Boating Weather: Clouds

Boating when it’s nice out is fun; boating when the weather conditions aren’t perfect is challenging. Boaters who most easily adapt to the conditions usually have a knack for anticipating them. Anticipating and giving yourself some time to prepare for or avoid bad weather will be  more intuitive if you become proficient at interpreting the changing cloud cover. Next time you head to your boat, take a good look at the cloud cover. Compare it to the weather forecast you’ve checked on TV, or on your smartphone. Compare what you’re seeing in real life to what the forecasters are telling you is on the way. Clouds indicate the kind of weather you’ll encounter when you’re on the water, but are also a visible sign of weather to come. A skilled boater should be able to identify cloud types, know what weather changes are associated with different types, and be in step with forecasts — or even a step ahead.

Rough weather can pop up quickly. Using your own understanding of clouds along with local forecasts from the pros will keep you safer on the water.
Rough weather can pop up quickly. Using your own understanding of clouds along with local forecasts from the pros will keep you safer on the water.

There are three basic groups of clouds based on structure and height, Cirrus clouds are high and feathery, stratus are the layer clouds which tend to be flat and uniform when covering most of the sky, and finally there are cumulus which are heaped, lumpy masses.  Within those groups are the principal types: Cirrus, Cirrostratus, cirrocumulus—20,000 to 40,000 feet; altostratus, altocumulus — 8,000 to 20,000 feet; and stratus, stratocumulus, and nimbostratus below 8,000 feet.  A further breakdown of cloud forms fall into 10 genera with many subtypes and combinations. For a more thorough description, visit the NOAA/NASA Cloud Chart (PDF file), or any edition of the International Cloud Atlas (PDF file). Or check out cloud types on Wikipedia.

Cloud types are significant only when you consider the timing of their development, structural changes, and particularly the sequence in which they occur. For instance, cirrus clouds are usually the first sign of an approaching storm, but to be a real indicator of approaching weather, they should increase in number and be followed by cirrostratus clouds.

If cirrostratus exist, an indicator of a storm would be for them to thicken and be followed by altostratus. Altostratus are followed by nimbostratus and precipitation that started with the altostratus continues. Storm conditions are present with both altostratus and nimbostratus.

The end of storm conditions is often signaled by breaks in the low clouds and clearing skies to the west. Since storms in the mid-latitudes generally move from west to east, a storm’s approach is indicated by clouds gathering to the west, and a storm’s passing by clearing in the western sky.

Cumulus clouds forming in the morning indicate moist conditions, and convection will likely continue during the day, resulting in more numerous and higher-building clouds by afternoon. Watch out for thunderstorms.

Altostratus clouds are the most reliable weather indicator. They almost always indicate a front approaching, and impending, continuous, all-day rain or snow, if they continue to thicken.

My tips for boaters interested in weather prediction are to combine your knowledge of local forecasts with cloud observations. Notice the wind direction of the clouds and their sequence. It will give you a leg up on being a better, more prepared boater.

For more about wind and weather, read these articles:

Boating Weather: Atmospheric Pressure

Pressure changes in our atmosphere are due to uneven heating of air. Wind flows from high pressure toward low pressure.
Pressure changes in our atmosphere are due to uneven heating of air. Wind flows from high pressure toward low pressure.

“Man lives at the bottom of an ocean of air,” reads the opening of the chapter on the atmosphere and its circulation in Knight’s Modern Seamanship. It goes on to say that sailors should be able to supplement official forecasts with personal observations in order to make intelligent decisions.

There are numerous sources of weather information these days, starting with the immediate convenience of weather apps on smartphones, Internet weather sites, and television weather reports. For deeper background, US Coast Pilots and the Sailing Directions, published by the Coast and Geodetic Survey of the Navy Department, along with Pilot Charts to name a few, are all helpful in providing useful information on prevailing winds, fog, ocean currents, and average weather conditions. These references, along with a broad understanding of how our atmosphere produces weather, will help you interpret local conditions and make those intelligent decisions about boating.

Gravity binds the gases surrounding our planet in layers. The lowest layer of the atmosphere is called the troposphere. Generally speaking it extends 6.5 miles up from the surface, has both vertical and horizontal air circulation, and is the area to which storms are confined. Our air is comprised of a mixture of gases, 77 percent  nitrogen, 21 percent oxygen, and one percent each of argon, carbon dioxide, and water vapor.  It is the water vapor in our air that is the most important in creating weather. That, and the heating of our atmosphere from convection, conduction, and radiation — and the fact that water has more capacity to absorb and hold heat than any solid or liquid except ammonia. These are the factors that help create thermal land and sea-breezes. The higher level of our atmosphere, the stratosphere, which is 10 to 20 miles from the surface, has virtually zero water vapor and hence, no storms.

Air is, well, as light as air, but the total weight of the atmosphere is enormous. If you were to weigh a one-square-inch cross section of our atmosphere from the surface to its outermost limits almost 70 miles up, it would exert 14.7 pounds of pressure at sea level. We typically measure atmospheric pressure with barometers, in inches of mercury or in millibar units. Fourteen point seven pounds per square inch is equivalent to 29.92 inches of mercury or 1013.25 millibars; this is the average, but atmospheric pressure at any location is constantly changing and it varies from place to place. Variations in pressure are due to temperature changes. Warm air expands, becomes less dense and has a lower atmospheric pressure. Conversely, cold air is denser, heavier, and has higher atmospheric pressure readings.

