Marine Weather on a Cruising Sailboat

This web page contains hints that we found useful as we attempted to unravel and master the mysteries of marine weather. Many of these came from studying weather using the Starpath Weather Trainer. Others came from studying various texts on marine weather, attending weather courses and seminars, and from observation and experience.

An understanding of weather patterns, how they differ in different areas, and how they change within those areas is an essential part of seamanship. We invested a significant amount of our time on this topic since we hoped (optimistically as it turned out) to sail whenever possible and minimize use of the engine. We also had no desire to get caught out in dangerous conditions. While we knew that Callipygia could get through pretty much anything, we were not so sure about ourselves as crew.

You can see how we collected and keep track of weather data in the Weather Notebook and the Deck Log. The Radio Page gives a schedule of Weather Nets and explains how we learned to use Weather Fax.

Table of Contents for this Article
1. General Information	2. Air Pressure	3. Fronts
4. Squalls	5. Cyclones	6. Winds
7. Clouds	8. Weather Forecasts	9. Sea States
10. Currents	11. Fog

1. General Information

Weather planning for smaller vessels is based on long-term data regarding conditions over an area, and knowledge of how the ship and crew perform under these various conditions.
1 cubic yard of water weighs a ton, so in rough weather if cockpit drains or scuppers don't empty quickly, a boat can quickly become unbalanced.
Most of the time in weather forecasting you are looking for good weather and not trying to avoid bad weather. Understanding weather allows you to find the most favorable winds and route so that you can catch favorable 15-20 knot breezes.
There is not much you can do to avoid a fast moving bad weather system when you’re on a passage going at 6 knots so best to see it coming and stay out of the way.
If the latest weather forecast suggests the need for significant course alterations, make half of it and then wait 6 hours for the next forecast and/or confirmation from your own observations before making the full course alteration.
In many cases, heading offshore for a coastal passage is equivalent to adding days to get into rougher conditions.
In the upper atmospher (jet stream), long waves create the storm track, short waves create the storm.
In the N Hemisphere, lows (cyclones) go anticlockwise and highs (anticyclones) go clockwise. Thus, put your back to the wind, your left hand out and 20 degrees forward to find the direction of the center of the low (Buys Ballot law--Remember BBBLL=Buys Ballot says Back Left to Low in the Northern Hemphisphere. In SH use Right Hand).
Weather planning for smaller vessels is based on long-term data regarding conditions over an area, and knowledge of how the ship and crew perform under these various conditions.
1 cubic yard of water weighs a ton, so in rough weather if cockpit drains or scuppers don't empty quickly, a boat can quickly become unbalanced.
Most of the time in weather forecasting you are looking for good weather and not trying to avoid bad weather. Understanding weather allows you to find the most favorable winds and route so that you can catch favorable 15-20 knot breezes.
The significance of dark nasty looking weather depends on which direction it lies in relation to the general direction that weather moves in your location.
Extratropical cylones form outside the tropics any time of year but most often in winter, are numerous (about 1,500 develop per year), form over land as well as sea, move west to east, span 700-1,500 miles, have cold cores, and have their strongest winds on the periphery, don’t have an eye, and have associated fronts. They often reach gale force but except in the North Pacific are rarely hurricane force.
Tropical cyclones form in the tropics generally in specific season, are relatively few (70 per year), form over water, move east to west for part of their lives, span 2-300 miles, have warm cores, have strongest winds (over hurricane force) near core, have a calm eye, and don’t have fronts.
Veer = shift (clock) to the right in both hemispheres. Back = shift to the left in both hemispheres.
On clear days, visibility is about 10 miles. The visible range of land is 1.14 x square root of the height of land above observer’s eye.
A ridge is an elongated area of high pressure, identified by the area of maximum anticyclonic curvature of wind flow. A trough is an elongated area of low pressure, identified by the area of maximum cyclonic curvature of wind flow. A trough is distinguished by its elongated nature as compared to a low (or cyclone) which is circular. A large scale trough may contain one or more lows.
Get a weather map and mark your position on it. Note the time for the map. From your log, check the wind speed, direction, and barometer. Figure out what the map suggests your wind speed, direction, and barometer should be. Compare notes. Mark your COG on the weather map, figure the next day’s run, and plot your likely position. Keep this and study it as new forecasts are issued. Listen to voice forecasts and fill in details. Compare with previous projections.
Isobars in the warm sector typically align with winds aloft (in 500mb chart). Surface storm track is usually 300-600nmiles north of and parallel to the 5640 meter contour. Surface lows and fronts generally move along surface at a speed 1/3 to ½ of the 500mb wind speed. Surface winds in a low are typically ½ of the 500mb wind speed.
The key issue to applying what you learn from weather maps is how fast is your vessel. The tactical use of map information always involves projected DR position at the speed your vessel can make in the projected wind and wave conditions. It is much like taking current into account when passing through areas with strong varying currents. Good weather tactics are always tied to good navigation skills. The captain has to make the trade off between safe and fast.

