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Reading the Weather
Your understanding of the weather can make the difference between a successful trip and a disaster.
Adapted from, Beating the Odds: A Guide to Commercial Fishing Safety, 7th Edition, Jerry Dzugan and Susan Clark Jensen, 2018
Do you know what causes bad weather? Blame it on cold, dry air meeting warm, moist air and other associated differences in air pressure.
Do you know how to get the most out of National Weather Service broadcasts? Can you read weather maps? Do you regularly make and interpret your own weather observations? Do you seek out “local knowledge” regarding weather and its effects on currents and sea conditions? These can all help you make wiser fishing decisions. Understanding weather terminology is the first step.
Weather Broadcasts and Maps
High and Low Pressure
The air in the earth’s atmosphere constantly moves to equalize temperature differences between the earth’s air, water, and land masses. As these bodies of warm and cold air circulate, they build up areas of low and high pressure—represented on weather maps by the letters L and H. Low pressure systems usually bring poor or worsening weather, while high pressure systems generally produce good or improving weather. However, both types of pressure systems can produce high winds and seas.
On weather maps, high and low pressure systems have lines drawn around them. These lines—called isobars—connect places of equal barometric pressure. Isobars not only show the shape of a weather system, but also give some indication of surface wind strength. Closer isobars mean stronger winds because there is a greater difference in air pressure in a shorter distance.
Beware of any squiggles seen in isobar lines around a pressure system shown on weather maps. They can become storms or hurricanes. A low pressure system of warm moist air that moves away from the equator and meets cool dry air from a high pressure system in a higher latitude can cause storms. An example is cold air from the northeast coast meeting warm Gulf Stream air.
A compression zone is where one pressure system (as in a low pressure system) runs into another slower moving pressure system (such as a high pressure system). This compresses the isobars. Wind can increase without much change in wind direction or a change in the barometer, as in the Santa Ana winds of the southern California coast where light winds suddenly switch to strong northerly winds. Smaller secondary lows sometimes form on the equator side of a low pressure storm, bringing more warmth and moisture with them.
National Weather Service Symbols and Contractions
High and Low Pressure Systems Over Eastern Asia, the Gulf of Alaska, and Alaska
Air Flow in the Northern Hemisphere
Animated Weather Map from windy.com
Online weather charts provide information on weather fronts and weather systems.
Using Local Observations to Supplement Forecasts
Combine official forecasts with your own weather observations to avoid as much heavy weather as possible.
Barometric Pressure
As a general rule:
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Falling barometric pressure means poor or worsening weather. The faster the fall, the stronger the storm.
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When a storm is upon you, the worst is over when the barometer starts to rise and the wind starts to switch from the south to the west, bringing windy, clearing weather. The wind direction can, of course, be modified by offshore winds out of bays, rivers, and passes.
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Rising pressure usually indicates clearing or good weather. The faster the rise, the sooner the clearing.
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When the barometer has been reading exceptionally high for a few days, weather changes occur slowly.
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Tides are somewhat affected by air pressure. The air in a high pressure system is heavier than in a low pressure, so it pushes down on the ocean more than a low does. Consequently, when the weather is dominated by a very high pressure system, the water level won’t fluctuate as much, and high tides are often lower than predicted.
During a strong low pressure system, the air doesn’t exert as much weight on the ocean, so the high tides can rise more. This, combined with the driving winds associated with intense low pressure systems, often creates damaging storm tides.
Reading the Barometer
Although a barometer does nothing more than measure air pressure, it can be a valuable tool, but only if you read it regularly, record the readings, and remember that air pressure is only one part of the total weather picture.
Although some barometers measure air pressure in inches, the National Weather Service measures it in millibars. See the millibar conversion chart at https://www.weather.gov/ama/conversions.
To get the most out of your barometer:
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Read it at least twice a day (morning and night are good times). Gently tap the barometer’s face, and watch which way the needle moves. The tapping releases the stored up friction and usually makes the pointer jump slightly up or down, although the needle will move by itself with larger changes of air pressure.
