The sport of sailing is probably more dependent on natural elements than any other human activity. That's because sailboats and sailors are entirely at the mercy of wind and waves, not to mention sun, heat, cold, current and water splashing in your face. The ability to predict and handle a variety of weather conditions will make sailing more safe, fun and successful.
The America's Cup and Olympic sailing campaigns are good examples of how weather information can be used to improve sailing performance. In these and other races, it's critical to know how the wind will shift and where the current is flowing. While cruisers and daysailors don't have the sophisticated weather technology of an America's Cup syndicate, anyone who goes out on the water in a sailboat still should know as much about the weather as possible.
Weather: The Big Picture
What are the causes of the weather and wind? Weather is actually movement and change in the bottom layer of the atmosphere. This layer is directly affected by differences in temperature and pressure caused by the effects of the sun on the varying surfaces of the Earth, as well as the angle and intensity of the sun. When air is warmed over land or at the Equator, for example, it expands and rises, creating an area of lower pressure. Cooler air over water or in the polar regions contracts and creates higher pressure. High pressure areas typically move toward areas of low pressure. This movement is one cause of wind.
The lower atmosphere is made up of a series of air masses, each of which has its own characteristics of temperature, pressure and water content. Maritime air masses form over the oceans and are usually high in moisture. Continental air masses form over land and are much drier. Depending on where an air mass has formed and traveled, it can be warm or cool. Within each air mass the weather is relatively steady, but when different air masses interact, we get changes in the weather.
Atmospheric pressure can be plotted with isobars, just as a topographical map shows the contours of the earth's surface. Each isobar line connects areas of equal pressure. High pressure areas can be thought of as mountains, for there is more dense air pushing down, while low pressure areas are like valleys. When the isobar lines are very close together, you have a "cliff," which means unsettled weather.
In general, wind flows from the mountains to the valleys. Because of the Coriolis effect (caused by the spinning of the earth), however, the wind flows clockwise around high pressure areas in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere). Conversely, the wind flows counterclockwise around low pressure areas (clockwise in the Southern Hemisphere).
Fronts -- When two air masses come together, the boundary is often a confused and unsettled area called a "front.". A cold front occurs when cooler, drier air wedges in under warmer moist air. It is usually fast-moving and often has a violent line of thunderstorms. The cumulonimbus (thunderstorm) clouds building high above the front are a warning signal that danger is near.
A warm front approaches more gradually as warm, moist air slowly moves in over cooler, drier air. High-level cirrus clouds usually indicate the approach of the warm front within a couple of days. The cirrus clouds are followed by lower and thicker clouds as the front gets nearer, until thick nimbostratus clouds bring extensive precipitation. Unlike the cold front, however, the passage of a warm front is not usually stormy.
Weather Indicators
When you are on the water, there are a number of telltale signs you can use to predict the weather that is coming.
1) Watch the west: In the Northern Hemisphere, the weather pattern generally moves from west to east. This means that if you want to know what weather is about to hit you, look toward the west.
2) Clouds: As we just discussed above, clouds are often the best way to tell what's coming. Here are some of the most common types:
Cumulus -- Puffy white clouds that typically form over land during fair weather.
Cirrus -- Very high, stringy clouds made of ice, usually indicating the slow approach of a warm front. Sometimes called "mare's tails."
Altocumulus -- Also known as a "mackerel sky." Wavy ridges of clouds (looking a bit like fish scales) indicating that a warm front is getting closer.
Stratus -- Very low, grey rain clouds.
Cumulonimbus -- Tall, imposing thunderheads that indicate the approach of a cold front.
3) A large halo or ring appearing around the sun or the moon is caused by high cirrostratus clouds at the beginning of a warm front and is frequently followed by lower and thicker clouds and then rain.
4) "Red sky in the morning, sailor take warning; red sky at night sailor's delight." This is an old saying that usually holds true. Red light is created when the sun's rays hit dust particles in clear, dry air. With the sun setting to the west, a red sunset means that there is clear, dry air coming. A red sky in the morning, however, indicates that the clear weather has already passed.
