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The Great Lakes are notorious for the violence and sudden nature of their storms. These environmental phenomena include:  electric storms, wind storms, sudden waves, and water spouts. A swim across a Great Lake is very much dependent upon the weather. For this reason, a crossing of Lake Ontario is reputed to be one of the toughest swimming challenges in the world.


Based on our experience with the Great Lakes, SSO has set out "minimum acceptable standards" for the forecasted weather for a proposed swim. These minimum acceptable standards have been established for safety reasons and weather meeting these standards can be far from ideal for a swimmer.


 "Minimum Acceptable Standards" 


 1.  No lightning or thunderstorms are predicted for the lake during the 24 hours after the anticipated start

 of the swim, with the recommendation that any thunderstorm predictions are located more than 50 km

 beyond the shores of the lake;

 2.  The waves are not more than one metre trough-to-crest;

 3.  The wind is not more than 10 knots (19 km/h); and

 4.  The water temperature is at least 10°C (50°F).


Pre-swim Weather Timeline:

12-24 hours prior to the anticipated start of the swim, SSO requires the swimmer's coach and/or manager, in consultation with the swimmer and the captain, to obtain the Swim Master’s confirmation that the weather is acceptable.

After the start time is confirmed, the coach and swimmer ideally should not change it unless there is a medical problem, or other extenuating circumstance.


After the confirmation of the weather, if the weather forecast becomes unacceptable and a postponement is necessary, the Swim Master will notify the coach/swimmer. SSO understands that considerable organization of people, equipment and time is involved in a marathon swim, and that a postponement can cause inconvenience and hardship; nevertheless, the safety of the swimmer and crew is paramount.


One-to-two hours before the swim is to take place, at the point of departure, the Swim Master will confirm that the equipment is in order and that the weather forecast remains acceptable.


For safety reasons only, the Swim Master has final authority on assuring that the swim meets required safety standards and, therefore, the Swim Master reserves the right to withdraw SSO’s sanction for the swim.

Notes on Weather

  • To ensure the best chance for success, the water temperature should be at least 18°C (65°F) and there should be minimal winds.

  • For 3-5 days after a heavy downpour or when the lake has been at 75°F (23°C) for a week or more, the bacteria count increases rapidly, especially near Toronto.  Please take extra precautions against nausea in these circumstances – e.g. very light pre-swim meal, stay well hydrated, use anti-nausea wristband, and don’t wash goggles in the lake water.

  • Heavy or prolonged rains swell rivers emptying into Lake Ontario and cause distortions in near-shore current patterns for days afterwards, e.g. the Humber River.

  • Wind currents are the dominant feature of surface circulation on the Great Lakes. Their speed and direction are dependent primarily upon the wind speed, wind direction and distance over which the wind has blown, as well as the depth and contour of the lake, the temperature difference between the air and the water, and the presence or absence of layers of water at different temperatures, known as thermoclines.

  • If the wind blows in the same direction for a sufficient time, the surface layer of water responds to the wind by following in the same direction. On Lake Ontario, within about eight km of the shoreline, both the wind and the current tend to move parallel to the shore. In general, the currents flow in one direction along the shore for about three to eight days, then reverse over a period of about a day, and then flow in the opposite direction along the shore for several days.

  • Surface current is estimated to be about 10% of wind speed. So a wind of 20 km/h would generate a 2 km/h current.

  • The height of waves is determined by the distance over which the wind has blown, together with the velocity of the wind.

 Weather Links

  • Internet links for daily marine buoy data:

Lake Ontario:

Lake Erie:

To select other marine buoys:

  • Links to maps of Surface Temperature which may be inaccurate under cloudy conditions:

Western Lake Ontario:

Eastern Lake Erie:


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