Many mariners are familiar with the methods of determining wind speed from a surface weather chart. Recall that a surface weather chart depicts isobars, or lines of constant pressure from which the position of highs and lows can be determined. Since wind is generated by horizontal pressure differences we can use the isobar spacing, which will indicate the magnitude of the horizontal pressure gradient to determine the wind speed. Whenever the isobars are closer together, the horizontal pressure difference is greater, and the winds will be stronger. When the isobars are farther apart, winds will be lighter.Â
Â Â Â If you sail, especially in a smaller boat, then you are aware that the wind speed is rarely steady, but instead varies on a short time scale. The amount of variation can range from very little in some situations, to a great deal in the case of extremely gusty conditions. The gustiness of the wind does not necessarily increase as the wind speed increases. It is possible to have stronger wind be rather steady with only slight variations in speeds, and it is also possible to have rather light winds, but with frequent gusts well above the average wind speed. For the sailor, the gustiness of the wind has a great impact on the process of sailing. This situation is particularly noticeable for the skipper of a small boat, hanging onto a tiller with one hand and a main sheet with the other. For these very small craft, a very gusty wind has the potential to quickly knock the boat over, damage sails, and in general make the skipper work a lot harder and be on edge constantly. On a larger sailboat, it is often the wind gusts that will lead to blown out sails and other equipment failures. So how can we get a handle on the gustiness of the wind?
Â Â Â First, we need to understand where the gustiness comes from. While wind is, by definition, the horizontal motion of air, we must remember that the atmosphere is three-dimensional. In general, in the middle latitudes, wind speeds tend to increase with height, meaning as one goes up in the atmosphere, the wind speeds tend to become stronger. Most of the motion in the atmosphere is horizontal, but there is some vertical motion, even though it is usually smaller in order of magnitude. When vertical motion is present, particularly downward vertical motion, the opportunity for downward momentum transfer exists. In other words, some of the stronger winds at upper levels in the atmosphere can be brought down toward the surface. This will lead to occasional short, but sometimes dramatic increases in surface wind speed, which are wind gusts.
Â Â Â The next question we need to ask is under what circumstances will this downward momentum transfer occur? This requires us to look at atmospheric stability, which is the propensity for upward or downward motion to occur. Without going into a long discussion of this topic, in general, the atmosphere will be less stable (or more likely to allow for vertical motion) when the low level temperatures are significantly warmer than the upper level temperatures. Note that it is usually the case that low-level temperatures are warmer, but they need to be warmer by more than the usual amount for the instability to exist. This will occur in a few situations. First, on a typical summer day, the warmest low-level temperatures will occur in the afternoon, and this is why the wind will tend to be a bit more gusty at this time of day. Second, whenever cold air moves over warmer water, the water will warm the lower levels of the atmosphere, which will produce instability and lead to increased gustiness of the wind. This will typically occur after the passage of a cold front, particularly off the east coast of continents where the air behind the cold front is likely to be colder, since it is of continental origin. This is especially true during the winter.
Â Â Â But the true key to larger scale gustiness lies at the upper levels of the atmosphere. In the situation behind a cold front where instability exists, it is more likely that there will be significantly colder air at upper levels, and this makes it more likely that the instability will allow for downward motion rather than upward motion. Remember that colder air is denser, and therefore heavier, so it will tend to sink in this situation. This sinking air will carry some of the higher upper level wind speeds toward the surface and lead to the gustiness. By examining weather charts at the 500 millibar level, one can get a handle on how gusty the wind will be. A good rule of thumb is that once a cold front has passed, in the colder air behind the front, up to 50 percent of the wind speed at the 500 millibar level can be realized at the surface in the form of wind gusts.
Â Â Â To summarize, use the spacing of the isobars on a surface weather chart to determine what the sustained wind speed will be, and then look for situations where atmospheric instability will exist to infer that the wind will be gustier than usual, and finally, when behind a cold front, look to the 500 millibar chart and use 50 percent of the wind speed from that chart as a guideline for how high the surface wind gusts will be.
About the AuthorÂ
Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years for a large private consulting firm in Massachusetts, he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991. A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine with his wife and two sons. Ken’s Web site is: www.locusweather.comÂ