London is notoriously cool and wet, a pattern that was even more extreme than usual in the weeks leading up to the Games.
High heat and humidity are the toughest weather conditions for athletes to deal with.
The U.K. slogged through persistently wet, cool weather over the month preceding the Olympics.
Forecasts look good for moderate temperatures and dry weather in London over the next week or two.
Among the high-profile events at the Summer Olympics in London are swimming, diving, and gymnastics -– all sports that happen indoors, where wind speeds, temperature and humidity are irrelevant.
As runners and cyclists increasingly hit the track and roads this weekend, the weather will become a much bigger performance factor. London is notoriously cool and wet, a pattern that was even more extreme than usual in the weeks leading up to the Games.
So how will athletes fair in the typically damp United Kingdom?
Humidity might be a concern for marathon runners and other endurance athletes who need to sweat to cool off, said Matt White, an environmental physiologist at Fraser University in Burnaby, British Columbia. Rain can also make roads slick for cyclists if it's been dry for a while. And heavy downpours can play with the minds of athletes who are used to training in dry conditions.
But compared to the heat typical in Beijing, Athens and other previous Olympics venues, London's moderate temperatures should, overall, be good news for competitors.
"If I was talking to a coach about London, unless they had a heat wave, I think the environment is not going to be a big determinant," Fraser said. "Rain is cooling, and if you're working hard, it can act like sweat evaporation."
When it comes to weather, by far the biggest – and most extensively studied – concern for athletes is a combination of high heat and high humidity. The human body at work uses sweat to dissipate the heat that builds with effort, but that sweat needs to evaporate to have a cooling effect on core body temperature.
As air heats up, it can hold more and more moisture. And hot, wet air severely compromises the ability of sweat to evaporate.
To prepare for the hot and humid conditions at the 2008 Summer Games in Beijing, which ended up not being as extreme as expected, many athletes went through heat-acclimation training beforehand.
Fraser helped the U.S. men's field hockey team prepare for the Beijing Olympics by putting players in a room heated to 35 degrees Celsius (95 Fahrenheit) and 75 percent humidity. For an hour a day for five days, the athletes worked out on a treadmill or an exercise bike in the heated room – giving their bodies a chance to adapt to exercising in hot and humid conditions.
With heat-acclimation regimens, an athlete's body learns to sweat sooner and to produce more diluted sweat, helping him or her simultaneously stay cooler and better retain electrolytes. Heart rates also stay lower, and body temperatures stay lower.
On the other end of the weather spectrum, extreme cold can make it harder for athletes to get muscles warmed up. When chilled, the body also tends to divert blood from the arms and legs to the brain, heart and internal organs. In water, that can happen at temperatures of about 10 C (50 F), Fraser said. But air temperatures need to be much colder to cause danger. Cross-country ski racers often successfully compete in conditions as cold as -30 C (-22 F).
London certainly won't be that cold, but the U.K.'s summer has been persistently cooler and wetter than normal for the same reason that much of the United States has been persistently hot and dry.
As the Arctic warms twice as quickly as the lower 48 states, there has been a weakening of the temperature gradient between high and mid-latitude regions, said Paul Douglas, meteorologist and founder of Weather Nation, a weather outsourcing company in the Twin Cities, MN. As a result, the jet stream is blowing farther north and less strongly than usual. That has caused weather patterns of all kinds to stall.
"What we're seeing increasingly is that the atmosphere seems to be getting stuck in a rut," Douglas said. "Weather systems are just moving slower."
In the four to five weeks leading up to the Olympics, Douglas said, a large ridge of high pressure settled over Greenland and Iceland, shoving storms southward and eastward to the U.K. and Scandanavia.
The good news for athletes who prefer drier conditions is that the U.K.'s cool, wet pattern began breaking down just before the start of the Games. Over the next week, models predict highs in the 60s to low-70s in London, with nighttime lows in the 50s and no signs that flooding rains will return any time soon.
"I don't think weather is going to be a huge factor at the games," Douglas said. "That may be wishful thinking."