This year’s pre-Olympics in Enoshima, Japan, starkly illustrated for me the reality of being on the water in extreme hot and humid conditions. One day we saw a temperature of 42°C on the coach boat, along with very high humidity. Daytime temperatures in August in Enoshima are regularly in the high 30s. But it’s not so much the temperature as the humidity that makes you uncomfortable.

Between races, competitors swiftly made for their coach boats and never have I seen sailors so fervently grasping for cold drinks, cold towels and putting bags of ice on their heads. Some briefly ripped off their buoyancy aids to add an ice vest for ten minutes.

Enoshima is a small island 50 miles to the south-west of Tokyo, connected to the mainland by a bridge. Built specifically for the Olympic sailing competition in 1964, it is a good venue. The tricky bit is the timing. The 2020 Olympic regatta (27 July to 6 August) takes place at the height of the tropical summer and the relative humidity is mostly above 70%.

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Most races in the test regatta were completed on schedule, with a few delayed due to the late arrival of a sea breeze. However, one thing we did see in August was a typhoon. It made landfall in the south-west of Japan 36 hours before the first race.

Forecasting typhoons is well developed in Japan – not surprising given the magnitude of their effect on everyday life. They occur mostly from July to October, mainly in August and September. About 25 typhoons occur a year and 11 approach Japan directly. This year Typhoon Hagibis struck in October, late in the typhoon season, and was the worst in decades with 140mph winds and flooding causing fatalities.

August is the month that sees the most action from typhoons as they originate to the south-east of Japan, track north-west then re-curve to the north-east. Generally the strong winds only last about three days, but the waves will last another 48 hours.

Enoshima is open to waves from the south: the next land to the south-east is Papua New Guinea over 2,000 miles away. The August typhoon resulted in racing being held in 1.5-2m waves on two days in the pre-Olympic event, including one very tricky day, which saw these waves with only 6-8 knots of wind.

Humidity and windspeed

Humidity is the relative amount of water vapour in the air and is the main cause of everyone’s discomfort during a hot spell, thanks to its ability to make temperatures feel much higher. When we perspire, the water in our sweat evaporates, cooling the body as heat is carried away. When humidity is high, the rate of evaporation and cooling is much reduced, resulting in it feeling hotter than it actually is.

When the relative humidity reaches around 90%, your sweat does not evaporate. In these situations, your body temperature may rise and lead to heat rash, cramps, heat exhaustion and eventually heat stroke.

The ‘heat index’ (see below) is a way of measuring how we experience temperatures. This ‘feels like’ factor is now being incorporated into weather forecasts. Just as the wind chill with cold temperatures and strong winds makes it feel colder, so in hot and humid conditions, temperatures ‘feel’ much higher. The heat index also measures temperatures in the shade, so sailors in direct sunshine could easily experience an additional 7-10°.

Humidity also affects windspeed. Many people are aware that as temperature rises the density decreases in the atmosphere (molecules get further apart). However, it’s less well known that as humidity rises, density decreases a little further (H₂O water molecules are less dense than the molecules they replace).

As humidity rises, our ability to cool down decreases, making the temperature ‘feel’ hotter than it really is

The result is that, in hotter climes, for a given wind speed, we sail a little slower due to the air being less dense. In meteorology, temperature forecasting is generally well advanced, but humidity forecasting certainly isn’t and adds an extra dimension to forecasting wind in the tropics.

When we see a slack pressure gradient (e.g. light winds in the morning) sea breezes develop the world over, from the equator to the edge of the ice at the poles. In tropical areas such as the east coast of Brazil, sea breezes can be strong and extend out to sea nearly 100 miles. In hot and humid areas like Japan, in very volatile air, sea breezes do still develop in the same way but usually only build to 10-12 knots.

The Japanese are working hard to alleviate the effects of the heat for athletes at the 2020 Olympics, but it is hard to do in sailing. Some national authorities requested that sailors be allowed to wear heart monitors during racing.

During training, the Spanish team have even been getting their athletes to swallow a pill that measures core body temperature. This information is then transmitted via Bluetooth to a device to measure how their athletes are coping with the extreme heat and humidity.

About the author

Mike Broughton is a pro race navigator who has won many titles including World and European championships. He is a qualified MCA Master to captain superyachts and previously had a successful career in the Fleet Air Arm flying Sea King and Lynx helicopters.

First published in the December 2019 edition of Yachting World.