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Techniques and Methods for Forecasting Passage Weather at Sea

Sections: Marine Weather Forecasts Maps Bulletins Weather Forecasting Instruments Using Observation

Coastal weather stations included in radio 4 shipping forecast and weather forecasting techniques and passage weather

Marine Weather Forecasts

Marine weather forecasts can be obtained from a variety of sources such as Television, radio, newspapers, and the internet. They provide generalized forecasts covering land areas with some offering more detailed synoptic charts showing pressure systems, isobars, fronts, and wind strengths. Marine weather forecasts are broadcast by public radio stations and may include storm and gale warnings. Specialist forecasts can also be obtained from coastguard services and coastal radio stations.

In the UK, forecasts for Port Areas are available by telephoning the Regional Met Office forecasting centres. These reports give present weather which is valuable if you are planning to sail in that area.  Another source is ITVs Teletext service giving forecasts for the next 24 hours for each sea area.

Weatherfax and Navtex systems provide marine weather forecasts specifically tailored for mariners. You require dedicated receivers and software to obtain these forecasts that is used on a PC connected to a radio. In addition, Navtex receivers provide a range of navigational and safety information. Receivers display the information on screen, but most have a built-in printer.

Each of the Shipping Forecast phrases has a precise meaning:

  • Gales indicates mean winds of Force 8, with gusts to 43 knots.
  • Severe gale indicates mean winds of Force 9, with gusts to 52 kts.
  • Storm indicates mean winds of Force 10, with gusts to 61 kts.
  • Imminent means 'arriving within 6 hours'.
  • Soon means 'arriving within 6 to 12 hours'.
  • Later means 'arriving more than 12 hours later'.

Weather Maps

[ Weather maps ] are a method of obtaining information about weather systems, direction of movement, and associated winds for forecasting passage weather. Obtaining weather maps and satellite weather pictures can be from many sources, including the internet. You can compile your own weather map using radio weather forecasts which include a general synopsis and predictions for particular sea areas. This includes recent recorded readings at weather stations of wind strength and direction, pressure, precipitation, and visibility.

Plotting this [ information ] on a chart, you are able to construct the pattern of isobars and the direction and speed of movement of areas of high and low pressure. Pressure is depicted in millibars, and wind as an arrow blowing away from the observed direction.  Oblique lines on the tail of the arrow represent the wind's force being numerically equal to twice the number of lines i.e. a wind force of 4 is represented by two lines, and force 7 by three and a half lines.

The broadcast synopses give the centres of pressure areas and the positions of fronts and record these in at the correct positions, denoting 'H' for high and 'L’ for low. Isobars are the lines on a weather chart joining areas of equal pressure. Draw in the isobars at 2-millibar intervals relative, to the pressure reading from each coastal station on the chart i.e. if a station reports a pressure of 1021 mb, and then the 1020 mb and 1022 mb lines go either side of that station.

A guide to the spacing of the isobars is the wind speeds.  The geostrophic scales at the chart top show the spacing for given wind speeds with this spacing labelled 'Beaufort force' on the scales.

Weather Bulletins

USA

Special VHF weather radios in the United States are equipped with severe marine weather alarms and receive continuous updated weather bulletins.  Local airfields and some coastguard stations contribute to this information service. The US National Weather Service broadcasts information on 102.40 MHz, 162.475 MHz and 162.55 MHz and ship borne VHF radios are able to receive one or more of these Wx channels.

United Kingdom

In the UK, sailors consult lists which give the times of marine weather bulletins on a variety of frequencies, ranging from the shipping forecasts on 198 kHz (1515 m) AM long wave to VHF forecasts broadcast by Coast Radio Stations. Because of frequent changes to UK broadcast weather forecast services consult the weather sections of current nautical almanacs for information on frequencies and times of Coast Radio and BBC forecasts.

Official British Met Office forecasts are broadcast by the BBC on long wave (198 kHz, 1515 m) at 0048 hrs, 0555 hrs, 1355 hrs and 1750 hrs.  The format and pace of delivery used is intended to permit a skilled listener to record details in meteorological 'shorthand' enabling the compilation of a personal weather chart and a pocket tape recorder come in handy for this.