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Record the reading, the time, and how much the pressure has changed since the last reading. This can go in your ship’s log as a permanent record.
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After you have taken your reading, set the moveable needle on top of the pressure needle so you have a departure point for your next reading. This will tell you whether air pressure is rising or falling, and how rapidly it is changing. Writing down the pressure is important since the needle can be moved easily, unintentionally.
Clouds
Observation of cloud types and movement is an important component of making your own weather predictions. Although this chapter does not detail the various types of clouds, some general observations
are:
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Thickening and lowering clouds signal the approach of wet weather.
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High, thin clouds can be an early sign of approaching bad weather.
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Fair weather will generally continue when cloud bases increase in height over land and mountains.
Fog
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Radiation fog occurs in near-calm, clear weather when the earth loses heat into the night air, cooling and condensing the air above it. Depending on the wind, radiation fog can be anywhere from two to several hundred feet thick. It begins to evaporate shortly after sunrise, with the lower layers going first, but is slow to clear over water.
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When warm, moist air blows over cooler surfaces such as land or coastal water, the result is advection fog. This type of fog may develop day or night, winter or summer over the ocean and is the most likely type to be encountered at sea. Unfortunately, advection fog does not dissipate easily, and usually doesn’t clear unless the wind shifts or increases dramatically in speed. Sunshine has no effect on advection fog over water.
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Steam fog, also known as arctic sea smoke, develops when air less than 10°F blows over warmer water. This type of fog is likely to occur where cold river water flows into the ocean or on inland bodies of water in the fall.
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If fog persists after 1 or 2 p.m. and no major weather changes have taken place, the fog will remain and probably become thicker.
Development of Advection Fog
General Weather Observations
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When the wind shifts to the west, the weather will generally clear.
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The weather will generally worsen when the wind shifts to the south or east.
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Fair weather will usually continue when there is heavy dew or frost at night, the moon shines brightly, and the wind is light. The latter conditions are also often associated with a falling temperature.
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The temperature will usually fall when the wind shifts into or continues to blow northerly or northwesterly, or when the barometer rises steadily in the winter. Conversely, the temperature will rise when the wind shifts from the west or northwest to the south.
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To locate the center of the storm, use Buys Ballot’s law of wind and pressure. In the Northern Hemisphere, face the true wind (not the apparent wind caused by the combination of the true wind and the wind caused by the vessel’s own forward motion) and stretch out your arms. According to the Buys Ballot law, the center of the low pressure will be to your right and somewhat behind you. The center of the high pressure will be to your left and somewhat in front of you. This method can help you track the path of a weather system.
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In coastal waters, weather is modified by many local factors including mountain ranges, islands, glaciers, and coastal currents. Learn your local weather indicators.
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Make weather observations often, noting both the current weather and any changes that have occurred.
In the Northern Hemisphere, locate the center of a storm by using Buys Ballot’s law. This law was formulated in 1857 by the Dutch meteorologist Buys Ballot, and is also known as the Baric Wind law.
Icing
Vessel icing is potentially so dangerous that you should not rely solely on NWS icing warnings. Instead, you must be able to recognize conditions that cause icing and know both how to avoid icing,
and what to do if it occurs.
Cold air, cold water, and wind create a deadly combination for vessel icing. Although icing can be caused by freezing fog, rain, or wet snow, the most common and dangerous source is from sea spray. You don’t need to be underway to suffer sea spray icing. Boats at anchor are also at risk if they are in an area subject to strong, cold, offshore winds, especially off bays or near canyons or river mouths.
If your vessel ices heavily and you are in a dangerous situation, call the Coast Guard and request an escort to safe haven.
