Why climate enters the training equation
The human body operates best within a narrow range of internal temperature. When the environment pushes that temperature up or down, the organism spends energy defending itself, and that shows up in pace, perceived exertion and recovery. Ignoring the season of the year is planning in a vacuum. The same 10 km run done on a mild 18-degree day becomes a different session at noon on a 34-degree summer day with high humidity.
The good news is that there is plenty of consolidated evidence on how climate affects performance and what to do about it. Below, the points with the strongest scientific support.
Heat: the 7 to 14 day acclimatization
Training in the heat without preparation is the highest-risk scenario. According to the position statements of the American College of Sports Medicine (ACSM) and the National Athletic Trainers Association (NATA), heat acclimatization takes 7 to 14 days of gradual, progressive exposure. In that period the body increases plasma volume and sweat rate, reduces heart rate and internal temperature for the same effort, and starts losing less salt in sweat.
Sweat rates and sodium replacement
During endurance exercise, sweat rates typically hover around 1.0 to 1.5 liters per hour, but vary a lot between people, and elite athletes can exceed 2 liters per hour. Body weight, clothing, equipment and acclimatization level change that number. Along with the water goes sodium, and here a useful detail appears: those not yet acclimatized sweat saltier.
- Acclimatized athlete: lower sweat sodium concentration, on the order of 30 mmol per liter (about 690 mg/L).
- Non-acclimatized athlete: saltier sweat, on the order of 50 mmol per liter (about 1150 mg/L) or more.
The ACSM position stand advises that fluids with sodium make sense in efforts exceeding 2 hours or in any situation of heavy salt loss. It is not about a specific diet, it is about replacing what leaves. The validated practical rule is simple: replace fluid at a rate close to what you sweat, without overdoing it. ACSM Position Stand: Exercise and Fluid Replacement (Sawka et al., 2007)
The risk of hyperthermia and the performance drop
Heat is not just discomfort, it is real risk. When internal temperature rises too much, the body diverts blood to the skin to dissipate heat, which reduces venous return and maximal cardiac output. The measurable result is a drop in maximal VO2 and performance. Studies show a reduction in maximal VO2 on the order of 7 to 8 percent in the heat compared with a temperate environment.
Humidity makes everything worse, because sweat only cools when it evaporates. Above about 60 percent humidity, evaporation becomes limited and the body loses its main cooling avenue. That is why a hot dry day and a hot muggy day are different challenges, even with the same number on the thermometer. Losing more than 2 percent of body weight in water already compromises performance, so stepping on the scale before and after long sessions in the heat is a cheap and revealing measure.
Cold: warm up well and keep hydrating
Cold calls for the opposite logic, but demands the same attention. Cold muscle is muscle more susceptible to injury, so the warm-up stops being optional and becomes a safety step. Raise body temperature progressively before pulling the pace.
Hydration remains important, even with less sensation of thirst. In the cold you sweat less, but still lose fluid, including through breathing, since cold air is usually dry. It is worth arriving hydrated and carrying water on longer sessions.
One warning about the airways: cold air can trigger bronchoconstriction and attacks in those with asthma. The American Lung Association advises that people with asthma use the rescue inhaler 20 to 30 minutes before exercising in the cold. If you have a respiratory history, talk to your doctor before training in low temperatures.
Layered clothing and energy expenditure in the cold
The solution for the cold is wearing layers, not a single thick piece. The classic system has three functions:
- Base layer: fabric that wicks sweat away from the skin and keeps you dry.
- Middle layer: thermal insulation, which retains the heat generated.
- Outer layer: protection against wind and rain, light and windproof.
The advantage of layers is being able to open or remove pieces as the body heats up during training. Regarding calories, there is a popular idea that cold makes you burn much more energy. Shivering does raise expenditure, but there is an honest caveat: during exercise, internal temperature rises and shivering is suppressed, so much of that supposed caloric bonus from cold disappears precisely when you are training. In other words, training in the cold is not a magic shortcut to burn more.
Adjusting the plan by season
Translating everything into concrete decisions throughout the year:
- Start of summer: set aside 1 to 2 weeks of gradual acclimatization before demanding full pace. Expect naturally lower performance in the first hot sessions and do not confuse that with loss of fitness.
- Very hot or muggy days: move the session to early morning or late afternoon, reduce volume or intensity and prioritize fluid and sodium replacement on long efforts.
- Start of winter: lengthen the warm-up, adjust the hydration expectation downward but do not zero it, and build the wardrobe in layers.
- Extreme cold or strong wind: consider moving training indoors, especially in high intensity sessions or for those with asthma.
Climate is a variable, not an obstacle. Whoever reads the thermometer and adjusts training and replacement reaps consistency all year, instead of suffering at the extremes.
