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The term “weather window” is frequently used in connection with ocean voyages. It’s that period eagerly awaited by voyagers when conditions are right to leave harbor and begin a passage, whether it’s a short hop or an ocean crossing.

This is a broad definition, and it will be interpreted differently depending on the situation. This is as it should be, since every ocean voyage is different, with vessels having disparate capabilities and crews having various levels of experience.

This means, of course, that the process of determining a weather window is a very individual process, and it should be approached this way by skippers and crewmembers, as well as professional meteorologists providing weather forecasts and routing services.

Defining your weather window should begin long before an anticipated ocean passage. The first step is to look at the size and type of the vessel and decide what are the upper limits of winds and waves that it can tolerate. In the case of a sailboat, the boat's upwind performance will determine what wind directions will be acceptable. The level of experience of the crew will also play a role in determining how much wind can be tolerated.

The schedule constraints for the passage will play a role as well. The skipper of a passage being undertaken for pleasure has more flexibility in shaping his schedule and will likely have less tolerance for strong winds and rough seas than a skipper who has a delivery timetable to meet or who has limited crew availability.

Defining what is considered acceptable will allow weather patterns to be evaluated objectively as the start of the passage nears, and it will prevent "bending" the acceptable parameters to fit the weather pattern, which could result in the vessel's getting into heavier conditions than originally intended.

For a longer passage, such as trans-oceanic voyages that are anticipated to take more than 10 days, long-range planning is essential. After determining what conditions will be acceptable for a particular passage, the next step for the longer passages is to determine the best time of year to undertake them. Pilot charts are very useful tools for doing this. Weather data collected over a long period of time are used to generate these charts. They are presented in a map format with the wind information shown by sector.

Separate charts are issued for each month. The wind information is broken down to show what percentage of the time the wind blows from a certain direction, what percentage of the time wind speeds exceed certain values (such as gale-force winds), and what percentage of the time winds are less than certain values. From the information presented in the pilot charts, it can be determined whether conditions along the proposed route will be acceptable, keeping in mind that the data is based on averages and that variations occur in any given year.

Expect variability

The percentages shown on the charts are a good measure of how much variability can be expected. By examining the charts for each month and exploring different potential routes, one can determine a route and a time of year that is likely (but not certain) to offer the best possible conditions for the passage. It is also a good idea to have a basic knowledge of the tropical cyclone (tropical depressions, tropical storms, hurricanes, typhoons) patterns of the region of interest. These systems are likely to show up on the pilot charts as very-low-probability events for a given point, but, as is well known, they can produce extreme conditions that are dangerous for ocean-going vessels.

Strong winter storms can also pose serious threats, but because they are larger and somewhat more frequent than tropical systems they will be better represented in the pilot charts.

Pilot charts are of some use for shorter passages as well, but a detailed analysis of them is not quite as critical. For passages of less than a week, particularly at higher latitudes, there is enough variability in weather patterns that a weather window may be found even though the pilot charts may show a low probability of acceptable conditions. The pilot charts should not be dismissed entirely, however.

As the time of the passage approaches, the actual weather situation should be evaluated. Examining current weather conditions for the area of interest and the surrounding region is critical to get a feel for where the weather systems are and how they are evolving with time. The weather conditions over the waters for the several days leading up to departure will also directly affect the sea state. These conditions can be gleaned from observations and weather maps widely available in newspapers, on television, or on the Internet. Forecast information must also be prepared and evaluated in order to determine whether or not a weather window will exist for the passage. This is an ongoing process, one that will begin several days before departure and continue up until the time of departure.

Preparing a weather forecast for an ocean voyage is somewhat like shooting at a moving target because the forecaster is not forecasting for a fixed geographic point but rather for the position of the vessel, which will be constantly changing. Therefore, it is imperative to know what the expected speed made good of the vessel will be and what variations weather and sea conditions may produce in the progress of the yacht. There is a variety of ways to obtain weather forecast information, including using forecasts from government agencies when they are available and appropriate, as well as hiring a private meteorological consultant to do the forecasting.

Forecast accuracy

Weather forecasting's current state of the art allows forecasts to be made for up to two weeks or so, but the accuracy of the forecast decreases as the time range of the forecast increases. Beyond a five- to seven-day period, the value of the forecast is mostly in predicted general weather patterns and not so much in specific predictions of the location of weather systems or the conditions they will produce. This means that the weather forecasts will be used quite differently depending on the length of the passage being planned. For an extended passage of 10 days or more, study of the pilot charts takes on a greater importance since accurate forecasts for the end of the passage are nearly impossible to obtain before the passage begins. In these cases, an accurate forecast prior to departure is essential to avoid unacceptable conditions through the early portions of the passage, though later in the passage it is possible that unacceptable conditions may develop that could not be foreseen before departure.

Using the pilot charts to choose the time of year of the passage can help reduce the possibility of adverse conditions but cannot rule them out completely. Longer passages tend to have a bit more flexibility in the route, and for this reason it is best to continue to receive forecast information while underway so that, if it appears that unacceptable conditions will develop during the passage, the route can be changed to minimize the impact of such conditions.

