I went for a 10 K run through the Delaware and Raritan Canal State Park in 90 degree weather. It was a struggle to get through it. This is the first extended period of heat and humidity in New Jersey this year-- 85 to 95 degrees Fahrenheit with high humidity. When it happens, I usually skip running for a day or two and try to do something else. But, I can't do that for an entire week and no self-respecting athlete should unless they are injured.
I think we are making the transition to real summer conditions in the parts of the Northern Hemisphere that experience four seasons. I see people getting their hair cut short. That makes you marginally cooler, but athletes need to take more aggressive action. Now is the time to be focused on getting down to the lowest Body Mass Index (BMI) you can sustain.
I've let myself float back up to 29 from a low of 26 or 27 at the end of the summer last year. That's not acceptable. By putting 10 pounds back on, I've reduced my VO2 max (maximal oxygen uptake in the bloodstream) and dramatically slowed my running pace. Getting rid of that weight again will help me to feel significantly better when I'm exercising in this weather.
The best approximation of VO2 max that I can find is the OwnIndex calculated through the Polar Fitness Test by most sophisticated Polar Heart Rate Monitors. I run and cycle with a Polar S-720 heart rate monitor. This is a really sophisticated unit designed to capture heart rate and cycling data (speed, altitude, cadence, power, etc.). It's a great heart rate monitor, but it's more sophisticated than many people need or want.
A good alternative is the Polar M52. This has many of the most important features for fitness and weight loss purposes (OwnIndex calculation through the Fitness Test, heart rate limits, and a coded transmitter), but doesn't have the cycling data capture options. As a result, it costs about half as much.
Heart monitors like the Polar M52 also provide an estimate of the number of calories burned for your exercise session. I find this estimate very useful when combining exercise with dieting.
There are less expensive heart rate monitors in the Polar line, but the M52 has the features that I find most useful for aerobic training purposes. If you're considering buying a less expensive HRM, make sure it has the features that you think you want. A lot of the less expensive ones only measure rudimentary things like instantaneous and average heart rate, time in a single target heart rate zone, and estimated calories burned. That's not enough for me, but it might be right for you.