2. Air Pressure

1-2-3 rule. If pressure drops 1-2mb an hour over a period of 3 hours, watch out. Then use the 4-5-6 rule (drop of 4-5 millibars in 6 hours) which means something major is headed your way, if this happens you can be sure you are in for a gale--or worse.
Reading and recording (on graph paper if you can) the pressure will ensure you are paying attention to air pressure. You need a barometer that not only tells you if the pressure is rising or falling throughout the range, but also accurately tells you what the pressure is. Without knowing the true pressure, weather charts are not as useful.
You need a barometer that is accurate throughout its range and it should be calibrated, it’s ok if it reads 3mb high, so long as you know that. Cost goes up with weight, and often just pay for more brass with no increase in barometer quality. Some of the best come in very thin brass and weight only 2lbs.
General rule of thumb in the middle latitudes: Low pressure brings wind, clouds, and rain; high pressure brings light winds and clear skies (although you can get strong wind between two highs or if a high is pushed up against a stationary low). However, in the tropics winds tend to increase with high pressure.
Diurnal variation is about 1-3mb with highs at 10 am/pm and lows at 4 am/pm. It is most prominent in tropics, in part because less pressure variation there.
You can estimate the wind by spacing between millibars. But also, remember that wind can double going through gaps in mountains or between islands (gap wind). Also, note that close spacing in tropics produces much higher winds than at higher latitudes.
There are 3 semi-permanent high pressure systems that really comprise the Horse Latitudes. They are stronger in the summer than in the winter. They are: Siberian High (over land), Pacific High, and Atlantic High (called Bermuda High when positioned westerly, Azores High when easterly). Their mean centers are betwen 30 and 40 degrees. You can tell if a high has become a “blocking high” that deflects or slows the usual west-to-east path of weather systems when: on the weather map they are in the correct place for the season; they are smooth and round with at least 2 isobars round them; and central pressure is +1030mb. The best spot to pick for getting round one is 2 isobars (8mb) from the center of the High. If the jet stream above develops an omega shape (omega block) the high can sit in the same place for a couple of weeks.
Lows are circular depressions which can develop steep pressure gradients; troughs are elongated low pressure systems which develop between 2 highs or more often as a projection out of a central low.
Low pressure systems tend to move in a direction parallel to the isobars in their warm sector.

3. Fronts

A front is the boundary between cold(er) and warm(er) air with the temperature and pressure differences producing strong winds at the boundary.
There are 4 types of fronts: Warm (Rain, leading to mist/fog, stratus clouds, temperature slowly rising, moves at 10-15 knots). Cold (Heavy rain at front, then showers, cumulonibus, rapid temperature drop, moves at 20-25 knots and much faster in higher latitudes). Occluded (steady rain then squally, stratus then cumulonimbus (Cb), moves at 10-15 knots, temperature may rise or fall). Stationary (intermittant rain then clearing, lowering stratus (St) then Cb, temperature slowly rising, moving slowly or not at all).
Wind shifts at passage of a a front in the northern hemisphere will be a permanent veer--when facing the wind, it will shift (clock) to the right. As a front approaches off shore, winds back as the wind builds, then suddenly veer and then either steady out or dissipate after the frontal passage.
Warm front has a classic cloud sequence. This is more obvious in summer than winter. Sequence unfolds over 12-36 hrs, usually a day. From cloudless sky, comes a 22 degree diameter (thumb to pinky stretched) halo round sun/moon showing ice crystals have invaded the upper air (?stratosphere). Then building cirrus appears coming over the horizon. [Note cirrus sometimes show up at the tail end of a passing cold front, so when you see them coming over the horizon from a clear sky, then it means warm front is coming.] The cirrus thicken into cirrocumulus and/or deepen into altostratus then stratus. Then it starts to rain (nimbostratus) at roughly half way timewise to the actual front and it will keep on raining until the front arrives. During this time the barometer will slowly drop and winds may back a bit. When the front comes, the winds will be gusty and suddenly veer. Once veered (front passes) wind speed and direction will remain steady as you move into the warm sector, and there will be stratocumulus or mixed clouds and some rain.
Cold front typically follows warm front in a day or so during which period you are in the warm sector. The cold front is seen as a wall of tall dark clouds. When the front arrives the winds will increase rapidly and be gusty and there maybe thunderstorms with the rain. At passage the winds will suddenly veer, and the skies will gradually clear. Often there is a clarity and freshness in the air after a cold front goes through.
Cold fronts can travel on their own without a warm front. But you almost never get a warm front without a following cold front.
Occluded fronts (where a cold front has caught up with a warm one) approach as a warm one does, but the front itself has characteristics of both–it is just plain bad weather.
Sometimes an isolated low or trough will approach much as a warm front does with winds backing round and building and then veering as the low passes. However the veer is gradual, not sudden as at a front.
When an isobar crosses a front it is V-shaped and points away from the low. You determine the expected wind shifts from this.

