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Boating Safety: Understanding Weather – part 7 of 16

I think everyone who has been on the water for a while has had an experience with an incorrect weather forecast. Honestly, forecasting the weather is not easy. One individual made the analogy that it is like predicting where your cigarette smoke will hit the wall across the room. There are so many variables in weather forecasting it is understandable how they can be off from time to time. For the boater however, an accurate forecast is important. It could mean the difference between a fantastic trip or a catastrophic one.

Speaking with a meteorologist with the U.S. Navy I found that long term predictions are not very reliable, however weather within the next 12-24 hours can be predicted with relative confidence. There are few basic weather tips boaters should know to help make their own decisions whether it is time to come in, or even go out at all.

The first basic principal is that warm air rises and cool air sinks. The sinking cool air moves across the landscape to where the warm air is rising producing the wind; these are referred to as convection cells. Land cools and heats faster than water. Early in the morning the land has cooled overnight and the sinking air mass over land blows seaward forming the land breeze. As the day warms, the land warms faster. Around mid-day the land has become warmer than the Gulf and the air above the land begins to warm. This warm air rises and the winds shift from land breezes to sea breezes. There is usually a lull, or dead wind, in between.

Where the warm air is rising the air pressure drops forming a low pressure area. Warm air holds more water vapor and is associated with higher humidity and rainy weather. The cool air sinking forms higher pressure areas where the air is low in water vapor and the weather is generally fair.

The warm rising air is generally where storms will form. As the warm humid air rises it eventually reaches a high enough altitude that the surrounding air temperature is cool enough for the water vapor to condense; this is known as the dew point and is where clouds begin to form. Thunderstorms, with high winds and lighting, can form from these and the captain should be aware of their formation.

The barometer is a durable and reliable instrument for monitoring the weather (photo: Rick O'Connor)

The barometer is a durable and reliable instrument for monitoring the weather (photo: Rick O’Connor)

The best way to monitor the formation of these low areas and thunderstorms is by using a barometer. These are durable instruments, pretty accurate, and have been used by mariners for centuries. Most have gradients in inches and millibars. A falling pressure indicates rain is coming. How heavy the rain depends on how fast the pressure is dropping. One rule to follow is that if the barometer drops 0.03”/hour or more bad weather will occur within 12-24 hours; again the faster the pressure is dropping the stronger the storm.

Another key to watch are the formation of thunderheads; clouds that form into thunderstorms. There are four indicators that a thunderstorm is forming. First is the high anvil. This cloud layer forms as the warm air rises and condenses. As the water vapor condenses the air becomes lighter and travels faster. Streaks of water vapor form the anvil at the top of the thunderhead. The faster the anvil grows that faster the air is moving and the stronger the storm is becoming. Second, the formation of cumulus clouds between the dew point elevation and the anvil. Third, the darkness of these cumulus clouds; which indicates heavy rain. And fourth are the roll clouds. These are the building cumulus clouds that form at the front of the thunderhead. They appear to be rolling out in front of the storm. As they approach the captain can expect drastic changes in wind speed and direction. If sailing, reef now. Wind speeds in an approaching thunderstorm have reached 60 knots! And you may experience strong down drafts.

Two questions that come up when you see a thunderhead building and your barometer dropping are: How bad is this storm? And how much time do I have? As we have said, the intensity of the storm is based on how fast the anvil is building and how quickly your barometer is dropping (remember the rule). The classic measure of distance is timing the difference between visually seeing lightning and the sound of the thunder. The basic rule is to take the number of seconds between seeing lightning and hearing thunder and multiple this by 0.2; this would be the distance in miles. Another question that occurs is “which direction is it moving?” Locally thunderstorms typically form in the NW or SW. Those forming in the NW tend to move ESE and will pass over or to the north of you. SW storms move ENE and tend to move over or to your south; keep an eye on it. The roll cloud tends to lead the storm. As the roll cloud approaches you will feel a sudden drop in temperature and wind increase. Time to batten down. In our area thunderstorms are more common in the summer and typically form over land in the afternoon; we may have as many as 4/week. Thunderstorms can form over the Gulf but these tend to form between midnight and dawn when the water is warmer than the land.

A thunderstorm builds over Mobile Bay (photo: Molly O'Connor)

A thunderstorm builds over Mobile Bay (photo: Molly O’Connor)

Another weather feature to watch for are water spouts; small tornadoes that form over water. They tend to be 20-200 feet in diameter and about 1000 feet tall. The upper portion typically moves in a different direction and speed than the lower section given the typical curved or bent look they have. Because of this shape most water spouts do not last more than 30 minutes.

Today’s mariner have an assortment of electronic devices that can tell you the weather forecast and even watch them form and move. An app that has been highly recommended by experienced sailors is SailFlow; the download is free. It is still a good idea to understand the “ole ways” to help avoid a bad day on the water. There is of course a lot more to learn about understanding the weather. Captains should consider one of the classes offered by the U.S. Coast Guard Auxiliary or the U.S. Power Squadron. You can also contact your local Sea Grant Extension Agent for more information.

Rick O’Connor   850-475-5230