If we draw lines on a map to connect spots that have the same or equal pressure, we call these isobars. Pressure gradient refers to how close isobars are together. A “strong” gradient means isobars are closer together. Wind speed depends primarily on the pressure gradient. Wind direction depends on the direction of the pressure gradient from high to low, and is influenced or deflected by the rotation of the earth. This deflection, right in the northern hemisphere and left in the southern hemisphere, is known as the Coriolis Effect. The final determination of wind speed and direction is friction; land causes more friction, water less, so you might surmise that winds blow harder over water.

Prevailing winds on a global scale. Illustration courtesy of WikiMedia Commons.
Prevailing winds on a global scale. Illustration courtesy of WikiMedia Commons.

Since uneven heating of the earth causes differences in atmospheric pressure, which generates winds, and the equator is subject to more heat than the poles, it stands to reason that the excess heat at the equator is the basis for our worldwide wind pattern known as the prevailing winds. I won’t go into specifics of the prevailing winds except to say that sailors usually try to take advantage of them, and understanding them will enhance your weather observations and boating acumen.

Understanding barometric pressure, and knowing that winds runs parallel to isobars, has helped mariners for many years. One example is expressed by Buys Ballot’s Law: At sea, in the northern hemisphere, when the barometer needle falls and a rotating (cyclonic) storm is expected, if you face the wind, the storm center (low pressure) will be  to your right. Armed with this knowledge, ship captains have long been able to steer courses to avoid the worst parts of storms.

Being familiar with the causes of prevailing winds and knowing how to interpret barometric changes are valuable tools in any boater’s skill set.

For more about wind and weather, read these articles:

Boating Weather: A Bit of Oceanography

Over many thousands of years, the outflow of the Hudson River has created the well-known undersea canyon.  Illustration courtesy of WikiMedia Commons.
Over many thousands of years, the offshore flow of the Hudson River has created the famous undersea canyon. Illustration courtesy of WikiMedia Commons.

I was fortunate to go to sea right after high school on a Woods Hole Oceanographic vessel, the Lulu, sub-tender for the deep submersible Alvin. I was only 18 and a mess-man in the galley at the start. I was quickly impressed with the relationship between oceanography and weather. My first trip took us offshore to the Hudson Canyon,  where the outflow of the Hudson River empties onto the continental shelf and carves its way to the deep abyssal depths of the ocean floor. Topside we encountered a brutal storm—the fury of which was, I learned later, due to the welling of cold water into the canyon from the ocean depths. My naïve happy-go-lucky whistling was the reason my shipmates gave for the storm, but I soon learned that the inherent physical attributes of the earth bear a more direct impact on weather than seaman’s superstition.

Understanding attributes like the ocean bottom’s topography, the ocean’s temperature structure, and the related atmospheric circulation of prevailing winds   and current patterns, will help you understand the big picture when you’re dealing with weather locally– even if you’re boating on a lake or river — specifically how storms are swept in a general direction due to these physical realities. This knowledge improves a boater’s ability to interpret weather changes and predict storms, and it starts with this big picture understanding.

All precipitation that falls on land originates in and from the oceans, and hence a description of oceanic conditions is vital to predicting weather. Later in my seagoing career, I was the designated weather observer during a circumnavigation. I’m hopeful my daily reports helped someone anticipate, forecast, and optimize safe boating.

In my next blog I’ll talk about atmospheric pressure, but if you’re interested in learning more on how oceanography impacts weather, I recommend Knight’s Modern Seamanship.

For more about wind and weather, read these articles:

Boating Weather: Thermal Sea and Land Breezes

All summer long I’ve been describing the southwesterly sea-breeze conditions typically found on Narragansett Bay to my boating guests. While the shape of the bay and the surrounding land contributes greatly to the result, the primary cause of the sea breeze is the rising of warm air and the sinking of colder, denser air. More scientifically, this kind of sea breeze is called a “thermal” wind and the same term applies to the reverse air-flow at night, or thermal land breeze.

As sunlight warms the land during the day, the air above the land loses density and rises, drawing in cooler, denser air from over the water. Illustration courtesy of WikiMedia Commons.
As sunlight warms the land during the day, the air above the land loses density and rises, drawing in cooler, denser air from over the water. Illustration courtesy of WikiMedia Commons.

According to Knight’s Modern Seamanship, water has more capacity to absorb heat than any solid or liquid except ammonia. Water’s unique capacity to store heat — but without a large change in its own temperature — accounts for the reason that land is warmer than its adjacent waters during summer. This affects large inland lakes as well as  coastal regions. Air over the land is heated, becomes less dense, and expands upward, pulling cooler air onshore from the surface of the surrounding water. These cooling sea breezes can penetrate inland for considerable distances (25 miles) even though they extend only a few hundred feet above the ground, while the same condition on inland lakes may penetrate for just a few miles.

