A regular blog reader (hi Gabriel!) left a comment wherein they mentioned wind against sea. My first reaction was that I knew right then that Gabriel was an experienced open water swimmer. My second reaction was to kick myself for not mentioning wind against sea previously.

Wind against sea is a condition that most open water swimmers (who are aware of it) dislike. Some aren’t aware of it and it can mostly be chalked down to local weather. Some few, like it (in theory anyway) for races in the short to ten kilometre range (though I generally dislike it for training, as it can be hard work). For long distance marathon swims wind against sea is an accepted fact of life, and generally not pleasant, though usually manageable.

As I mentioned above, racing in wind against tide, doesn’t mean that I like choppy water (I don’t, I prefer rough or calm) it’s just that I like to think others will hate it more. I’ve previously equated it to cycling time trials into a head wind. Every cyclist likes a time trial on a flat course on a warm day, but a wind day makes some give up early. new and intermediate open water swimmers and triathletes need experience with and develop skills in swimming in rough water (fortunately, I’ve written a How To swim in rough water).

What is wind against sea?

The ZC2 buoy in the English Channel amid wind against tide chop

Well, it’s quite simple: Wind against sea means a breeze or stronger wind blowing in one direction while the tide or current is running in the opposite direction. As such it’s also more often called “wind against tide”. The two clash to form a choppy rough surface which is disruptive  to swimming. Using sea instead of tide does actually show that it can happen on currents are well as tides.

Examples include the annual Cork to Cobh marathon swim in Ireland, which sometimes suffers wind against tide for the final mile, as the course swings east past the island of the Hawlbowline Naval Base on a dropping tide, and the swimmers who are swimming on a dropping tide in the estuary, are suddenly exposed to wind across the large expanse of Cork Harbour, Europe’s largest natural harbour.

Similarly English Channel swims which start on a high tide regularly suffer wind against tide during the second (ebb) south-east direction tidal leg of a crossing. The Manhattan Island Marathon Swim can have coastal onshore blowing up the estuary against the ebbing flow of the Hudson river once swimmers round Spuyten Duyvil to head down toward south Manhattan in the last leg of the swim.

In many cases with a swim of sufficient duration or during a particular it is possible to predict wind against tide conditions. Tides last about six hours and fifteen minutes∗ and all locations have a prevailing wind direction, which explains the above examples.Also worth remembering is the onshore winds are more common on coastal areas in mid-day and afternoon in warm weather as the temperature gradient between land and sea reverses. With early morning calm offshore condition, the breeze blows from land to sea and calms the waters, but as the Sun warms the air over land the flow reverses to cause “sea breezes” which blow from sea to land. The warmer the coastal climate the earlier sea breezes will develop during the day as land temperatures change daily whereas the sea is more thermally stable.

∗ Semi-diurnal tides, that is, two (almost) twice-a-day tides of almost equal height, both high and low. The period is more accurately about 12 hours and 25 minutes, but that varies slightly by location in semi-diurnal locations and some diurnal locations only have one of each tide per day. It is the twenty-five minutes doubled that moves high tides back by about fifty minutes per day. See this previous post for a consideration of tides, a subject to which I’ve been considering returning.