It’s hard for any discussion of sailing—long-distance or ocean-crossing passages in particular—to not quickly turn to the topic of weather and storms.
Every human activity involves non-zero risk, and sailing is no exception. However, there are several tools prudent sailors can employ to mitigate and minimize risk from weather.
These tools include planning, forecasts, and observations.
By far the most vital strategy for sailors to avoid bad weather is planning to be in the right place at the right time. This means scheduling the voyage to occur in the most ideal season for the location.
Generally speaking, an ocean has its cyclone season during its summer months. Sailors can plan more precisely, though, by consulting what’s known as “pilot charts” to determine the best time of year for a passage.
Although pilot charts vary from source to source, they typically will show, on a regional and month-by-month basis, aggregate historical information such as prevailing wind speed and direction, currents, and frequency of storms.
A tool such as the Ocean Atlas app functions as a pilot chart, and offers more interactive planning capabilities.
It also pays to be aware of global weather patterns such as the El Niño Southern Oscillation (ENSO), which influences the sea surface temperatures that feed storms and other major weather patterns. Sailors can sign up to receive free monthly updates of ENSO status by emailing firstname.lastname@example.org.
Weather forecasting capabilities have improved dramatically in recent decades. The prudent mariner will study weather forecasts prior to departure, and, ideally, while underway.
Depending on the location and access to internet, very detailed graphical forecasts can be obtained easily prior to departure. Obtaining forecasts and other weather information while underway is more involved, but highly worthwhile. In remote locations or more than a few miles from shore, weather information must be obtained via radio or a satellite service.
Sources for forecasts while underway include:
- Text. Some satellite-based devices (such as the Garmin inReach) may offer very basic text forecasts. This is perhaps better than nothing, but is so limited it’s not recommended as a primary source for sailors.
- VHF radio. If you’re within range of the VHF WX (weather) stations broadcast in the U.S., basic voice forecasts are available.
- HF radio. Also known as “SSB” or “shortwave.” The exact capabilities and data available will depend on the radio and other equipment, but voice forecasts, weatherfax synoptic charts, gridded binary (“GRiB”), and satellite imagery are possibilities.
- Satellite. A satellite data connection generally offers the highest fidelity weather information available.
Each forecast source comes with its own considerations and limitations, which are imperative for the sailor to understand before relying on it.
First and foremost, understand how the accuracy and reliability of a forecast decrease sharply the farther into the future it goes.
But more subtly, for example, that GRiB files are not reviewed by meteorologists, whereas synoptic charts are. And that the now-time conditions depicted in many weather apps reflects what’s known as a “zero hour forecast” and not necessarily actual real-time observations or conditions.
Some other tools prudent mariners should consider include:
- Satellite imagery. Satellite imagery shows atmospheric moisture which results from vertical instability such as fronts and squalls, and cyclonic storms. It’s especially useful when planning to transit the intertropical convergence zone (ITCZ, colloquially known as the “doldrums”).
- CAPE. Some forecast sources offer a data point known as convective available potential energy or CAPE. CAPE can correlate with vertical atmospheric instability, which may portend squall and rain activity.
- 500 MB. Upper air observations and forecasts—in particular for the 500 millibar pressure contour—can be used by advanced mariners to make educated guesses about longer-term weather, especially for mid-latitude voyages. For details see the book Heavy Weather Avoidance and Route Design by Ma-Li Chen and Lee S. Chesneau.
- Weather router. An excellent resource is a weather router, which is often an experienced and professional meteorologist who will provide one-on-one service to help with planning, watch the weather, and alert about upcoming conditions. This may be an attractive option because not only will a sailor benefit from their expertise, but also a router typically has access to more and higher fidelity data to interpret. Ideally the weather router has sailing experience and understands how conditions affect boat operation, comfort, and safety.
The right software and service (such as PredictWind and Weathermuffin) can make a tremendous difference to the task of obtaining and analyzing weather conditions and forecasts while at sea. There are a wide variety of weather software options, with varying levels of ease-of-use and tech savvy required.
In addition to the above, a sailor’s own observations of local conditions can play a critical role in weather awareness and avoidance.
- Barometric pressure. A barometer—or even better, a digital barograph—is a vital observation tool. Rapidly falling pressure can indicate a forming or approaching low pressure system.
- Wind shifts. If the wind rapidly changes strength or especially direction, the forming or approaching of a low pressure system may be indicated. Buys-Ballot’s law may be used to estimate the bearing of the system.
- Clouds. Learning to “read” the sky is a valuable and rewarding skill. Recognizing an approaching warm front will help a sailor prepare (especially for those followed closely by a cold front), as will sighting a row of cumulonimbus clouds on the horizon.
- Radar. Because water droplets reflect radar, it’s a valuable tool for observing squalls, especially on dark nights.
- Temperature. If the temperature is observed to change rapidly, it could be due to a front.
- Change in sea state. Because ocean waves travel faster than storms, if a new swell is observed, it could indicate a strong wind or storm in the distance.
- Dangerous semicircle. If a cyclonic storm can’t be avoided, the sailor can strive to avoid the dangerous semicircle of the storm to reduce adverse conditions encountered.
- Sea surface temperature. Observation of sea surface temperature can aid in suggesting if conditions are potentially favorable for tropical cyclogenesis, the development of cyclonic storms. Generally a sea surface temperature of at least 26 °C (79 °F) is one of the required conditions.
Although, depending on the type of observation and the speed of the boat, there may be limited options for avoidance by the time the observation is made—making keeping up with forecasts while underway all the more important.
With proper planning, forecasts, and observations, prudent sailors have many tools at their disposal for increasing the chances of avoiding bad weather while sailing.