Nomogram charts that help predict ice accumulation are an invaluable aid and should be posted in the wheelhouse when icing is possible. Nomograms show that significant icing begins when the air temperature is less than 28°F (the temperature at which saltwater freezes), water temperature is less than 46°F, and winds are greater than 25 knots.
Nomograms of Superstructure Icing Conditions Due to Spray, Showing Icing Conditions for Vessels Heading Into or Abeam of the Wind
Light Icing: Less Than 0.3 inches/hour (0.7 cm/hour)
Moderate Icing: 0.3–0.8 inches/hour (0.7–2.0 cm/hour)
Heavy Icing: Greater Than 0.8 inches/hour (2 cm/hour)
Icing is dangerous because:
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It increases the vessel’s weight. This both increases draft and reduces freeboard.
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The accumulation of ice on the vessel’s superstructure and rigging raises the vessel’s center of gravity and reduces stability.
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It can cause the vessel to list if the accumulation is uneven.
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It alters the vessel’s trim.
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Speed and maneuverability are both reduced.
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The wind has more of an effect on the increased surface area of the rigging.
Although icing has a different impact on every vessel, vessels that are smaller, are marginally stable, take more water and spray on deck, have large deckhouses, or have heavy topside equipment or fishing gear are at a higher risk of capsizing due to ice buildup. Icing is most severe when you are abeam or heading into the wind.
Figuring Ice Weight on a Vessel
If you can estimate the surface area of the iced portion of
your vessel, you can easily calculate the weight of the accumulated ice.
One cubic foot of saltwater spray ice weighs 60 pounds, which means that a 12" × 12" × 1" slab weighs 5 pounds.
To figure out how much the ice on your vessel weighs, you
need to know both the surface area affected (area), and the speed at which the ice is building (thickness). If you are in conditions causing ice to accumulate at the rate of one inch per hour, how much would the ice weigh on a 10' × 20' area on your vessel’s house after 1 hour? After 5 hours? The following equation will tell you:
Weight = area (in square feet) × thickness (in inches) × 5
pounds
Just plug your numbers into this equation:
The area (for this example) = 10' × 20' = 200 square feet. The thickness (for this example) = 1" after 1 hour, or 5" after 5 hours. This means that after 1 hour, the weight of the accumulated ice on this 10' × 20' portion of the house = 200 (area) × 1" (thickness) × 5 pounds = 1000 pounds. The weight after 5 hours = 200 × 5" × 5 pounds = 5000 pounds.
The weight after 5 hours = 200 × 5" × 5 pounds = 5000 pounds. The actual weight on this vessel will be greater in both cases because we only took into account a portion of the vessel. This extra weight will have a tremendous effect on how the vessel responds to handling and the sea conditions.
If Icing Is Anticipated
When the air temperature drops, consult nomograms to anticipate and avoid icing. When icing is possible, your main goal is to reduce your risk. Several possible actions exist:
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Run to a protected port.
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Head offshore to warmer waters.
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Slow your speed and alter your course to travel with, rather than into, the wind and seas.
When Icing Cannot Be Avoided
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Establish and maintain a radio communications schedule.
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Ready ice removal equipment (mallets and baseball bats) and rig lifelines on deck.
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Stop fishing. Store and secure fishing gear below decks or keep it secured and covered on deck, especially crab pot webbing and coils of line, which are notorious for collecting ice. Cover with tarps if possible. If the situation warrants, your best course of action with crab pots may be to throw them overboard. Deck equipment should also be covered.
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Lower and secure the cargo boom.
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Prepare emergency and lifesaving equipment.
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Make sure scuppers and discharge pipes are clear.
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Batten down watertight doors, manholes, and hatches.
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Check deck lights to make sure they work. Icing will worsen at night if the temperature drops.
Combatting Icing
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If you cannot safely return to port, reduce your speed and alter your course so you are traveling downwind.
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Crewmembers working on deck should wear winter clothes and a personal flotation device, and should be snapped onto a lifeline. They should be especially alert to the danger of frostbite.
