Know the Weather When You’re Boating

Written By:  cws-admin April 9, 2016 at 3:27 pm

Secrets in the Sky

the sky

Every boater knows the weather can make the difference between a safe and pleasant cruise and a harrowing adventure. Here’s how to know what’s coming.

Boaters, whether cruising inshore or offshore, have the responsibility to know what conditions are expected for the duration of their intended voyage. They learn to pay attention to the forecast and know what parts of the information contained in it will have an impact on their boating, and they make cruising plans accordingly. But it’s the smarter boater that also learns the telltale signs in the sky, and can spot the weather patterns as depicted by the weather charts.

It’s a prerequisite of the captain’s job, before leaving the dock, to know what advisories, watches, and warnings are contained in the marine forecasts that are in play throughout a cruise.

The most important weather reports are generated and issued by the National Weather Service (NWS), the largest field office of the National Oceanic and Atmospheric Administration (NOAA). A critically important function of the NWS is its marine forecast program, and some of the most useful parts of that are the marine advisories and warnings, which we need to discuss in detail.

The lowest criterion for a marine warning is a marine advisory called a “Small Craft Advisory” (SCA). A SCA can be issued for wind, waves, and even ice. The criterion for the issuance of a SCA varies, and is specific to geographic areas of the coastal and inland waters of the U.S., including the Great Lakes. For winds, the range will be somewhere between 20 to 33 knots. Keep in mind, this refers to sustained winds, determined by wind averages of one minute. Also, keep in mind that wind gusts lasting just a few seconds—and that are sometimes up to 40 percent higher than sustained winds—are generally not within the realm of accurate forecasting, although some West Coast offices do forecast them, and base their advisories, watches, and warnings on those figures.

The criteria for wave heights, defined as “the average height of one third of the waves present,” will also be variable. For example, the NWS office in Morehead City, North Carolina, can issue a SCA for wave heights of 5 feet, while the office in Eureka, California, uses a benchmark of 15 feet. The reason for this variance is the constituent mariner base for those geographical locations that the NWS serves, whether it is recreational power boaters in Morehead City or the commercial fishing industry in Eureka. A SCA may also be issued when sea or lake ice exists that could be hazardous to small boats.

Clearly, a SCA precludes most recreational vessels from leaving the dock and should direct most of those underway to seek shelter as soon as possible. The next category up from a SCA is a Gale Warning (34 to 47 knots sustained wind), then up to a Storm Warning (48 to 63 knots sustained), and finally, a Hurricane Force Warning (64 knots and higher sustained or gusts). These warnings should leave no doubt as to whether or not a recreational boater thinks he should chance an encounter with these higher winds, much less strategize to take on their wave-height conditions, which drastically increase and are exponentially higher than a linear increase within SCA wind ranges.

While there is no precise definition of what the physical size of a small craft is, all vessels should be aware of the conditions associated with a SCA advisory. And as noted, each geographic forecast area of the U.S. has its own separate criteria for SCAs, which have been based on casualty or vessel damage (in other words, the standards have been written in blood).

Weather Underway

Ocean waves

As with any critical element of safe boating, be it maintenance or navigation, conditions need to be checked regularly and logs should be kept. Weather, too, should become part of your onboard routine. Simply put, boaters need to observe weather conditions at least every three hours, then log them. Data points that generally prove useful include observations about the sky, including cloud types and estimated amount of cover, pressure, ambient air temperature, dew point, and seawater temperature, visibility, barometric pressure, and wind and sea-state conditions.



Clouds offer visual evidence of the conditions within the atmosphere. They are indicators of present weather conditions as well as those of the near future from six to 24 hours out. Clouds are made up of water droplets, ice crystals, or both. Their makeup varies with temperature, and they are found mostly in the lower portion of the atmosphere called the Troposphere. There are a few exceptions where clouds can extend beyond the Troposphere (typically in Midwest states) but boaters are not likely going to experience the same type of severe weather along the U.S. coastal sections.