Other Areas

Forecasts are based on specific sea areas extending from Iceland to northern Spain can be received in all the localized areas.  Books of charts with the ability to record forecasts are obtainable from the Royal Yachting Association and the Met Office.

Format

Every broadcast begins with a list of gale warnings followed by a general synopsis with the position and movement of pressure fronts and depressions for the next 24 hours.  24 hour forecasts are given for each sea area, commencing with Viking, and continuing in a clockwise fashion, concluding with South East Iceland. Details include wind direction and Beaufort speed with expected changes and weather and visibility, followed by reports from coastal stations of wind force and direction, visibility, barometric pressure and change.

Weather Forecasting Instruments

Barometers

The primary weather forecasting instrument aboard is a [ barometer ] which is calibrated by calling the local coastguard and obtaining the present correct reading then adjust as necessary.

The pressure readings direction and rate of change is the primary  consideration as rising or falling pressure and the speed of change indicates an imminent weather change. Pressure readings taken every hour are recorded in the logbook.

A barograph, a recording barometer, makes continuous readings of pressure recorded on a paper graph which provides a ready reference. Traditional barographs are susceptible to the motion of the boat but electronic versions are available.

Anemometers

To be able to measure wind speed and direction is valuable and can be obtained by an electronic instrument with a [ masthead vane and anemometer ] , or by a [ hand-held cup anemometer ] and a hand bearing compass.

When using hand-held instruments, take readings on the windward side of the boat. These instruments provide apparent wind direction and strength, so record boat speed and heading and correct for true wind speed and direction. Integrated instruments provide true wind speed and direction if connected to an electronic compass. Record the true wind information in the logbook on an hourly basis.

Psychrometers

Electronic logs may have a temperature sensor built into the transducer allowing the user to record sea temperature being a guide to the risk of fog. Sea temperature information is more useful when used in conjunction with readings from a [ psychrometer ] . This weather forecasting instrument has a dry bulb thermometer for measuring temperature and a wet bulb thermometer for measuring humidity. Use it in conjunction with tables enabling the calculation of the dew point temperature of the air. Comparing dew point information and the sea temperature is a weather forecasting method to obtain a much more accurate assessment of fog risk.

Weather Observation and Forecasting Techniques

If unable to receive useful forecasts at sea, use weather forecasting techniques such as your own observations to monitor the weather. Once becoming used to the passage of weather, you are able to predict the speed and severity of approaching fronts.

If in tropical latitudes, monitor the barometer expecting to see gradual diurnal changes. If there is a significant pressure drop and the cloud increases at high altitudes and a building ocean swell, expect an approaching storm. The centre of the storm will come from the direction of the sea swell. In temperate latitudes, approaching depressions can be predicted by falling pressure and a wind that backs initially under building high cloud.

Wherever in the world, use [ clouds ] as a technique to forecasting weather:

  • High clouds are related with weather systems up to six hours away.
  • If wispy and white, fine weather is imminent.
  • Clouds lifting and dispersing indicate that good weather is approaching.
  • Clouds at a lower level relate to current weather.
  • If dark, heavy, and lowering, poor conditions and rainfall are likely.

A way of forecasting rain is to observe if there is a halo around the sun or moon. This haze is caused by the refraction of light by ice crystals carried in moisture-laden clouds.

Beaufort Scale

Before weather forecasting instruments, wind strengths were stated on the [ Beaufort Scale ] devised by Admiral Francis Beaufort in 1805 to describe the effects of wind. The Beaufort Scale initially related the wind strength to the amount of sail a tall ship could carry being later modified to include the effects of wind observed on land and at sea.

The Beaufort Scale is used to define wind strengths at sea, with the Beaufort Forces defined as speed in knots. The descriptions are useful now allowing an observer to judge wind speed on land or at sea without the use of instruments.

Forecasting sailing passage weather.

weather map symbols
weather map
masthead anemometer
handheld anemometer
psychrometer
barometer
Cloud Types
beaufort scale