Why does that cold bottle of water feel so amazing after a blazing summer workout?
You’re hot, the water and bottle are cold. Simple, right?
Turns out there are four methods for body heat loss: radiation, conduction, convection and evaporation.
Radiation
Radiation is the emission of electromagnetic heat waves, in this case from our bodies. We usually run hotter than our environment, so we lose heat as part of the ever-present quest for equilibrium. On the flip side, this is also what warms us when we stand in the sun, especially when our skin is cold.
Conduction
Conduction is direct heat transfer from one object to another. An example would be from the cold bottle of water to our hands or face, as above. We can also lose heat to the ground, an ice vest or to still, not moving, water.
Convection
Convection is heat loss via rapid movement of air or water around us. For example, wind can make us cooler because it’s blowing away the warm layer of air immediately surrounding us and replacing it with a new layer, which we heat and which is in turn blown away to start the process again. Moving water, such as a river or a current, has the same effect.
Evaporation
Evaporation is the cooling mechanism most athletes are familiar with. When we sweat, water evaporates off our skin, transferring heat into the air around us. The evaporation of one liter of water removes 580 kilocalories of heat from the body. This is an adult human’s most effective way to lose body heat. The ability to sweat increases during puberty, so sweat rates and the related cooling power will be lower in pre-pubescent athletes than in adults.
How can you use this info in cold weather?
- Block wind to limit heat loss via convection.
- Wear wicking clothing so that sweat isn’t trapped next to the skin, limiting evaporation.
- Cover exposed skin to limit radiation.
- Put layers between you and any sports tools that can conduct heat away from you: paddles, sticks, poles, bats, etc.
- Wear a hat to limit radiation from the head. Don’t use one with a large brim, which will limit radiation from the sun.
How can you use this info in hot weather?
- Try to exercise during the cooler parts of the day. The hotter and more humid the weather, the harder it is to lose heat via radiation, convection and conduction. This means we are dependent on evaporation.
- If you can’t head out during the cooler temps, try starting your exercise session cold. Consider either by pre-cooling with an ice vest or finding a cooler environment to get ready in (think a very air-conditioned room).
- Wind is your friend, up to a point. It will help you cool off via convection, but it can also impede performance if you’re having to run, bike, or ski directly into a headwind. Adjust your pacing goals appropriately.
- Try to exercise during the less humid parts of the day. The more humid it is, the less effective evaporation becomes. So if you have to decide between exercise in the morning with cool but humid temps or in the evening with warm but drier temps, take the evening workout!
- Wear wicking clothes, which transfer heat and water from the skin to the air effectively. Cotton and linen tend to trap the water, keeping it from evaporating.
- When done training, wear a hat with a large brim to limit radiation from the sun – unless wearing the hat and trapping your heat will make you hotter.
For more information on environmental conditions that affect sweat rate and hydration needs, see the blog on Environmental Factors that Affect Sweat Rate.
And here are some things to consider if exercising outside in the fall or winter.
If you have questions, reach out here!
REFERENCE: Chapter 25: Exercise and Thermal Stress. In: McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy and Human Performance. 8th Ed. Baltimore: Wolters Kluwer Health; 2015: 618-621.