Using a Weather Router

Here are four bits of hands-on advice from Bob Jones of Ocean Marine Nav Inc. about using a weather service on your next voyage.

Hint: Communication is key.

1. “Depending on how far offshore they’re cruising the minimum they are going to need is a cell phone,” Jones says. “Some cruisers start off cruising along coastal and inland waterways and they’ve got to have a cell-phone connection to contact the weather router.”

2. “If they’re even thinking about going offshore for any period of time, including Catalina and the Channel Islands or the Bahamas, areas like that, you’ve got to have a satellite phone as back up,” Jones says. “Satellite comminucations are much better than they’ve been in the last several years, much more stable. There are lots of varieties of satellite phones to choose from and the prices are coming down. I’ve spoken to people in the Indian Ocean and the Mediterranean Sea and anywhere in-between so [I know] they work really well.”

3. “Onboard, downloadable e-mail systems or Internet systems let you view forecasts like mine, which I can send directly to you via a satellite hookup,” he says. “You can download it at certain times of day or have it constantly on, depending on where you are. And you can get graphics with that.”

4. “Being able to speak to a router who can give you some general information regarding the pattern or the routing options that you have available is important,” Jones says. “Speaking to somebody in weather forecasting who’s familiar with the local conditions can be helpful too, and also, depending on the navigation, they can give advice to get you through the more challenging weather patterns that you come across.”

Clouds are classified by height as defined by their vertical elevation (a cloud’s bottom as seen by boaters) and appearance (horizontal and layered or vertical in extent). Latin words are used to define a cloud’s height and type. For example: Stratus Clouds (from the Latin word stratomeaning “layered”) are low in height—below 6,500 feet and layered in appearance. Altostratus clouds (from the Latin words alto meaning “middle” and strato) are midheight—ranging between 6,500 and 18,000 feet and appear layered. Cirrostratus clouds (from the Latin cirromeaning “curl of the hair” and strato) are high in altitude—18,000 feet and above, and look like layered, curly hair. Put together the Latin to figure out the cloud layer and cloud type.

Clouds and wind are one of the two most important factors in short-period weather forecasting from local conditions alone.

Wind and Pressure

Relating to wind is the very important barometric pressure, which measures the weight of the total column of air above a given level in the atmosphere. The unit of measure used in the U.S. generally is the millibar (mb), though at airports, pressure is reported in inches of mercury (1 bar, or 1,000 millibars is equal to 29.529983 inches of mercury). The numerical average value for surface pressure in millibars is 1013.2 mb for sea level, but the pressure can range between 940 mb to 1050 mb. The instrument that boaters can use today in measuring this pressure that is becoming more widely accepted is the digital barometer. The variance in numerical pressure throughout the course of a 24-hour day is dependent on both large-scale weather systems and their impact on local-scale weather. This variance of pressure should be recorded in the weather log at least every three hours (consistent with the physical station plot of pressure changes called pressure tendency).

Logging is key because the pressure tendency record of three-hourly barometric pressure values will tell a boater specifically what the pressure is doing, be it falling, rising, or steady. Pressure tendency relates to air motion, both horizontal and vertical. For air motion in the horizontal, one can easily relate to that, as we feel it (wind) and see its impact directly on the ocean surface, in the form of waves. The vertical component of air motion is much less sensibly apparent, but relates to cloud formation or dissipation. Falling pressures coincide with cloud formation; rising pressure coincides with diminishing clouds. It is important for the boater to note the rate of rise and fall of pressure to determine how quickly winds will increase or decrease, and how fast clouds will form and diminish. For example, if barometric pressure falls 1 millibar per hour over a six- to 24-hour period, it will result in rapidly deteriorating weather including clouds, precipitation, and especially a serious increase in wind speed (as much as a 40-knot increase).

Such rapidly changing conditions only highlight the importance of logging barometer readings every three hours and noting the change. In conjunction with sky and wind, and sea-state observations, this will give a boater a confirmation of what the short-term weather will be over the next six to 24 hours, as compared to what the forecast may have been understood to be before leaving the dock.