At night, radiational cooling may cause the land to become cooler than air over the water, so air-flow in the opposite direction — a land breeze — occurs. Without the sun, nighttime differences between land and water temperature are not as dramatic, and hence the land breeze is typically not as strong as a sea breeze.

The thermal conditions in Newport, Rhode Island are so regular that you can almost set your watch by them. The southwest sea breeze comes in around noon and produces fair-weather clouds. Regardless of the overall weather pattern or wind direction, the sea-breeze effect is so dominant that it blows until the land cools around sunset. And all this is because of the uneven heating of land and water, or more specifically because water has a heat capacity of two or three times that of land.

For more about wind and weather, read these articles:

Boating with Weather Apps for Smart Phones

Eight of us are on  board, sailing on Narraganset Bay, RI, when a tremendous thunderclap gets our attention. A wicked looking massive black cloud arcing bolts of lightning is descending from the northern end of the bay. “Quick,” says the skipper, “who has a smart phone with a radar app?”

smartphone weather apps

Five people start scrambling for their phones. They independently track the path of the oncoming storm and concur it will pass just north of us.

I’m used to looking at the local weather forecast online before I go boating. I routinely compare various websites such as the Weather Channel , Weather Underground  and AccuWeather  for hourly conditions prior to leaving the house, but I’ve never seen such a wide array of phones and apps on board as this.  Weather updates from all the major brands had us comparing apps long after returning safely to the mooring.

WeatherBug radar overlay
WeatherBug’s map interface with a radar-loop overlay. The user can drop pins on the map to mark places and waypoints.

I haven’t had enough exposure to all these apps yet, and since lots of other people study this, I’ve added some helpful links to get you started. What I do know is, smartphone weather apps saved our bacon on the bay that day!

Marine Weather Apps for Your Smartphone (

Best Weather Apps: 10 we recommend (

Review of Marine Weather Apps for Apple and Android (

Weather Apps for Iphone (Weather Channel)

Weather Apps for Android (

Top 5 Free Weather Apps for Android (Samsung Center blog)

Give us your opinion and comments on the best weather apps for boating.

For more about wind and weather, read these articles:

Heavy Weather Boating

storm waves
Heavy weather can be frightening for some, exhilarating for others, and death or destruction to the unprepared.

Two incidents recently reminded me of things we all should know about boating in rough weather conditions. The first involved my participation on someone else’s boat in a Leukemia Cup charity event run by the New York Yacht Club (NYYC). While the event was held within the confines of Narragansett Bay in Rhode Island, the weather conditions were hardly ideal—a driving rain and 25 knot winds with frequent gusts over 30. Given the dreary forecast, the club did a nice job providing a weather update early in the morning and advising the participants not to venture out until a decision was made to cancel or postpone the start of the event. After a delay of an hour ashore, NYYC reminded skippers that the decision to race was theirs alone.

The rain let up just before noon so our mixed crew of experienced and newbie sailors set out from Newport harbor even though the wind continued to howl. Our skipper gathered the crew and explained basic man overboard procedure, although we did not practice it. Then we raised our mainsail, with a reef in it to reduce sail area, under the lee of Gould Island. Next to us, also raising sail under the lee of the island was Interlodge, a fabulous looking 52-foot IRC racing machine. I was completely surprised to see that this boat’s towering flattop carbon-fiber mainsail had no reef points at all, meaning that it had no way to reduce sail area other than not putting it up at all. Nevertheless they spun away, heeling heavily, toward the start. By the time we got to the starting area we noticed that only 15 out of the 60 boats registered for the event had shown up. By the time the first gun went off, accompanied by a 30- plus-knot gust, several more boats, including us, had prudently retired and headed back to harbor.

This incident reminded me that we are not in control of the weather; that discretion, proper equipment, and crew preparedness are the only things we can control, and that we are usually never completely prepared. Did I mention that while several of us were wearing our own inflatable harnesses and PFD’s, some weren’t? And the skipper never made mention of where the boat’s PFDs were stowed.

storm sailing
Being prepared and equipped for being caught in a storm is entirely different than voluntarily exposing your crew, guests and boat to one.

The next incident happened when I got back to work the following day. Apparently, the storm was bad enough farther up the coast that it caused the yacht club where I work to cancel launch service while I was off trying to race. The waterfront director relayed to me that several club members who wanted to get to their boats were angered by this and one actually challenged him about the protocol for cancelling the service. I was pleased to hear there is no such protocol, just a judgment call that people in responsible positions have to make sometimes. The lesson here is that you should never be pressured by anyone to do something you think is unsafe, like electing to go out in a storm.

Heavy weather can be frightening for some, exhilarating for others, and death or destruction to the unprepared. I’ve sailed across oceans and been caught in storms where we had no choice. Hopefully, you’re in control of your own actions, the safety equipment on your boat, and ultimately the preparedness of your crew and passengers. Practice man overboard and other emergency practices, indicate to the crew where your safety equipment is stowed and, if you have a choice, don’t fool with Mother Nature—you can’t win, you can only survive.