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Remove ice first from life rafts, aerial wires, navigation lights, scuppers and discharge pipes, stays, shrouds, masts, rigging, superstructure, deckhouse hatch, anchor hoist, and net ports.
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Check the bilge regularly.
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If ice keeps building up in spite of your de-icing efforts and your roll gets lazy and sluggish, you’ve got too much ice on board. If your vessel ices heavily and you are concerned about your safety, call the Coast Guard and request an escort to safe haven. Have the crew ready to abandon ship. Sound drastic? Boats roll over very quickly, and you must be totally prepared with immersion (survival) suits on, life raft ready to launch, and no one below decks. This precaution is not pushing the panic
button, only being sensible and ready.
Combine your observations with official forecasts to avoid as much heavy weather as possible. Seasoned fishermen know that heavy weather can turn a relatively uneventful trip into a life and
death situation.
Tropical Weather
Tropical climates bring their own weather risks, such as cyclones, waterspouts, and lightning.
Tropical Cyclones, Hurricanes, Typhoons
Tropical cyclones (sustained winds in excess of 64 knots) in the Atlantic are called hurricanes, and in the Pacific they are called typhoons. Call them what you will, they are masses of air that spiral around (counterclockwise in the Northern Hemisphere) a low pressure center and they are every mariner’s nightmare. Fortunately with modern weather predicting tools such as satellites and various ways
to get weather reports, tropical cyclones can be planned for. Unfortunately prediction is difficult, because the direction they take can change.
In 2012 the 108 foot replica of the tall ship Bounty left port and headed into the path of Hurricane Sandy, resulting in the loss of the ship and two lives and putting rescuers at great risk. Since the destructive potential of cyclones is so severe, weather warnings should always be taken seriously and if sheltered port is within reach, it should be taken. It is better to lose your vessel on land than at sea.
If you cannot seek shelter, avoid crossing in front of the track of the cyclone, since you may be slowed down by seas and the cyclone may speed up. It will be critical to navigate away from the center of the depression and avoid the right front quadrant (in the Northern Hemisphere) of the cyclone. In this dangerous quadrant, the storm’s wind speed blows in the same direction as the speed the cyclone is
moving.
A hurricane with 110 knot winds moving at 20 knots will produce 130 knot winds. Conversely on the other side of the center when the 110 knot winds are blowing into the direction the hurricane is moving at 20 knots, you will experience a reduction of winds to 90 knots. This may be small comfort, but it will better your odds. Most importantly, respect severe weather forecasts. As one fishermen said who lost his vessel in 80 knot winds and was lucky to survive, “If the weather report says ‘storm warning,’ don’t go. It’s not worth it!”
Waterspouts
On May 8, 1980, a waterspout in San Antonio Bay, Texas, sank a shrimp boat. One crewmember was lost and the other two crew were injured. Waterspouts occur in the Great Lakes, on the west and east coasts, and most frequently in Florida. They are caused by cold air moving over warm water.
Waterspouts fall into two categories. Tornadic waterspouts develop downward in a thunderstorm, and
form or move over water. They are associated with severe thunderstorms, and are often accompanied by high winds and seas, large hail, and frequent dangerous lightning.
Fair weather waterspouts are generally not associated with thunderstorms, are more common, and usually not as dangerous. They develop on the surface of the water and work their way upward. Fair weather waterspouts form in light wind conditions, so they normally move very little.
Although usually not as strong as tornados, waterspouts have damaged and sunk ships. The best way to avoid a waterspout is to move at a 90 degree angle to its apparent movement. Never move closer to investigate a waterspout and beware of flying debris.
Lightning
Lightning Hit Fishing Boat But Crew Safe: “The family … was relieved to hear that their son and three other crewmen aboard the Whitewater II are doing OK and heading back to Charleston after the 63-foot commercial fishing vessel was struck by lightning. The report said that the Whitewater II’s crew contacted the Coast Guard in Jacksonville, Fla., via radio about 4 am Wednesday and reported that a lightning strike had knocked out their navigational equipment on Monday.” Source: thesunnews.com and WorldFishingToday.com, 4/29/10.