4. Squalls

A squall is a cumulonimbus cloud gone out of control. Once the anvil appears, it is decaying.
The motion (direction it is moving) of a squall is veered about 30 degrees from the ambient wind direction. The first wind will hit you from the direction of where you see the squall. Watch the bearing of the center of the squall to decide if you’re on a collision course, or if you will pass in front of or behind it. As with a big ship, try to pass behind it, since the winds in front of a squall are much much much stronger than the winds behind.
The strongest winds come with the first heavy rains. If it starts raining lightly, you have missed the worst of it. Squalls are over in 20-30 minutes. The strongest winds are at the leading edge (the cold dome) which extends about 2 miles out in front. The taller and blacker they are, the worse they’ll be. Winds at the leading edge are typically 20-30 knots, with gusts to 40 knots.
Once you’ve been through a few squalls, you will be able to judge their intensity potential. Note that squalls produce wind in downdrafts. Sometimes a “downburst” of much greater intensity occurs (micro or macro depending on diameter) where winds can reach 100 knots. The danger from a downdraft is the suddenness and force with which the wind strikes. This is magnified if the windspeed is intensified to downburst levels.
Instability lines are long (possibly discontinous) lines of thunderstorms, may be 100s of miles long and 10-50 miles wide. They form usually in the warm sector of an extratropical cyclone.
Squall lines are mature instability lines, containing several squalls, typically 100 miles ahead of a fast moving cold front.
Waterspouts are tornados occuring over the sea. They are not uncommon in the Bahamas and have been sighted as far north as Puget Sound. They occur in severe weather, and touch the water for about 10 minutes. They descend from the base of cumulonimbus, primarily in the area of squall lines moving ahead of a cold front. They are less severe than tornados, but nasty, and typically are short lived with the base touching the water from a few to 10 minutes.
In the tropics on the ocean, squalls most often happen at night. Look at the clouds at sunset to get an idea of what’s coming. If they are stratified or stratifying there isn’t much instability in the air. If their cummulating there is and squalls are more likely. You rarely get an isolated squall. They travel in packs.
In forecasting squalls, the thing to look for is vertical development ie cloud base to top relative to cloud height (sea to cloud bottom). If the height of the clouds is less than the ceiling, you just get big cumulus, there won’t be enough instability to get cumulonimbus. If the height of the clouds is greater than the ceiling then watch out.
To judge the severity of an approaching squall take note of the height of the cumulonimbus. the height of the ceiling, and the darkness of the cloud. Taller, lower, and darker are worser.

5. Cyclones

When you get the first circular isobar around a low, it is called a cyclone. Need not be a storm. 2 circular isobars generally mean a storm is developing. The important thing is to notice the first circle.
To avoid a tropical storm or extratropical low, head off in a direction that is forward of perpendicular to the direction the storm is travelling. The angle depends on the ratio of your speed to the storm's speed of travel. [See storm avoidance in our "Using Radar" notes.]
The RHS of the path a Northern Hemisphere storm is the dangerous semicircle, with winds veering and pushing you into the path of the storm. The Left Hand Side is the navigable semicircle, with winds backing and pushing you out of the way. Also, on the Right Hand Side the rotary winds are increased by the speed of the storm’s forward motion, and on the LHS the rotary winds are decreased by the speed of the storm’s forward motion. Finally, the dangerous side is the direction in which the storm is likely to recurve and move towards.
Surface storm track is typically 300-600nm north and parallel to the 5640 meter contour on the 500mb map.

