Selecting the Right Weather Equipment
by Daniel Piltch, Marine Computer Systems
 
(as originally published in Professional Mariner Issue #51) 
  

 

The options available for gathering information about weather have grown at an astounding pace. At one time, the big challenge was to predict the weather - what conditions would you be facing the next day? Now, with technological advancements and plummeting prices the big challenge is not predicting when you'll face harsh conditions, but rather how to avoid heavy weather in the first place.

With the abundance of weather information available onboard, it's convenient to break it down into a few different overlapping categories. First, what type of weather information are you looking for?

area coverage: local or synoptic
validity: short-range or long-range
time period: analysis (current) or prognosis (forecast)
How will you get this information delivered to your vessel?

VHF
MH/HF/SSB
Satellite
How will this information be presented?

voice
text
weather chart
satellite imagery
Given these choices, there are several options that make sense to put together. For instance, receiving local, short-range current conditions is easiest done over a VHF voice broadcast, but this method would be inadequate for receiving high-seas, long-range forecasts due to the limited reach of a VHF broadcast and the limitations of voice as a medium for conveying information. Similarly, it would be overkill to use a satellite to transmit local voice weather information to a small area, but it would be appropriate to use this medium for a high-seas transmission. Generally, the type of information is matched to an appropriate means of delivery. Expect to get more detailed weather information from the more advanced channels. A satellite image contains much more raw weather data than a voice broadcast.

The most common ways to receive weather information are:

voice broadcasts over VHF or SSB
weather charts over SSB and sometimes by satellite
textual broadcasts over Inmarsat C or Navtex
imagery over satellite, and charts, text, and imagery over the Internet via a landline or Inmarsat connection
 

VHF NOAA Weather Radio

The vast majority of vessels rely solely on the local VHF voice broadcasts for their weather information. While this may suit a local captain who never ventures further than his home harbor and never leaves port for more than a day at a time, the information available on these broadcasts is limited by the nature of how it's delivered. It's a great way to quickly hear about the local conditions and what to expect in the short-term. It's not such a great way to understand the larger prevailing weather patterns and derive your own conclusions based on the conditions you actually find outside.

VHF broadcasts are prepared locally, by a nearby local forecast office of the National Weather Service. The reports are transmitted over NOAA Weather Radio on the VHF weather frequencies. NOAA or the National Oceanic and Atmospheric Administration is the National Weather Service's parent organization. The nominal range for these broadcasts varies with the local terrain, but 20-40 miles from the transmitter is about average, with antennas on higher ground often reaching out to about 60 miles. The National Weather Service publishes a marine service chart for each region of the coast, listing weather radio transmitters, data buoy locations, and schedules for both SSB voice weather and weather fax. For $1.25, this is a very useful chart. You can order one from:
Distribution Division (N/ACC3)
National Ocean Service
6501 Lafayette Avenue
Riverdale, MD 20737-1199
(301)-436-6990
(800)-638-8975
(301)-436-6829 FAX

 

Weatherfax Charts

For a richer look at the weather, you'll need to look at a weather chart. For US waters and adjacent high seas areas, these charts are produced by divisions of the National Weather Service, and can be obtained over the Internet, or over a single sideband radio via weatherfax, or via geostationary satellite broadcast.

The charts typically cover various time periods such as current weather (also known as an analysis chart), 12 hour forecast or prognosis, 24 hour forecast, and so on up to a 96 hour forecast. The charts are prepared in black and white, with major significant weather features denoted. High pressure and low pressure systems are clearly indicated as are frontal boundaries and many other weather features. A sample weather chart is shown in figure 1.