These are just a few of the ways that we, as amateur meteorologists, can predict weather while we're on the water. If you want a more accurate forecast or a report before you go sailing, try a professional weather service.
Obtaining Weather Information
Television, newspapers and radio reports can give you a general prediction for the day. Some use detailed weather maps showing isobars and fronts, while others seem more concerned with Nielsen ratings than the actual weather. In either case, the information is large-scale and usually not very detailed about the sailing conditions in your local area.
The National Weather Service broadcasts continuous weather reports over VHF frequencies 162.400, 162.475 and 162.550 megahertz. They give the present wind strength and direction, temperature, sea conditions, visibility and tides, as well as information on the movement of fronts, precipitation and fog. They also give local and regional forecasts for the next twenty-four hours, plus an extended forecast. Advisories or warnings are broadcast, and thunderstorms are tracked.
Many marinas and yacht clubs post visual weather warnings to alert boaters to adverse weather conditions. The signals you may see are:
One triangular red flag -- Small craft warning (18 to 33 knots wind)
Two red flags -- Gale warning (33 to 47 knots wind)
One square red and black flag -- Storm warning (48 to 63 knots)
Two square red and black flags -- Hurricane warning (64 knots +)
If you ever see two red and black squares flying at your local club, I'd suggest you head home and batten the hatches.
Other sources of weather information include local airports, flight service stations (for pilots) and weather facsimile machines that draw synoptic maps. Perhaps the most valuable source is your own experience and observation. Learn what the weather patterns are in your sailing area by making observations on the clouds, barometric pressure, temperature, wind direction and speed, and humidity. I keep a racing notebook that includes a record of the weather and what the wind does every day I go sailing.
How Weather Affects Wind
As we saw before, wind is caused by air moving from areas of high pressure to areas of low pressure. The greater the difference in the pressure and the closer the isobars are together, the stronger the wind will be. We also learned how wind circulates around high and low pressure areas due to the Coriolis effect. The direction of the wind at your location will depend on two things:
1) the positions and movements of pressure areas; and
2) local thermal effects
Here are some of the basic types of breezes:
High pressure -- A high pressure system generally brings cooler air from the north. You've seen this many times on crisp, clear fall days when there's a chill in the air, and the leaves are blowing all over. This breeze usually blows from the northwest and is very puffy and shifty.
Low pressure -- A storm system typically brings winds from the northeast. These winds are usually very strong and steady, and are typically accompanied by rain and higher tides.
Seabreeze -- This is a very common breeze in shore areas, caused by differences in temperature between the land and water. During the day, the sun heats the land more quickly than the water, causing the air to rise above the land. This draws in cooler air from over the water. As this moist air moves over land, it gets hotter and rises, forming cumulus clouds. In order to have a strong seabreeze on the surface, there must be a gradient wind aloft in a direction opposing the seabreeze. This carries the heated moist air out over the water where it cools and subsides, joining back in the thermal cycle.
Land breeze -- As evening approaches and the sun goes down, the land radiates its heat until it is cooler than the water. This creates higher pressure over land, so air begins to flow out to the lower pressure area over the water. This night-time breeze is not nearly as strong as the seabreeze because the temperature differences between the land and water are not as great.
Wind Characteristics
One thing you will learn about the wind is that it's always changing. Even when the breeze seems very steady, an experienced sailor will pick up subtle changes in velocity and direction. Here are some of the ways that the wind changes:
Oscillations -- The wind is almost always shifting back and forth, covering a range of anywhere from a few degrees to as many as 40 degrees. This is due to a) vertical instability of the air (e.g. right after a cold front) or b) the wind passing over a land mass. If you've ever sailed in a place like the Charles River in Boston, for example, you'll get some great shifts as the wind comes through all the skyscrapers.
Persistent shift -- Sometimes the wind shifts generally in one direction due to the passing of a weather system, the geographical influence of land or the Coriolis effect on a seabreeze.
