below the temperature of the sea air, the air contracts and sinks, resulting in a seaward outflow of breeze. It’s pretty reliable.
The big-picture patterns are reliable too, because the whole planet is constantly warmed by the sun, although unequally as there is more warmth in the equatorial regions and less at the poles. This creates belts of high and low pressure. Winds blow from (cooler) belts of high pressure toward equatorial (hotter) belts of low pressure. And wind circulates counterclockwise around lows and clockwise around highs in the Northern Hemisphere (reversed in the Southern Hemisphere). Rotational forces of the earth deflect this circulation, resulting in seasonal trade winds. Crossing guides and pilot charts provide this empirical information.
I once asked intrepid cruising friends what kind of weather forecasting they used in search of favorable conditions, since most of their circumnavigating was done (by choice) without an engine or electronics. As if giving directions to the corner store instead of crossing oceans, she answered simply, “We just ride around the stationary highs. We watch the barometer to keep an eye on the 1020 millibar line.” The ancients didn’t have the benefit of satellites and computerized data either, and they crossed oceans the same way in search of a quartering breeze.
Transient surface weather systems are trickier to predict, driven across the planet by the undulating movement of upper atmosphere air, the Jet Stream. In the Northern Hemisphere, moving surface areas of low pressure generally advance from west to east, with the path bending slightly poleward and winds circulating counterclockwise. Winds circulate clockwise and slightly outward around highs. Surface-level boundaries between highs and lows (different air masses) cause frontal passages. We use synoptic charts to find them. A source for information is the website of The National Weather Service Ocean Prediction Center.
Fronts associated with Northern Hemisphere low-pressure areas generally have characteristic behaviors. Cold fronts, with a falling barometer and often dynamic gusty winds, can arrive with squalls or thunderstorms. Southwesterly winds will shift abruptly to the north/northwest and sudden cooling temperatures occur after passing. On weather charts, a cold front is marked with a blue line of triangles pointing in the direction of travel. Warm fronts—with rising temperatures and light rain or fog—will have wind backing south to east, then veering to the southwest after passing. A warm front is marked with a red line of half circles pointing in the direction of the front.
Highs and lows are depicted like rings of an onion, with lines of equal barometric pressure, called isobars. The closer the isobars surrounding the center, the more wind circulates—clockwise and outward around a high, or counterclockwise and inward around a low. If a low looks like a tightly coiled spring, with isobars closely packed together, watch out. If a high is represented by isobars spaced far apart, light or no wind exists. Directional arrows show paths of moving systems, and wind arrows indicate surface winds. Studying the charts, we can anticipate changes in wind direction. Place your vessel’s position on the chart, study the paths of systems and visualize your planned route.
Here’s an old-school exercise called Buys-Ballot’s Law: A Northern Hemisphere observer facing into the true wind will locate a high-pressure center to their left (and slightly ahead) and the low-pressure area will be on the right (and slightly behind). The rule is reversed in the Southern Hemisphere.
You can further practice your prediction skills by drawing a diagram with wind vectors spiraling counterclockwise around a low’s center on a piece of paper. Add your position. You can visualize wind shifting as systems move and your relative position changes. Study the wind vectors and you’ll either see wind veering (a clockwise shift) or backing, counterclockwise.
Tropical storms and hurricanes are the ultimate lows and we have high-tech help to avoid them. Use it and stay well out of harm’s way. Remember the sinking of the Bounty with two lives lost in Hurricane Sandy in 2012, and the loss of 33 mariners aboard El Faro in Hurricane Joaquin in 2015? The National Transportation Safety Board concluded these tragedies occurred because both vessels ignored available data. In light of those sobering losses, here are a few mechanics to consider.
We know a Northern Hemisphere hurricane rotates counterclockwise with strengths of at least 64 knots. We also know its track can be fast and eccentric. This is why it’s prudent to chart the storm’s broadcast position and track and record changes in your wind observations and barometer. One side of a hurricane is stronger than the other, because the velocity of the hurricane moving along its track is added to the velocity of the wind circulating around it on one side and subtracted from the circulating wind on the other side. As seen from above, the right-hand side of the storm’s track is the “dangerous semi-circle” and the left-hand side is the “navigable semi-circle.” A vessel on the right side of the low experiences the wind speed plus the forward velocity of the storm, creating higher winds and bigger waves. On the left side, the wind speed and wave heights are lower because the net wind speed is the wind speed minus the forward velocity of the storm. I pray that none of us ever has to choose the best hurricane maneuvering tactics, but it goes like this.
Plot your location and the storm’s center. Update the plot as new reports are received. Record the wind regularly, watching for shifting winds. If it veers, shifting right, it confirms you are on the right-hand side of the track line. That’s the dangerous semi-circle and you should put distance between you and the passing storm by placing the wind and waves on your starboard bow at the safest speed possible, letting the storm pass astern of you. If the wind’s back, shifting left, you are on the navigable side of the low, with less wind and smaller seas. You should place the wind and waves on your starboard quarter, allowing the storm to push you away from the stronger center.
I recommend Chris Parker’s Marine Weather Center’s excellent forecasting and routing services. Cruisers can receive it by SSB, email or voice. Plus, because of the latest technologies, today’s satellite weather is easier to attain than ever.
Wind projection is essential seamanship and gives us agency onboard. Use it to your advantage and to avoid the worst. No matter which weather forecasting services you opt for, combine those with your own wind predicting skills to bring it all together and travel safely.
This article was originally published in the August 2021 issue.