 

To get these charts over the World Wide Web, visit the site: http://weather.noaa.gov/fax/marine.shtml. To get the charts via email, see the instructions available at http://weather.noaa.gov/pub/fax/ftpmail.txt. Since Internet access at sea isn't practical for many vessels, many mariners rely on weatherfax charts via SSB. This method has been around since the 1950s and hasn't changed significantly since then. In the US, charts are prepared by either the Marine Prediction Center or the Tropical Prediction Center. The charts are then sent to the US Coast Guard for transmission from one of the following stations: Marshfield, Mass. (near Boston), Belle Chase, LA (near New Orleans), Point Reyes, CA (near San Francisco), Kodiak, Alaska and Honolulu, Hawaii.

Each station maintains a regular daily schedule. Every chart is simultaneously broadcast on a number of different frequencies, with the hope that at least one of them will be appropriately clear for a given location at a given time of day. See figure 2 for a sample schedule from the Belle Chase, LA station. The schedule for each station is printed on the back of the corresponding marine service chart mentioned above, and can also be found on the Internet at http://www.mpc.ncep.noaa.gov/radiofax/. In January 2000, the National Weather Service issued a 94 page publication entitled, Worldwide Marine Radiofacsimile Broadcast Schedules, which contains information about all known transmitting weather fax stations. The publication is available for free on the Internet at http://www.nws.noaa.gov/om/marine/radiofax.htm and will prove useful for any International passages. For information on other radio transmissions, consult the British Admiralty's multi-volume publication: List of Signals, which contains information not only about weather fax stations, but also about many other radio transmissions of interest to the mariner.

While there are still a limited number of dedicated units that focus solely on receiving and printing weather fax charts such as the Furuno Weather Facsimile Receiver/Printer, most people choose to interface their single sideband (SSB) radio with their computer instead. The computer based option offers a number of advantages over the dedicated receiver/printer. With the dedicated units, you're limited to seeing the image directly as it arrives off the radio waves. There is no opportunity to clean the image or tweak it to be clearer or more readable. However, by using a computer and software to receive the images, you have the ability to perform many different operations to try and improve the quality of the incoming weather chart. As an example, consider the image in figure 3. This is an image of a chart as it was received onboard from the Belle Chase/New Orleans transmitting station. By using weather fax software to clear away some of the "snow" or "static" in the image, and re-aligning the image, we can improve the image to look like it does in figure 4.

Two common computer based weather fax systems are Xaxero's Weather Fax for Windows (distributed by Coretex in the US), and Ocean and Coastal Environmental Sensing (OCENS) Corporation's WeatherStation 2000. Both of these programs allow you to connect a single sideband radio to an onboard computer, and manipulate the chart after it has been received. WeatherStation 2000 builds on these capabilities by also allowing you to do some basic satellite imagery manipulation as well.

 

Weather Satellites

Satellite imagery is becoming increasingly common on commercial vessels as it is virtually the only way to get a real-time picture of what the weather is doing at a particular moment in time. There are two types of satellites that relay weather information - geostationary and polar-orbiting satellites. Geostationary satellites orbit the each at about 22,300 miles above the surface. They move at the same speed as the earth, so they seem to hover over the same spot on the equator constantly. Because of their great distance from the earth's surface, their signal is relatively weak by the time it would reach a shipboard antenna. Any antenna that expects to get a useful signal would need to be stabilized in such a way that it consistently points directly at the satellite, despite the rolling and pitching of the vessel. The antenna also needs to be of sufficient size to capture a strong enough signal - typically three feet or more.

Polar-orbiting satellites, however, orbit the earth at only 500 miles above the surface. They orbit in a more or less north-south plane, and constantly transmit pictures of the slice of the earth's surface that's directly below the satellite. Because of their closer proximity to the surface, useful antennae can be much smaller and do not need to be stabilized. Consider the quad-helix antenna in figure 5. This antenna, from OCENS, is capable of receiving imagery from any one of three of NOAA's polar orbiting weather satellites.

In addition to showing weather patterns, satellite imagery is also capable of providing infrared images of cloud tops, land masses, and the sea surface. These infrared images can provide the temperature of various features within fractions of a degree. This is especially popular in the commercial fishing fleet which uses satellite imagery to get real-time accurate readings of the temperature of the sea surface. Using this type of imagery, ocean currents, surface temperature boundaries, and eddies show up very clearly. Note the strong temperature boundary shown in figure 6.

