Strategy: Unlocking the Mystery of Gradient Wind

In previous columns, we discussed the importance of understanding the gradient wind as a basis for predicting where the next shift will come from (“Strategy,” April ’98). Similarly, the gradient wind can help you predict the strength, direction, and character of the sea breeze.

You will recall that the gradient wind isn’t a real wind--it’s a theoretical wind based on the change in air pressure from one location to another. All this talk of theory may be a little heady, but it turns out that the actual wind 1,000 or 2,000 feet above the surface is normally pretty close to the gradient wind. Knowing the wind at that level may help an airplane pilot, but it isn’t much use to those of us who are sailing around at the surface!

Finding out the wind at 1,000 or 2,000 feet above the surface is slightly easier than determining the gradient. Over U.S. coastal locations, the National Weather Service launches weather balloons daily to observe the wind, and observations of winds aloft are available for locations all over the country. Coastal Doppler radar sites now observe wind aloft, too. For really big events like the Olympics, I’ve even launched my own weather balloons. You can do this, too, with some helium-filled party balloons. Track the balloon as it rises, and you can tell how the wind changes with height, both in direction and speed.

For the more sophisticated weather folk out there, computer models directly forecast the gradient wind and winds aloft. You can access this information through the Internet. For example, wind aloft forecasts are available for Nantucket along the southern New England coast, as well as Southern California and the Great Lakes region. Also, the winds aloft forecast can be obtained online through a data system reserved for registered aviators. So if one of your crew is a pilot, you’re in luck.

One advantage of knowing the direction of the wind aloft is that it doesn’t vary from place to place as much as the actual surface wind does. For example, the gradient in one location can be representative of the gradient wind for locations within 100 miles or so. This is particularly true when weather systems are more or less stationary. So, the winds aloft measured by the National Weather Service weather balloon at one location can often be used to describe the gradient for the larger area.

In general, the gradient wind changes much more slowly than the surface wind. However, it does tend to change over the course of a day’s worth of sailing, so it’s wise to take this into account. To do this, it’s necessary to understand how the gradient relates to the large-scale weather features we observe on a typical surface weather map.

First, let’s talk about highs and lows. In the northern hemisphere, gradient flow is clockwise around highs and counter-clockwise around lows. If you’re sailing on a day when a high-pressure cell is passing from west to east, just north of the course area, the gradient will veer right. For example, if the high was going to be northwest of you in the morning and northeast of you by evening, you can expect the gradient to be from the northeast initially, then veer to east and then southeast by the end of the day.

When we consider the behavior of gradient winds around a front, a more complicated case results. Let’s take a look at a typical cold front. Ahead of a cold front, the gradient wind is typically from the southwest. Behind the cold front, the gradient is typically from the northwest. If the forecast calls for the passage of a cold front during the day, the National Weather Service forecast will usually call for southwest winds shifting to the northwest after the front passes. The real case is rarely this simple, however.

The gradient around a cold front backs as the cold front approaches, then veers abruptly with the passage of the front. Then, the wind will often gradually back again within the next hour or two, effectively wiping out a portion of the abrupt shift.

Let’s say you’re sailing on a day that a cold front is approaching. As the front approaches, the winds initially will be southwesterly. As it gets closer, the gradient wind will back left toward the south until just before the front passes. Immediately after the front goes by, the wind veers abruptly to the northwest. Then as the front moves away, the wind backs gradually toward the west-northwest.

Remember that the actual surface wind typically will be left of the gradient wind by 10 to 15 degrees over the open ocean or as much as 20 to 30 degrees on an inland lake. Also, the actual time scales of gradient wind changes can vary substantially season to season. In the summer, gradient changes tend to be very slow. In the transitional seasons of spring and fall the gradient wind will change quickly, and often more dramatically.

One additional consideration is the case where local winds are different from the gradient wind, such as a sea breeze. A sea breeze is a good example of a locally generated wind, which at times directly opposes the gradient wind. The strength, direction, and character of as a sea breeze attained during a race can be substantially influenced by the gradient. For example, for an offshore gradient, the more perpendicular the wind is to the shore, the harder it is for the sea breeze to develop. In cases where the gradient is blowing hard offshore, the gradient can keep the sea breeze from developing at all or cause it to form farther offshore.

Whatever conditions you’re sailing in, the gradient plays an important roll in defining the wind you observe.

Coffee and Gradients

It’s Friday morning, the first day of your class championship, and you’ve just dragged yourself out of bed. What are you going to do? Turn on “Good Morning America” or “Today?” No way! It’s time to check the weather and build your own forecast. Fire up your percolator, your computer and your modem, and while you’re waiting for the coffee, step outside and look skyward. Check for low clouds (cumulus or stratus), as their movement often will indicate the relative direction and movement of the gradient wind. If the clouds are flat and uniform, expect the wind movement to oscillate slow and have few gusts. If the clouds are lumpy or vertically developed, expect more random windshifts and greater gusts. If it’s calm at the surface but the clouds are moving at a relatively fast pace, expect those stronger winds to mix down to the surface later.

Coffee’s on! Fill your cup to the brim and dial up your favorite weather websites. A couple of my favorites are:wxp.eas.purdue.edu, www.rap.ucar.edu/weather/index.html and maps.fsl.noaa.gov/RUC.real_time.cgi.

First, look at the surface pressure map and remember that the gradient wind is approximately parallel to the isobars on the map. Isobars close together indicate a stronger gradient wind. Isobars farther apart indicate lighter gradient. Next, look at the surface wind forecast maps. These usually indicate the gradient wind and do not have the resolution to forecast local winds. Correlate these with the winds you observe and you can develop an idea of what the wind will do.

Finally, check the satellite picture. Extensive cloud cover means that the gradient wind will likely dominate. Scattered clouds means more mixing and more variable wind and local breezes are possible.

Armed with this knowledge, you’ll have the confidence to tell your skipper or crew that one side of the course will be favored all day. If you call it, you’ll be a hero. If you’re wrong, well, there’s always tomorrow.