Do you listen to the forecast regularly and do you know and understand the terms used by the forecaster? To know how rapidly a low pressure system is moving or when the various types of weather are going to arrive is very useful and the RYA Weather Forecasts booklet G5/02 will be a great help. When you hear the weather forecast on the radio, you should be aware of the meanings of the terms used:

Gale Warnings
Wind is expected to reach force 8 (34 kns) or over, but gusts of 43 kns may occur. If severe gale warnings are given, the mean wind speed is expected to reach force 9 (41kns). Storm warnings include winds of 48 kns +, force 10 +.

When the weather is likely to arrive

  • Imminent – within 6 hours of the warning
  • Soon – 12 hours from the time of issue
  • Later – 12 hours + from time of issue

Visibility

  • Good – more than 5 nautical miles
  • Moderate – 2 – 5 nautical miles
  • Poor – 1000 metres to 2 nautical miles
  • Fog – less than 1000 metres

Speed of movement of pressure systems

  • Slowly – up to 15 kns
  • Steadily – 15 – 25 kns
  • Rather quickly – 25 – 35 kns
  • Rapidly – 35–45 kns
  • Very rapidly – over 45 kns

Those of us who rely on the synoptic chart for an idea of the weather need to be able to interpret not only when the fronts are going to arrive with their possible attendant wind shifts, but also what wind strengths. By using the geostrophic wind scale you can get an idea of the wind strength in the area you will be making passage. If there is no wind scale on your weather map, you can still use the scale, by interpretation, given in the RYA Weather Forecasts booklet.