Marine weather tips

Here are a variety of tips that mariners can take advantage of when assessing the weather conditions they are experiencing and/or are forecasted to experience:

Knowing where the low is…

If the wind is blowing at your back, the lower pressure is on your left side and higher pressure is on your right side.

Settled for a reason…

Many coastal communities were originally settled because they were sheltered from the worst of the wind. Mariners should remember that wind and other marine conditions reported from inhabited coastal sites may be significantly different from those occurring over open water.

Just outside the harbour…

Mariners who are sheltering in a bay or cove should always heed a marine forecast of heavier winds and waves just outside the bay. Even over short distances the sea state can change and mariners may unwittingly venture from a safe shelter into dangerous seas.

Start of a sea breeze…

One of the first clues to the onset of a sea breeze is the sudden dissolving of low cloud just offshore. This is a sign of air gently subsiding in preparation for feeding the first drift of air onto the shore.

Tidal influence…

Mariners are advised to plan routes that take advantage of the stage of tide (ebb-flood) and to use weather forecasts along with local observations of wind and sea to avoid dangerous spots. 

Wind speed from cloud speed…

A good rule of thumb in guessing the maximum gusts in an approaching thunderstorm is to add 15 knots to the estimated speed of the cloud; add another 10 knots if the cloud base is very low over the water. For example, assume a thunderstorm is moving in from the southwest: if the lower cloud approaches on a prevailing wind of SW 10 knots, then the expected first-gust speed is 25 knots, or 35 knots if the cloud base is low and black.

Role of swell…

A threatening sky with increasing black clouds and rain is generally not part of a large-scale wind system, if it is not preceded by swell. Therefore, any wind encountered should only be temporary. However, increasing swell for the direction of advancing storm clouds suggests an approaching low with a large area of strong winds.

Look upstream…

Northwesterly winds can develop very quickly in the wake of a passing storm. The winds start in the west and spread east, so mariners can forecast their arrival by heeding observations from locations upstream.

Waterspouts…

Although some immature waterspouts are very small, they should be avoided because they can change to more violent systems without warning.

Slowly or away…

A vessel that is moving slowly or away from the prevailing wind and waves will experience far less icing than a vessel that is heading quickly into the wind and waves.

Clues to pack ice…

Mariners experiencing smaller waves than expected should suspect that pack ice is close by. Also, the sighting of marine mammals (such as seals) which seem far from shore is another indication that pack ice may be near.

Fresh water chops…

With a rapid shift or increase in the prevailing wind conditions, choppy white water can be seen over fresh water before the same conditions develop over salt water. 

From sheltered to exposed…

Vessels operating near small islands should pay close attention to the weather forecast since rapid weather changes may occur and wind shifts can quickly change a sheltered coast into an exposed one. Add to that the complexities of many coastal effects and the conditions can quickly become dangerous.

Wind opposing the current…

Rough seas can develop when winds blow in the opposite direction of the current, especially during periods of large freshwater run-off events.

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