I live in a coastal cold to mild climate with temperatures in the low 50s to low 60s average and people suffer heat prostration when it gets to 70 a few times a year.
However, in the summers I stay at a summer home where temperstures are between 100 to 110 fahrenheit.
After I train in this heat for awhile, i race considerably better -- one minute faster for my 5k and 2 min for my 10k.
On top of that, i did some statistical analysis and found that on the average, the runners that come from warmer climates have much better times than runners from colder climates or runners from coastal cities.
This would explain the horrible track performances in cities like fort bragg, mendecino, eureka and especially san francisco where there are like at least 20,000 runners and at least 30 high schools which year after year after year have on average the worst times for high school runners for a major city in the entire state of california.
I don't believe it has to do with bad coaching or no talent because that wouldn't make any sense when you take times of 100,000 or more runners being slower. The only explanation is that people just do not get as much trainung results in cold temperatures as they do in hot climates. And as a person who goes through both extremes every year, running in temperatures 50 degrees warmer each summer i notice, sense and feel the difference as well as run faster times when i train in the heat.
So i am highly recommending that serious runners get out in the heat to get better performances.