6. Wind

When the boat is moving, the direction of the true wind is always aft of the apparent wind, regardless of the point of sail. The faster you go, the more the apparent wind moves forward. The difference between true and apparent wind is largest when sailing with the wind on the beam. It is easy to misinterpret what’s happening to the true wind. Watch the catspaws/surface ripples closely. Stare at them!
Whenever the true wind changes direction, you know the isobars have moved. This is a signal that something is happening. It is easy to miss wind shifts when sailing, so always note the true wind as well as the apparent wind. A wind shift of 10 to 20 degrees can easily be lost when sailing on a reach by speed changes. Therefore, if boat speed picks up determine what wind attributes caused it. Was it a wind speed change or a change in wind direction--or both? Get in the habit of frequently deducing true wind direction.
True wind and apparent wind are essentially the same when boat speed is below 20% of wind speed.
Pressure gradient alone does not give wind speed, curvature of isobars also affects it.
You know you are in the trade winds when, around 30 degrees N you find steady 15-20knot winds from the NE/E with large wellorganized seas and small puffy cumulus following the wind.
Land influences wind on the sea nearby and can have a greater influence than all but major weather systems–and it will affect how those play out. Therefore, understanding the influence of land on the wind is essential for coastal sailing. Influences are two types: large scale regional effects that are included in local forecasts; and small scale effects that are not (though these may be referred to in the Coast Pilot). Another way of categorizing influences is into those caused by temperature and those caused by topography.
Land and Sea Breezes are caused because land heats faster than water from sunlight, and cools faster in its absence. Note “sea breeze” means from the sea, a “land breeze” means from the land. In the absence of other weather patterns, these can cause significant winds. A sea breeze begins gently soon after sunrise and veering as it builds it gets strongest (about 10 knots) in late afternoon. After sundown, a land breeze then develops overnight. Land breeze typically is much lighter than the associated sea breeze, unless there are mountains in which case it may be stronger because of more and colder air falling down the mountainside at night. Classic sea breeze effect is found along straight featureless shorelines. In the morning, effects are noticed ½ -1 mile from shore, by afternoon effects are noticed as much as 10 miles out.
A sea breeze in a confined area flowing through a mountain gap can reach 20-25 knots on a regular basis. These are usually well known and in planning, you can and should count on them to take advantage of or avoid them. In some areas gap land winds can reach more than 40 knots.
Typically, sea breezes produce little puffy bits of cumulous at the top of a mountain in the afternoon. If you don’t see them as usual, and a sea breeze doesn’t come as expected, it means a low out to sea is sucking the air away. So watch out.
When sailing along the shore in a 10-15 knot sea breeze, expect twice that wind at the entrance to a constriction (bay or river mouth). Look for white caps as you approach any turn or inlet.
Land obstructions impede offshore winds to a distance about 8-10 their their height offshore. Rule of thumb, you will feel the wind diminish when you can span the obstruction with a hand width held at arms length. Also, a strong leeward wind will be deflected by a cliff, which will provide some protection.
Gusts bring stronger winds shifted to the right. This gives you a lift (let's you head up) on the starboard tack but is a header (makes you head off) on the port tack. A lift is a windshift away from the bow, a header is a windshift towards the bow.
Unless the wind is strong and the seas favorable it is usually slow and precarious to sail dead downwind. It will be faster to reach a downwind destination to reach up a bit and jibe periodically.
Winds from a storm passing over a constricted waterway are influenced by the topography. Generally storm winds will be deflected by the mountains to flow up or down the waterway, not across it.
The wind near a flat shore will be more perpendicular to the shore and near a steep shore more parallel to the shore than the wind further off shore.
Wind curls round a point. Head in to tack close to the point to get lifts from the wind curls, rather than heading out to tack.
Wind speed will be higher for isobars with a distinct curve around a high, or lower for isobars curving similarly around a low. Note these predictions only work for winds outside the tropics. Trade winds are typically stronger than predicted.

