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Weather instruments - What should you have on board?

Jun 27, 2011

The suite of weather instruments you should carry while voyaging

Attractive and functional, a barograph will keep a record of pressure changes

Attractive and functional, a barograph will keep a record of pressure changes

Courtesy Robert White Instruments

Weather instruments are cool! There is something about quantifying different weather parameters that sparks an interest in many of us. Of course, for ocean voyagers, there is a more significant need to know many of these parameters since the weather conditions, and possible changes in the weather conditions, have a direct impact on the activity. Indeed, the safety and well-being of those who commit to an ocean voyage hang in the balance.

On the other hand, a full complement of high-quality weather instruments can be very expensive and is not possible on many vessels due to space considerations. But there are some very reasonably priced options available that will meet the most basic needs. I will not recommend specific brands or models of instruments for this article, but rather will provide a general overview of the types of instruments and ranges of options available, and will comment on which instruments are truly necessary and will deliver the most useful and critical information.

Wind speed and direction
Wind speed and direction affect all boats in that these parameters help drive the local wind wave field. Of course, for sailboats, the wind is the primary method of propulsion, so it takes on an added importance. Wind speed can be observed without instruments by using the Beaufort Scale, which allows the appearance of the sea state to be converted into wind speed. Also, by examining the local ripples or wavelets on the water, the wind direction can be discerned, and it will be perpendicular to the waves.

Moving up a bit to a device other than one’s eyes, the old fashioned tell-tale also gives information about the wind speed and direction. These can be as simple as strands of yarn tied onto shrouds. It is important to realize that these simple devices show apparent wind, not true wind. In other words, they factor in both the actual wind speed and direction and the air motion across the deck generated by the motion of the vessel. Handheld anemometers to measure wind speed are also available, ranging in price from very inexpensive to moderate levels. Users need to face into the wind while holding these devices at eye level to read the output.

Sensors for wind speed and direction can be installed on the top of the mast with a display in the cockpit or at the nav station. Wind speed sensors (cup or propeller types can be used) typically spin a generator, and the electric current produced drives the display, so no external power is needed for an analog display. A masthead wind vane will turn a switching device, which will display the wind direction below, but these instruments typically require a power source. Some masthead sensors are now available in wireless versions, eliminating the need for cables through the mast. Again, the output generated by these devices will be apparent wind. If the output is sent to an onboard computer which also keeps track of the vessel’s heading and speed, the conversion to true wind can be done electronically.

Converting apparent wind to true wind is not absolutely necessary for short term knowledge of the wind affecting the boat, but knowing the true wind speed and direction is important when one wants to know how the wind velocity at the position of the boat fits in with weather systems in the area, or if a report is made to other vessels in the area that do not have the same heading as yours, or to shoreside interests. The conversion is a vector operation, keeping in mind that the wind direction shown by an onboard instrument needs to be changed to a true compass direction before the conversion is undertaken. This is done by adding the compass heading of the boat to the wind direction shown on board, assuming that the instrument is aligned such that wind coming from the bow is shown as a 000 (north) heading. If the result is greater than 360 degrees, then just subtract 360 from the result.

Temperature
Temperature can be sensed in a general fashion by the crew. If it feels chilly, then crew members will break out the fleece! It it feels warm, then layers will be shed. If a more specific knowledge of temperature is desired, there are many options for inexpensive thermometers. Some of these include a remote sensor attached by a short cable to a display device, which might have the capability to record high and low temperatures for a period of time. Wireless sensors are also available. Temperature can also be recorded as part of a sling psychrometer observation (see next section). On larger vessels, it may be desirable to have a display unit that matches other display units, like those for wind speed and direction. These units will be more expensive, but not necessarily any more accurate. Connecting to an onboard computer for more extensive tracking of temperature trends is possible as well.

One other note about temperature — if you are voyaging farther offshore, you may want to have the capability to measure the water temperature. This is particularly important if you will be moving through the Gulf Stream or other currents. These devices can be as simple as a thermometer designed to be lowered over the side on a line, much like a swimming pool thermometer, or as fancy as a thru-hull sensor providing data to a remote display unit matching other such units on board and connected to a computer.