Evaluation of weather forecast information should be the primary tool for making a departure decision for a shorter trip. Frequently, shorter passages don't have much, if any, flexibility in routing, so the choice of when to depart will dictate the weather and sea conditions for the entire passage. For both shorter trips and more extended passages, when making decisions based on weather forecast information, it is important not to forget what has been defined as "acceptable conditions" for the passage. It is very easy to get caught up in the preparation for departure and the evaluation of weather forecasts, and this is why it is a good idea to set the parameters for acceptable conditions well before this time arrives.

There are other factors that will need to become part of the determination of a weather window. These include local conditions at the departure port, such as tidal currents, and whether or not it is safe to navigate out of port during darkness or other periods of poor visibility. The effects of ocean currents, such as the Gulf Stream, on the passage also need to be considered, particularly the combined effect of these currents and winds from certain directions, which can produce rough seas. Air temperature is also a concern, particularly when it is cold enough for freezing spray to occur.

A passage scenario

The use of a simple example can illustrate the complexities of the process for determining if a weather window exists. Assume that a 70-foot ketch will be leaving Newport bound for Bermuda in early October. The yacht will make about 180 nautical miles per day; therefore, the passage should take about three and a half days. The crew is experienced, but the yacht has had extensive rigging work done that has just been finished. The owner of the yacht wants the vessel in the Caribbean by mid-November. The captain has decided that gale-force winds could be acceptable if they were abaft the beam and not accompanied by seas of more than 10 feet, but that winds of 50 knots or greater would not be acceptable. Also, an extended period (more than one day) of headwinds would be unacceptable.

Rather than go through the details of the weather forecast process, consider the hypothetical weather scenario depicted in the accompanying illustration. Notice that a storm will move through New England coastal waters on Monday and will pass east of Atlantic Canada on Tuesday. A ridge of high pressure will move through the region of the rhumb line later Tuesday. Another low will develop along the southeastern U.S. coast and move quickly north-northeast toward the New England coastal region through the Tuesday-Wednesday time frame, and the cold front extending south from this low will move east into the vicinity of Bermuda by early Thursday. The front will slow down east of Bermuda on Friday, and another low may develop along the front.

Assuming that the yacht is ready to leave on Monday morning, the determination of when to depart will be based on determining when a weather window exists. A Monday departure does not fit the criteria as northeast gales are likely in the Newport area. By themselves they could be acceptable, but seas are likely to exceed 10 feet, which is not acceptable. Also, a departure Monday would bring the yacht into the Gulf Stream early on Tuesday with gale-force northerlies, and this would produce very rough seas. A Tuesday morning departure would feature northwest gales, but seas would not be as high and would probably be within the acceptable range. Winds would diminish through the day, and as the yacht moved through the Gulf Stream on Wednesday winds would be fairly light in the morning with seas therefore remaining acceptable. Winds would pick up from the south Wednesday afternoon, but this would not result in rapidly building seas in the Gulf Stream since the winds would be in the same direction as the current. Winds would remain nearly on the nose through Wednesday evening but would become more southwesterly through Wednesday night as the cold front approached the yacht from the west. Winds would shift to the northwest as the front passed the yacht later Wednesday night, but wind speeds likely would not exceed 25 knots through this period. Northwest-north winds would probably prevail through Thursday and through Friday as the vessel moved into Bermuda.

This analysis indicates that a weather window exists for a Tuesday departure. Wind and sea conditions fall within the acceptable range.

Another factor to be considered includes the recent rigging work on the vessel, which will be seriously tested for the first time. A Tuesday departure will certainly test this rigging. However, with winds diminishing after the first 12 hours or so, lighter winds would allow adjustments or repairs to be made if needed, and, in the extreme case, would allow a return to Newport with acceptable weather conditions.

Gulf Stream conditions

The conditions in the Gulf Stream are always a concern, and in this case relatively quiet conditions are likely to prevail. The amount of upwind sailing is less than ideal, but it falls within acceptable parameters. A departure on Wednesday would lead to the vessel having to deal with stronger winds in the Gulf Stream and a longer period of winds on the nose; departure later in the week would lead to rougher conditions in the Gulf Stream since the extended period of northerly winds over the stream would lead to building seas.

Although the weather analysis seems rather straightforward in this case, keep in mind that the forecast process has been greatly simplified, and that possible variations in the forecast have not been presented. In particular, there may be a great deal of uncertainty regarding the development of low pressure along the front east of Bermuda later in the week, which could have significant impact on the vessel at that time. The point of the example, though, is to illustrate that, even in a simplified situation such as this one, the determination of whether or not a weather window exists is complicated.

It is certainly worth considering using a forecasting service to help with the process of dealing with weather. Even if a skipper has some meteorological knowledge and feels comfortable evaluating the weather situation, the input of professionals can be very helpful in determining the appropriate weather window.

By Ocean Navigator