7. Clouds

There are 10 basic cloud classifications (genera) that all mariners should know. High (base above 15-20,000 ft approx depends on lat): Cirrus, cirrocumulus, cirrostratus. Middle (base is 6,500-15,000 ft approx): Altocumulus, Nimbostratus and Altostratus. Low (base is below 6,500): Cumulus, Stratus, Cumulostratus. And Nimbocumulus which can reaches up to high sometimes.
In addition to the 10 cloud genera, there are 14 species, 9 varieties, and 9 features. If you have a grip on these, they pinpoint exactly where you are in a weather system.
Clouds by themselves don’t tell you a lot. Clouds plus barometer are better. Clouds, barometer, and wind shift is best–preferably with a weather forecast. A progression of cloud changes is necessary for forecasting–it is the sequence of cloud formation that tells you what might happen.
Clouds are generally great for confirming what’s happening now, rather than for forecasting what’s going to happen–except for squalls and the development of tropical storms where reading the current clouds is invaluable. Once in a squall it is rain more than clouds that tell us what is happening.
Layers of stratus imply stable air. Cumulus implies instability with vertical motion. If the height is bigger than the base, watch out.
It is often hard to tell what kind of clouds you have, there may be a number of different types mixed up.
Low dark clouds, mixing cumulonimbus and nimbostratus mean an approaching cold front, a squall line, or a squall (or the bar of a hurricane).
Any change in the clouds which is not towards fair weather cumulus (cumulus humilis) means the weather is deteriorating (probably).
Watching speed of clouds aloft helps tell if a coming front is severe or weak.
As a rough rule, there is wind under cumulus. If stuck in light winds in a high, try to inch towards the clouds. Another way to find wind is to climb the mast with binoculars and look for roughness on the water surface. Generally low clouds move downwind slightly veered from the surface winds. Cumulus means there is an updraft under it. Thus as you go under, you will experience a wind shift, then back to the original direction. If there are rows of clouds you may experience oscillating winds–they shift under the cloud, then go back, then shift again. Watch for this and going downwind learn to predict when/if you need to jibe and jibe again.
Apparent cloud height decreases with distance off (the angle of view decreases). To estimate cloud heights it is best to do it with those more directly above you.
Rain clouds (nimbo–) are dark, sometimes black, on the bottom. As they fill up with rain they get flat bottoms, which then get raggedy again once the rain is over. Pay attention to the bottom of these clouds.
Anvil top means the cumulonimbus rose so high the top winds blew its top off. It also means the worst is over.
There are two basic types of clouds: stratus (layers) and cumulus (puffy). Often in the evening you get stratocumulus (puffy stratus). In the morning if you have this, watch to see if it moves towards cumulus or towards stratus. Cumulus means fair weather. Stratocumulus can form into bands or waves to show directions of winds above, but you really need cirrus or cirrostratus to tell high wind direction. Mix of stratus/cumulus features is called stratocumulus when low, but layers of altocumulus can look very similar at middle heights.
Individual puffs of cirrocumulus (high) are smaller than in altocumulus (middle) which are smaller than cumulus (low). Practice trying to get a feel for cloud height.
Stratus which hit the ground are fog or haze. You generally can’t see the sun or moon through stratus, but you can more often see a watery sun through altostratus. Altostratus are the best sign that bad weather is possible (strong winds and long periods of high seas). They indicate a coming low before the barometer starts dropping. They usually tell you bad weather is coming in ½ a day or so.
For telling wind direction, the wind is perpendicular to the wave pattern (just as with water). Cirrocumulus are best, altocumulus can indicate high wind direction. This is the direction the weather is traveling towards you. Waves in altocumulus look like rippled sand. Waves in altocumulus look more like rows of sheep.
If you can see cirrus moving, then there is a strong wind aloft. If the cirrus is torn or raggedy that indicates strong winds too. Strong winds aloft tend to bring more severe storms.
When looking for land from the sea, look for clouds that have formed above it from the sea breezes.

8. Weather Forecasts

Keep a tape recorder near the SSB for recording weather information.
Coordinating the information you get from weather sources is important. Make a sheet that lists the local and UTs, in UT order, of weather broadcasts/faxes. Then note the data on the sheet so that what you collect makes a coherent picture.
When you get a weather map, plot your position on it or draw the weather features on a sheet of clear plastic and lay it over your chart.
With a calibrated barometer and wind instruments, you can judge where you are on the weather map. Practice doing this. Set your course based on forecasts so long as your own observations confirm what the weather map says. Weather maps are based on shipboard observations, and there will be fewer from bad weather areas. When a map forecasts a low or front to cross our path, we can be sure it will. Our job then becomes to figure out when that will happen and how the wind will change when it does.
In mid-latitudes to discern whether weather is likely to change, look at the forecast map for the next day. The location, condition, and stability of highs is the main issue for predicting general conditions over a large area. Highs are either moving or stationary/blocking. Moving lows will disrupt the anticipated wind –however a blocking high may stop a low or a front or more likely deflect it polewards.

