Clearly, the main concerns are staying hydrated and keeping cool throughout warm weather workouts. As you will see, staying hydrated and keeping cool are two sides of the same coin. When you are at rest, such as sitting in a sailboat, heat is removed from your body in four ways. Some body heat is simply radiated from your skin into the environment. Some body heat is convected away from your skin by air currents, such as the wind that moves the sailboat. Some body heat is conducted through your skin to cooler surfaces, such as putting your hand on the metal mast or placing your feet in the water. The remainder of the excess body heat is evaporated from the skin as perspiration is changed to water vapor.

However, when you exercise, such as running, cycling or playing tennis, almost all of your body heat is lost through the process of evaporation. Of course, for evaporation to occur, there must be perspiration, and sufficient perspiration is dependent upon adequate hydration. Simply stated, you must consume plenty of water (or other fluids such as fruit juice) to safely perform exercise activities on hot or humid summer days. De-hydration is extremely serious because it prevents body cooling leading to potentially catastrophic heat illness.

To attain, maintain and replenish adequate levels of hydration, you should consistently consume 8 to 12 glasses of water on a daily basis. More specifically, you should drink one or two glasses of water prior to each summer exercise session. If your workout lasts longer than 30 minutes, I recommend drinking fluids during the activity at 15 minute intervals. For example, if you are bicycling for one hour you could drink 2/3 bottle of water at 15, 30 and 45 minutes. This would require carrying two water bottles on your cycle.

As soon as you complete your exercise session replenish your water loss, as you will undoubtedly lose more water than you drink during your workout. A good rule of thumb for standard summer training is to drink two glasses of water within several minutes after completing your workout.

In addition to proper hydration, you must make sure that perspiration can evaporate freely from your skin. Wet skin has minimal cooling effect until it evaporates, which uses body heat to change water into water vapor. The first step for enhancing the evaporation process is to wear lightweight and loose-fitting clothing. The second step is to wear porous and breathable exercise apparel to facilitate air transfer. Under no circumstances should you wear plastic or rubberized suits, as these impervious materials prohibit evaporation, impede performance, and greatly increase the risk of serious heat illness. Contrary to claims, plastic suits do not produce fat loss but they can cause excessive water loss and heat buildup.

Another approach to hot weather exercise is training in the early morning or late evening when the temperatures are lower and the sun's rays are less direct. You may also try walking, running or mountain biking in shaded areas such as wooded trails. On exceptionally hot or humid days, I recommend substituting a swimming workout, or exercising on a treadmill, cycle or stepper in an air-conditioned fitness facility.

If you are a tennis player, take frequent water breaks between games and use a wet towel to cool-off your face, neck, arms and hands. You should also wear a hat to shield your face/neck if you are in direct sunlight for long periods of time when playing tennis, golf, or softball.

By making hydration a high priority, and by paying attention to proper attire, exercise times, and activity areas, you should enjoy safe and satisfying summertime workouts. However, at the first indication of heating up or drying out stop exercising immediately, get into the shade and get fluids into your body. If symptoms persist, seek medical attention without delay, as heat illness is a serious condition that can have fatal consequences.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA. He is editorial advisor for many publications, including Shape Magazine, Prevention Magazine, Club Industry Magazine, and Men's Health Magazine, and author of several fitness books including the new releases, Building Strength and Stamina and Strength Training Past 50.