Water Temperature 11

The Channel's water temperature limits the crossing-season to late June (c 14C) to September (19C or 20C). The coldest months, January and February, 5-7C. An Olympics competition pool, by comparison, must be 25C to 28C.

Hypothermia, generally a concern below 10C, may occur at 20C in prolonged exposure as the core body temperature can crop to 35C (95F). The tell signs include claw-like hands unable to touch thumb-to-pinkie or, counter intuitively perhaps, feelings of Euphoria.  Mental cognition also goes but harder to judge when swimming. If observing, the lips or upper-back may turn blue or purple.

Hypothermia can occur anytime and rapidly, the swimmer losing muscle control, all the while aware s/he could be drowning. If to hospital, warm blood transfusions are given as the body cannot warm itself to normal.  For this reason, a cold water partner is critical - advise, of course, I rarely follow. 

This past winter, with a friend, I swam in the Thames west of the Teddington Lock where the river is non-tidal, flowing generally south-eastward from its source near Oxford. It is also less developed and, for long stretches, makes me think of Tom Sawyer's Mississippi River with its old river boats and unkempt shores. The Thames drops to 1C in winter so a wetsuit, thermal gloves, booties and cap are mandatory.

I do no anticipate 17C on the Channel swim being a problem; equally comforting, the water during the six-hour qualifier was 15C.

Photo of the Thames River at the Barnes Bridge :