9. Sea States

The sea state changes at the shelf. It is a good idea to draw the shelf on the chart with a highlighter and keep track where you relative to it, and noting what you experience at it.
When trip planning, find out what direction the prevailing swells are in the region. Even with a favorable wind, going beam to seas is to be avoided. And, some anchorages become untenable with a swell from a certain direction.
For an individual wave, the length in feet is the time between the two crests in seconds to the fifth. For a series of waves, the average wavelength in feet is 3 times the average time between crests squared.
Swells come from storms that are coming or long gone. They are generated from a long way away.
Significant Wave Height is the average height of the highest 1/3 of all waves. (SWH)
In a wave, water doesn’t move with the wave, but travels in a circle beneath the crest. In big seas, this water can be moving circularly at 2-3 knots.
At sea, waves look bigger than they are, and in photos they look smaller than they are.
Wave height builds whenever waves are slowed down by shallows (½ their wavelength), opposing wind, or opposing current.
Extreme storm (rogue) waves are much higher than the highest waves in a series, are most likely caused by a downburrst hitting the trough of a larger than average wave.
SWH generally reaches ½ the wind speed in a fully developed sea. Most probable wave height is ½ of this, and average height is about .6 of SHW. About 1 in 2000 waves will be twice the SWH.
As a rule it takes duration in hours equal to the wind speed in knots for seas to fully develop.
In still water, you see white caps with wind speed of about 10 knots, in a contrary current 8 knots, and a following current 12 knots.
When a weather report says waves of 6 feet, they are referring to SWH.
Typically waves break when their height = 1/10 of their length, although they look menacing at 1/18.
Waves when fully developed travel about .9 times the wind speed.
Swells are separated from the wind that created them and have uniform height and length and are very wide. Waves are what we experience along with the wind that is creating them and have many heights, widths and lengths. Ripples are small wavelets that are experienced with (or close by) a wind of 1-3 knots, they look like fish scales. all three combine to make up the sea state.

10. Currents

Approaching any harbor, careful monitoring is needed for sideways set of the tidal current.
In most places where there are strong land/sea breezes as from land constrictions, there are strong currents from water constrictions. Figure both of them into your planning, since there will be times when they run against each other creating very nasty conditions. The wind and the current generally will flow in the same or opposite directions.

11. Fog

Fog: 2 types, sea fog brought by the wind and often windy and foggy, and land fog (radiation fog) comes from the land, usually calm and burns off. You can predict fog is you plot the temperature/(relative humidity too?) and dew point for that temperature against time.
In the fog, no-one has right of way. Nav Rules require proper lookout, safe spead, and take all actions to avoid collision. If you have radar you must use it properly. Fog signals: 1 4-6 second blast every 2 minutes for vessel underway. You also hear engines. If you hear vessel forward of the beam, stop. Make your own fog signal, one long and 2 shorts (for a stopped vessel???) Try to make connection on Channel 13–give your GPS position if poss. Note that sound direction in fog is very unreliable. A good radar reflector is a must. You are not allowed, under the rules, just to drift unless you are at anchor. You need to have sails up and/or engine on so that you can move if necessary even though you may be stopped. If you think you are going to be collided with, head into the danger–you then can maneuver side to side if necessary, or take the blow on the bow which is much safer than on the beam.
If you anticipate much travel in fog, you must get radar. There are no two ways about it.
Nav rules require earlier avoidance action in fog if you have radar so that you never get near enough so that there is any risk of collision. The rules require, in a nutshell, that if the object is forward of the beam, turn right. If it is aft of the beam, turn away from the target. ie, turn right for all targets except one on the starboard quarter. If the target is forward of the beam, slowing is also a good idea. Maneuvers, as always, should be early and obvious (60 degrees).
Local forecasts are more reliable for fog than for wind. Fog is forecast from temperature and humidity changes, which are locally measured quite precisely. Wind is forecast from distance between isobars which are only measured more globally.
Fog comes in different types. Moist air moving over a cold surface is advection fog. Sea fog is this kind of fog where wind blows air from warm on to cold surfaces such as from warm or to cold currents, deep water welling ups, etc.
Sea fog can cover large areas and moves with the wind. It can be foggy and windy at the same time. A sea breeze can bring a thick fog, which does not burn off. Radiation fog typically is formed in still condition as the air cools over land at night and reaches the dew point; it burns off as the sun rises. You need to know what kind of fog you are in.

Bill Dillon (KG4QFM)
and
Pat Watt (KG4QFQ)
This page was last modified on April 18, 2010

About Marine Weather