Boating Course Weather

1. Boating CourseWeather Canadian Power & Sail Squadrons

2. General As an operator of a small vessel you can not safely ignore the weather. Even an elementary knowledge of theoretical meteorology can add greatly to piece of mind, comfort, and competence while afloat.

3. There are professionals whose job it is to collect, analyse, and interpret weather data and to disseminate this information to users.

4. The object of this section is to provide a basic explanation of weather hazards and to describe marine weather forecasts and warnings, and how to obtain them.

6. Publications that will be useful on the British Columbia coast Marine Weather Hazards Manual and The Wind Came All Ways – Owen Lange

7. Mariner’s Guide – West Coast Weather Services. These publications are available through Environment Canada’s Meteorological Service

8. For those who wish further instruction in meteorology, the CPS Fundamentals of Weather is recommended.

9. This presentation is available for download from my personal web site. The address is: weathercat.ca

10. Weather Hazards Five Phenomena that can be hazardous to operators of small boats.

11. Wind • Waves • Poor Visibility • Lightning • Hail

12. Wind

13. Wind – The atmosphere in motion. Measured in knots for marine use (1.15mph, 1.85km/h). Winds may be steady or gusty. A gust is a sudden increase in wind speed of 10 knots or more, lasting only a few seconds.

14. A squall is an increase in speed that lasts 2 minutes or more. Both gusts and squalls may also be accompanied by a change in wind direction.

16. A cat’s paw is the name given to the patches of ripples betraying the presence of slightly more wind on an almost calm day.

17. In a similar manner, an approaching patch of darker, disturbed water on a windy day can reveal the approach of a gust or a squall when there may be no other clue to its presence.

18. Only apparent wind is felt on a moving boat. This is a combination of the wind and the boat’s movement.

19. An example would be a wind from directly astern. In this case the boat’s speed is subtracted from the true wind speed to give the apparent wind speed.

20. There is a very good article on the web relating to apparent wind, you can find it at the following address: http://johnellsworth.com/writing/nautical/understand_appwind/understand_appwind.html

21. Wind without waves is usually only a problem when manoeuvring in a confined area. If a boat is well secured, it would take very strong winds to cause damage.

23. Sudden unexpected changes in speed or direction can cause problems however. Marine winds are greatly affected by topography, and tend to follow the shore line.

24. Expect changes in the wind when leaving protected areas or when approaching prominent topographical features. (Local knowledge)

25. Showers may be accompanied by gusts or squalls, whether or not a thunderstorm is occurring. When a shower approaches, prepare in advance for a short period of strong and gusty winds.

27. Waves

28. Waves – Energy transferred from the air to the water. Note that the water will move at only about 3% of the wind speed.

29. Wind Waves – generated by the wind blowing over the water surface. Swell Waves – left over wind waves that have moved away from their source area.

31. Wind wave heights are directly related to the wind speed, time (duration) of the wind, and distance (fetch) it blows over the water in a straight line.

32. Stronger winds require shorter fetches and durations to raise the same wave, and a storm force wind can produce 1-2m waves only a mile offshore in about 15 minutes.

33. Wave Trains Significant wave height is the average of the highest third of the waves Most frequent wave height………0.5 X sig wave height Average wave height……………..0.6 X sig wave height One wave in 10…………………..1.3 X sig wave height One wave in about 1000 …………1.7 X sig wave height Maximum wave …………………2.0 X sig wave height

34. Wind waves may be superimposed on swell waves.

35. Waves that oppose a current, such as at a river mouth, have a shorter wavelength, are steeper, and break more often than waves that do not oppose a current.

37. Waves moving into shallow water (depth less than 1.5 times the distance between adjacent crests) also shorten and break. Both conditions may be hazardous to small vessels.

39. Poor Visibility

40. Fog Fog is cloud that forms at ground level. The term is commonly used to describe any reduction in visibility.

41. Radiation Fog Requires clear skies, light winds and sufficient moisture, conditions most commonly found under a ridge of high pressure in the fall.

42. Called radiation fog because it is caused by radiational cooling. The air cools overnight and will become saturated given sufficient moisture.

43. The winds must be light but not calm, a bit of mixing is required to form radiation fog, otherwise the condensation will result in heavy dew.

44. Radiation fog is often thin and patchy and tends to form in, or flow into and fill low lying areas.

45. Dispersion usually begins as the sun warms the ground which then warms the air near the surface.

47. Radiation Fog

48. Advection Fog Horizontal movement of air. 80% of sea fogs are this type.

49. Contact with a cooler surface causes a moist air mass to cool below the dew point and fog will form.

50. This type of fog is usually widespread, deep and persistent. It may last for days and may not dissipate until the wind changes direction.