Offshore -- A breeze that's blowing from the shore onto the water (e.g. a high pressure wind) is generally oscillating. As you get closer to shore, the wind is puffier and shiftier, and the water is smoother.
Onshore -- An breeze blowing toward shore, such as a seabreeze, is usually fairly steady and often persistently shifting. It also brings waves and chop that increase in size with fetch and strength and duration of breeze.
Friction effects -- Wind near the water surface is slowed and shifted in a counterclockwise direction compared to the wind aloft. When the wind passes over land, it is slowed as much as one-third and backed as much as 30 degrees. That's why you'll sometimes get a change in wind direction as you sail toward land.
Geographic effects -- Air behaves essentially as a liquid, reacting to topographical features on land in predictable patterns. Wind will pour down a valley rather than climb over a mountain. By studying the land around your sailing area, and observing the wind on the water, you can develop an understanding of the geographic effects on the wind.
Current
Current is the horizontal flow of water relative to the bottom and can be caused by several different effects. In coastal areas, the most common current is a result of high and low tides. The gravitational pull of the moon creates a high tide bulge on the earth's surface following the moon as it rotates around the earth. Another bulge occurs on the opposite side of the earth. This results in a constant flow of water as the oceans rise and fall.
An "ebb current" goes out (from high to low tide), while a flooding current comes in. There are many places in the world where the tide rises and falls as much as 20 or 30 feet.
Pressure systems can also cause tidal currents. High pressure areas push down on the water surface and cause water to flow toward an area of lower atmospheric pressure. The wind also produces currents on large lakes and enclosed bays. A strong breeze blowing from the same direction for several days will push a lot of water into or out of a bay or toward an end of a lake. When the wind subsides, there will be strong currents in the opposite direction as the water returns to its normal position.
Rivers obviously have a strong current flow caused by gravity. Near the mouth of a river you are likely to experience both tidal conditions and the flow of the river. On the Hudson River, for example, tidal effects are felt as far as 35 miles north of New York City.
Measuring Current -- There are several ways to tell the direction and strength of the current. Before you even go out on the water, there are tide tables and current charts that will give you a large-scale idea of what the current should be doing.
When you are out on the water, get in the habit of looking at the movement of the water on buoys or lobster pots to determine the current's direction and approximate strength. To measure current velocity more accurately, drop a floating object into the water next to a fixed object and start your watch. Take an educated guess at the distance it travels in a set period of time. One hundred feet per minute is roughly equal to one knot of current.
Anchored boats can also give you an indication of the current direction and strength, especially if they are hanging in a direction different than where the wind should be pushing them. When we race on Chesapeake Bay out of Annapolis, for example, we keep an eye on the freighters anchored out in the bay. When they swing around in a direction opposite the wind, we know that the current is changing direction out in the deeper channel.
The surface of the water will also give clues about the current. If the wind and current are aligned, the water is smoother than when the current runs against the wind. San Francisco Bay is one of the more extreme examples of this. On summer afternoons, the seabreeze often builds to more than 20 knots coming right through the Golden Gate Bridge. When the strong currents are ebbing out the Bay, there are often violent standing waves that make it very challenging for small sailboats.
The boundary of two areas with different current is often visible as a disturbed line on the water. Sometimes you can see all sorts of weeds and flotsam along this line. Finally, you can determine current by sensing how your boat and others are moving relative to fixed objects to determine current. If you are aiming at a buoy, for example, but you have to keep changing your heading, chances are you have a cross-current.
Effects of Current -- Current is very important to racing sailors who must constantly compensate in order to get around the race course as fast as possible. In general, their goal is to sail in areas of more current when it is favorable and less current when it is unfavorable. Starting line and mark rounding situations become even trickier (and lots of fun to watch!) when a strong current is running.
Even when cruising or daysailing, it's important to understand the effects of the current. If you are trying to sail from point A to point B and there is a cross current, you cannot simply aim at point B. The current will carry you well downstream of your destination. Therefore you must sail a course aiming into the current so your boat actual travels on a straight line from A to B. It may even be wise to stay up-current of that line in case the wind dies.