from
The Sailor's Weather Guide
by Jeff Markell
Chapter 3: Onboard Observations
VISUAL OBSERVATIONS
Meaningful visual observation of weather phenomena requires practice, practice, and more practice. However, it is simple, easy, rather fun, and soon becomes second nature. Just as with instrument observations the most important factors with which you are concerned are changes over time.
Clouds
Of all the onboard observations you will make, those of clouds will be the most helpful to you in supplementing the information you have received from Weather Service. Clouds provide extremely clear and informative visual signals as to atmospheric conditions and activity. The various cloud types that you will observe while sailing are discussed and illustrated in detail in Chapter 10. They fall into three fundamental groups: cumulus, stratus, and cirrus.
Cumulus clouds, as noted above, are produced by air from low levels moving upward and cooling. Air that moves up from surface levels is immediately replaced by additional air moving in horizontally at the surface. The sailor feels this replacement as wind.
The small fair weather cumulus “puff balls” result from weak, upward airflow, which has little effect on surface air movement. However, the larger and taller ones are visible indicators of more vigorous vertical air movement. The sailor on the surface will find that when cumulus clouds are growing bigger and higher, they are often accompanied by winds that are becoming stronger and more erratic in both intensity and direction. Beneath cumulus clouds, conditions often change rapidly. Squally winds may alternate with calms while showers alternate with clear and sunny periods.
In contrast, stratus clouds are normally accompanied by winds that remain fairly steady both in intensity and direction. In a sky with stratus clouds overhead, changes will occur slowly. If they are thickening and lowering, it will probably rain, but not immediately. When they are thinning and moving higher, clearing may follow, but, again, not right away.
If Weather Service has predicted a major storm, high, thin cirrus clouds are likely to be your first indication that they were right,
particularly if they gradually get thicker and cover more and more of the sky. However, in fair weather they may appear for a while, and then dissipate. They may appear and dissipate several times in the course of a day without an appreciable effect on the weather. In any event, when they do indicate changing weather on the way, the change is distant. There’ll be no need to batten down hatches for a while yet!
Waves
Another plainly visible indicator of traveling weather disturbances is sea condition—wave height, speed, and direction (see Chapter 6).
Waves are built up by the action of wind on the ocean surface. In fair weather, a sea that is calm in the early morning may develop swells of 2 to 3 feet by mid to late afternoon when the day’s wind is blowing at 10 to 15 knots. These waves will come from the same direction as the wind that built them. Typically as the wind dies down at the end of the day, the waves die as well.
The longer and harder the wind blows, the larger the waves will be. The strong winds of a storm passing over large areas of water produce giant waves that radiate out over the sea for hundreds of miles from the storm area. As they move, they gradually become longer and lower until they finally die out. When you see and feel long low swells from a direction different than the wind, or when there is little or no wind, these swells are showing you the direction of a distant storm. That storm could be one that is predicted to be on its way toward you, it may be one that has already passed, or often those swells are from a storm that will never come anywhere near you.
It is not uncommon to encounter wave trains from two or more directions at once. Quite often you will see and feel waves produced by the local wind along with swells from a second direction. If you’ve been following the weather reports, you may know where those swells are coming from, and why. If not, it might be worthwhile to find out.
Visibility
The distance of visibility at sea in a small boat is difficult to determine accurately. The clarity of the horizon, or the lack of it, is one fairly useful gauge. When visibility is ten miles or over, the horizon is a very sharp, clear line, and sea and sky colors are distinctly separate. When visibility is down to about five miles, the horizon is indistinct and fuzzy. Sea and sky colors are graying.
The causes of changes in visibility are so varied that the distance of visibility at any particular time is not going to be a great help to you in weather forecasting. Poor visibility is produced by both natural causes and by manmade pollution. However, limited visibility under cover of stratus-type clouds is an indicator of stability. Weather changes will be slow. With conditions stable and winds light, it is always well to consider the possibility of fog.
Extremely clear, cool air with scattered small cumulus clouds following a rainy period usually signals the start of a spell of good weather. Very good visibility, but with large towering cumulus clouds moving in, signals a likelihood of sudden squalls, gusty winds, and generally unstable conditions.
In order to draw any meaningful indications for the future from present visibility conditions, one must view them in context with our other indicators: clouds, winds, temperature, and atmospheric pressure.
LOCAL FORECASTING
Weather forecasting for a particular locality on a particular day starts with a basic understanding of the average weather during that season of the year. Start by acquainting yourself with the available data for the area in which you operate. Chapter 4 will provide a beginning. Your own old local weather logs, and those of other sailors, can also be very helpful. There are several additional sources for this information.
Coast Pilots
One is the volume of the Coast Pilot that includes your area. These books are published by National Ocean Survey (NOS), the same agency that publishes your charts. There are nine volumes altogether; however, it is unlikely that you will need more than one. A list of the books and the areas each covers is shown in Figure 3-13. There are four for the U.S. East Coast, one for the U.S. West Coast and Hawaii, one for the Gulf Coast, Puerto Rico, and the Virgin Islands, one for the Great Lakes, and two for Alaska.
You should have the Coast Pilot for your area aboard for navigational purposes anyway. It provides a vast amount of information impossible to include on the charts. In addition, it gives summaries of average weather conditions as well as various types of unusual weather that have been known to occur, or recur in the area from time to time.
Pilot Charts
You should also consider getting the Pilot Charts that cover your operating area (these were mentioned and illustrated in Chapter 2, Fig. 2-9). That means either a set for the North Atlantic or the North Pacific. A set is twelve, one for each month of the year. Each chart provides information on prevailing wind directions and velocities, ocean current directions and velocities, calms, fogs, temperature and barometric pressure averages, and a description of average wind and weather conditions for that month.
Topography and Local Knowledge
Most boating, sailing, and fishing is done in coastal waters where nearby land features often significantly affect local weather. Mountains, deep valleys, forested areas, desert areas lacking in vegetation, or industrial developments creating smoke and fumes, all may produce highly localized weather phenomena.
Your local experience plus that of other knowledgeable mariners in the area will tell you the normal time, direction, and strength of the local sea breeze/land breeze cycle. The wind, temperature, humidity, and cloud conditions that often precede formation of thunderstorms, as well as their usual tracks, are often known locally. Fog and smog producing conditions are commonly familiar to those in the vicinity as well.
Making a Local Forecast
To make your local weather forecast, you will do, on a small scale, the same thing that is done by the forecasters at Weather Service. First review all of the information available to you up to the minute:
1. The latest forecast from Weather Service, preferably the current VHF-FM broadcast.
2. Your knowledge of the mechanics of weather as discussed in later chapters.
3. Your own recorded local observations made during the most recent 24 hour period.
4. The results of your background study of normal and known abnormal conditions in the general area at this season of the year.
By following conditions over several days you will develop a feel for how the weather is presently moving. Your forecast now is going to be whatever is reasonable to expect in light of all available information.
Looking at what you have observed plus the observations, summaries, and forecasts of Weather Service, make a forecast for the next 24 hours and write it down. Next day, verify it against what actually happened. Right or wrong, for practice, go ahead and do the same thing again. If you were substantially right—fine! If not, review all the information on which you based your forecast to see if you missed some significant indication.
If your results are not outstanding the first few times, do not be discouraged. In spite of all the scientific equipment and procedures involved, weather forecasting is partly a science but also partly an art. As with other arts, skill follows practice.