Nothing feels as exposed as being the only object on the water during lightning. According to the most recent (2000-2005) data survey by BoatUS Marine Insurance claim files, the odds of your boat being struck by lightning in any year are about 1.2 in 1000. The odds are almost 3 times greater in Florida. In a strike, people and equipment can be damaged, although a strong, properly bonded system will minimize damage and injury. The strike can hole your hull or blow out a through-hull fitting. Electricity can jump from one metal object to another, so avoid touching any metal. Stay in the center of the vessel. Put portable GPS and radios in a metal box like a microwave. After the strike, attend to the injured and prepare abandon ship equipment in case it is needed.
Reading the Weather
When these secondary lows combine with the primary low, severe
weather can develop. In the Northern Hemisphere, detailed maps also show that air flows clockwise around high pressure systems, and counterclockwise around low pressure systems. This air flow is caused by the earth’s rotation and is reversed in the Southern Hemisphere.
Fronts
Cold and warm air masses do not tend to mix, so they usually have a boundary—called a front—between them. Fronts are important to track because that is where low pressure systems develop, with their accompanying deteriorating weather.
Meteorologists use symbols to distinguish the four kinds of fronts: cold, warm, stationary, and occluded. Cold air moving toward warm air is called a cold front, and is represented by a line with triangles on its leading edge. Warm fronts, depicted by a line with semicircles on its leading edge, occur when warm air moves toward cold air. When a cold front overtakes a warm front, the result is an occluded front, and is shown by a line with both triangles and semicircles on its leading edge. If a front becomes stationary, the semicircles and triangles are drawn on opposite sides of the line.
Storms
Although land formations can alter a weather system’s course, the systems generally travel from west to east in the Northern Hemisphere. This, too, is caused by the earth’s rotation.
The National Weather Service (NWS) issues three categories of wind warnings: small craft advisories, gale warnings, and storm warnings.
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A small craft advisory is a prediction for sustained winds (more than two hours) from 18 to 33 knots.
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Gale warnings forecast winds from 34 to 47 knots.
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Storm warnings predict winds of 48 knots to 64 knots.
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Typhoon or hurricane force winds 65 knots or greater.
The wind forecast also includes a prediction on wind direction.
The NWS issues sea height forecasts for coastal areas—for average sea height conditions to be encountered in open coastal waters unless otherwise indicated. The forecasted sea height is the average height of the highest one-third of the expected waves—although waves can occasionally combine and peak out at twice the forecast value. Sea height values do not take into account areas of normally higher or steeper seas found near bars, shoals, or restricted entrances into sounds or inlets.
Listening to Forecasts
Carefully listen to weather broadcasts at least two or three times daily and use either an actual or mental map of your area to picture where the weather fronts and systems are located and how they are moving in relation to you. Write down the pertinent information; memories are weak. Even better, use a small voice recorder to record the forecast for replaying.
Marine weather broadcasts often include a synopsis of weather systems and a 12 to 24 hour outlook. Sometimes a three to five day outlook will be included. For a list of NWS marine weather radio sources, see https://www.weather.gov/marine/wxradio.
Additional Weather Resources
If you have internet connectivity, you can get up-to-date offshore weather observations from the National Data Buoy Center at https://www.ndbc.noaa.gov/. This website has links to weather buoys around North America and the world. Weather buoys typically record wind speed, direction, and gust speeds; barometric pressure and the pressure trend; air and water temperatures; and wave height. There are usually observations for the past 24 hours and historical data available so that you can observe trends.
In addition, don't forget about commercial weather forecasters, such as wundergound.com and windy.com provide long range forecasts and a variety of maps and animations that are useful for anticipating weather conditions.