Humidity
In addition to gauging personal comfort, humidity is an important factor in the formation of fog, especially when voyaging over relatively cold water. In particular, whenever a warm moist air mass moves over cold water, fog will often form. The most frequently used measures of humidity are relative humidity, which indicates the amount of water vapor present in the air as a percentage of the maximum amount of water vapor possible at a given temperature (warmer air is able to hold more water vapor), and the dew point, which indicates the temperature to which the air must be cooled to reach saturation (100 percent relative humidity).

There are instruments that can display relative humidity and dew point directly available both in handheld and bulkhead mounted models, and connections to onboard computers are also possible. One of the most accurate ways to measure humidity, however, is through the use of a simpler instrument —  the sling psychrometer. This instrument is usually relatively inexpensive and consists of two thermometers mounted next to one another with a handle on one end which allows the instrument to be whirled through the air. One of the thermometers has a muslin wick covering its bulb which is moistened prior to whirling, and as the water evaporates from the wick, the temperature of this thermometer drops to the wet bulb temperature. Once both the dry bulb (the one without the wick) and wet bulb temperatures are known, it is a simple matter to use charts or tables to determine both the relative humidity and the dew point.

Pressure
Barometric pressure is unique among weather parameters in that it is the only one which cannot be readily sensed by humans (aside from perhaps some who claim that their joints ache more when the pressure changes quickly — though this connection has never been scientifically proven). We are able to sense differences in temperature by feeling warm or cold, differences in humidity by how quickly our perspiration evaporates, and we are able to feel how strong the wind is blowing and observe the direction. However, we don’t have an innate sense of whether the pressure is high or low at any given moment. Thus we need an instrument to measure this parameter — a barometer. Mercury barometers, which were the standard through the 19th century and into the early 20th century are almost never used any more, and the aneroid barometer is now the standard.

These instruments typically consist of the aneroid sensor (a chamber enclosing a vacuum which is deformed based on the pressure) connected to a pointer which indicates the air pressure on a dial — all enclosed in a cylindrical metal case. Another option is a barograph, in which the aneroid sensing unit is connected to a pen which traces the barometric pressure on a chart mounted on a rotating drum. This allows the trends in the pressure to be easily seen. More recently, electronic sensors have been developed which allow for a digital output.

A barometer is probably the most important weather instrument to have on board. While all the others offer useful information, knowing the atmospheric pressure, and, more importantly, how it is changing over time, provides essential information about how weather systems affecting the boat are behaving. Rapidly falling pressure can indicate a strong low pressure system or a strong front approaching the vessel, typically with strong wind and inclement weather. Rapidly rising high pressure can indicate that these features are moving away, but that winds will remain strong for a time. By using your onboard barometer in combination with available weather charts, a more clear picture of your position with respect to weather systems is possible.

There are some handheld barometers available, but the best choice for most vessels is a bulkhead mounted model. These are available in diameters from about four inches to seven inches, and there are many choices within a reasonable price range. If you have the space, a barograph is a nice upgrade, but at a significantly higher price. If you choose a barograph, make sure that it is designed for marine use, which for some barographs requires the installation of a marine damping feature. This will keep the pen arm from bouncing around as the boat moves through the water. If price is no object, consider a bulkhead mountable electronic barograph. These instruments will have a digital display of the current barometric pressure as well as a graph of the trend over the past couple of days recorded either on a long term paper chart or an LCD display. Some of these instruments have additional features, such as the ability to electronically store long term data, and an alarm feature which provides alerts when the rate of change of pressure exceeds a preset value.

What to choose
If only one instrument can be chosen, my suggestion is a good quality bulkhead mounted aneroid barometer. A barometer gives data that cannot be estimated without an instrument, that provides important information about where the boat is located within the larger weather pattern, and that helps predict short term weather changes. By recording the barometric pressure as part of regular log entries, trends can be followed without the need for a more expensive barograph. If the budget is limited, then after obtaining an aneroid barometer, consider adding the instruments shown in the table. If you have a more liberal budget, and the space on board, then upgrading to higher-quality instruments with more aesthetically pleasing displays will be possible, and many of these will have the capability to connect with onboard computers that will give you the ability to comprehensively monitor, track and archive just about all weather parameters.

Ken McKinley is a professional meteorologist whose consulting business, Locus Weather, is based in Camden, Maine.

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