Satellite imagery is often best used in conjunction with the weather charts received by weatherfax. For example, consider the image in figure 7 which shows a chart that arrived via weather fax, showing a long cold front stretching to the southwest off the US east coast. This information alone is valuable to anybody in the vicinity. But within that cold front - there isn't any indication of where the severe weather is. The thunderstorms typically associated with this weather pattern could occur anywhere along the front. Now consider the image in figure 8 which was obtained from SeaStation 2000 using the antenna shown earlier - this image shows the same cold front, but it has additional detail because it shows the cloud top temperatures. The green area highlighted by the black arrow is significantly colder than the surrounding clouds. This suggests that it is a taller group of clouds which in turn suggest greater convective activity and a greater likelihood of bad weather. This kind of detailed information isn't available on any weather fax chart, and can only be found by analyzing satellite imagery.

Inmarsat C Weather

As more and more vessels are outfitted with Inmarsat C systems as part of GMDSS compliance, more mariners are getting used to receiving the high seas text forecast several times each day as an email message on their Inmarsat C terminal. The IMO has arranged for meteorological services in various countries to broadcast the relevant high seas forecast for their part of the world on the Inmarsat C system. This can be critical information as the delivery system is one of the more reliable systems available. Needing only a small, non-stabilized antenna Inmarsat C systems such as Trimble's Galaxy or Sentinel units (figure 9) work in just about any conditions, almost anywhere on the planet (with the exception of extreme latitudes). Though weather charts and imagery are not transmitted on this system, the information included in the high seas forecast would allow you to re-create a weather chart using a small scale chart and a pencil.

Internet Weather

The Internet has rapidly become a primary source of weather information for many mariners - both recreational and professional. It is very common to download a pre-departure batch of the latest weather charts and forecasts to have on file before leaving port. Though the download-before-departure approach has diminishing returns as you're at sea for longer periods it is invaluable for short passages.

While some of the Internet charts have exactly the same content as those available via weatherfax, the clarity of the Internet charts is always crisper than those coming in over the radio. Another advantage to downloading your charts is that you can get the charts you need when you choose to receive them; you're not limited to a prescribed schedule that would force you to wait for a number of hours to get all of the recent charts you might need before leaving port.

The information available on the Internet is typically much more detailed than that available while underway. The quantity of information is greater as are the number of different sources of information. For example, there is far more information available on tropical storms and hurricanes on the internet than there is available over commonly accessible media at sea (SSB, weather satellite, Inmarsat).

Consider the following short list of URLs that might be useful:

 

Web Site Name Internet Address (URL) Description
Marine Prediction Center (National Weather Service) http://www.mpc.ncep.noaa.gov/ Excellent source of analysis and forecast charts. Original source of weather fax charts.
National Hurricane Center and Tropical Prediction Center http://www.nhc.noaa.gov/ Excellent source of up-to-date information about existing and developing hurricanes. Also has marine weather for tropical areas.
Marine Computer Systems http://www.MarineComputer.com/Weather/WeatherLinks.html An almost all text (no images) list of many, many different web sites providing marine weather information.
Gocruising.Net http://www.gocruising.net/ A text-only page with links to text-only weather information.
National Data Buoy Center http://www.ndbc.noaa.gov/ Provides detailed information on weather conditions at several buoys, including historical data and graphical plots.

 

 

In summary, there are many different ways to get weather information while underway, and many different ways of using the information that you gather. Matching the right equipment to the right vessel is perhaps the first step, but should by no means be the last. The next step is often participating in a good heavy weather avoidance class (typically 5 days). And of course, there will never be any substitute for experience - the more weather you're able to analyze, the more accurate your interpretations will be and the safer your vessel will be.
 
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