The Same Run Feels Completely Different in May Than March
Run the same three-mile route on a 45°F March morning and again on a 75°F May afternoon with 70 percent humidity, and the difference is startling. The pace that felt comfortable in March requires what feels like maximum effort in May. Heart rate climbs higher and faster. Sweat soaks through in minutes. The body that felt capable of more in March hits a wall that wasn’t there before.
This isn’t imagined difficulty or lack of fitness. The physiological demands of exercise in heat and humidity are genuinely, measurably greater than the demands of the same exercise in cool, dry conditions — and understanding why explains both why early summer training feels so hard and what to do about it.
The Body Has Two Jobs at Once
The fundamental challenge of exercising in heat is that the body must simultaneously supply working muscles with oxygenated blood and cool itself by directing blood to the skin surface for heat dissipation — and these two demands compete for the same limited cardiac output.
During exercise in cool conditions, the cardiovascular system has a relatively simple job: deliver blood to working muscles. The heart increases output to meet the demand, and because ambient temperatures are lower than body temperature, heat dissipation happens almost passively through radiation and convection from the skin surface.
In hot conditions, the body adds a second urgent demand: cooling. To dissipate heat, blood must be routed to the skin, where it releases heat to the environment through radiation, convection, and the evaporation of sweat. This diversion of blood flow to the skin comes directly at the expense of blood flow to the muscles. The muscles receive less oxygen per unit of effort, produce more lactate at lower intensities, and fatigue faster.
The heart compensates by beating faster — sometimes dramatically faster. Heart rates at the same pace in hot conditions can run 10 to 20 beats per minute higher than in cool conditions, a phenomenon called cardiovascular drift. This is the physiological reality behind the experience of feeling like you’re working much harder for the same pace: you actually are.
What Humidity Adds to the Problem
Temperature alone makes exercise harder. Humidity makes it dramatically worse, through a mechanism that is physiologically distinct from the temperature effect.
The body’s primary cooling mechanism during exercise is sweat evaporation. When sweat evaporates from the skin surface, it carries heat away from the body with remarkable efficiency — evaporating one liter of sweat removes roughly 580 calories of heat from the body. This system works beautifully in dry conditions, where sweat evaporates almost as quickly as it’s produced.
In humid conditions, the air is already laden with water vapor and cannot accept additional moisture as readily. Sweat evaporation slows dramatically. The sweat that beads and drips off the skin rather than evaporating provides almost no cooling benefit — it has left the body without removing its heat. The body responds by producing more sweat in an attempt to increase evaporative cooling, which accelerates dehydration without proportionally increasing cooling.
The combined effect of impaired evaporation and continued sweat production is what produces the oppressive quality of humid exercise: the body is working frantically to cool itself and failing, core temperature rises faster and higher, and cardiovascular stress escalates. The heat index — the combined heat-humidity metric in weather forecasts — captures this physiologically meaningful interaction. A 90°F day at 30 percent humidity is hard to exercise in. The same 90°F at 70 percent humidity is genuinely dangerous for sustained outdoor exertion.
Why May Is the Hardest Month for Outdoor Exercise
May sits at a specific and challenging point in the seasonal exercise cycle. Temperatures have crossed the threshold where heat significantly affects performance, but most people have not yet developed the physiological adaptations — collectively called heat acclimatization — that make hot-weather exercise manageable.
Heat acclimatization is a real and substantial physiological process. Over 10 to 14 days of daily exercise in the heat, the body develops measurable adaptations: plasma volume expands, increasing the blood volume available to simultaneously supply muscles and cool the skin. Sweating begins earlier in exercise and at a lower core temperature, getting ahead of heat accumulation rather than responding to it. Sweat rate increases, producing more evaporative cooling. Heart rate at a given effort decreases as the cardiovascular system handles the dual demands more efficiently.
A fully acclimatized athlete can sustain efforts in heat that would cause heat illness in an unacclimatized person of equal fitness. But acclimatization requires time — and in early May, most people have essentially no heat acclimatization. Every warm day is a first-day-of-summer physiologically, even if it’s the tenth warm day of the month.
This is why the same fitness that carried you through a comfortable March training block produces miserable, seemingly effortless runs in May. Your fitness hasn’t changed. Your heat acclimatization — which was zero in March and is still near zero in early May — has become the limiting factor.
How to Adjust Training for Early Summer
The evidence-based response to hot-weather exercise isn’t to stop training or to push through as if conditions haven’t changed. It’s to train with adjusted expectations and practices that allow the body to build acclimatization safely.
Run by effort, not pace. The most useful adjustment is also the most psychologically difficult: accepting that the same effort that produced a certain pace in March will produce a slower pace in May, and that this is correct and appropriate rather than a failure. Using a heart rate monitor to train at effort-equivalent zones rather than pace targets keeps training productive while avoiding the mistake of pushing pace in heat — which drives heart rate to unsustainable levels and increases heat illness risk.
Acclimatize deliberately and gradually. The adaptations of heat acclimatization develop in response to heat stress — you have to be in the heat, exercising, for them to occur. Fifteen to 20 minutes of easy outdoor exercise in the heat each day, even below your normal training intensity, triggers acclimatization. Build from there over two weeks. Trying to maintain full training volume and intensity from the first warm days accelerates neither acclimatization nor fitness and significantly increases injury and illness risk.
Shift timing. Morning exercise, before the day’s heat has built and before pavement has absorbed hours of solar radiation, is dramatically more physiologically manageable than afternoon exercise in May. On days forecast to reach 80°F or higher, the difference between a 6 a.m. run and a 5 p.m. run can be 20°F of air temperature and substantial humidity difference. Early morning is consistently the most favorable exercise window from May through September.
Pre-cool and hydrate proactively. Drinking cool water before exercise — not just during — reduces the initial thermal load the body must manage. Some research supports pre-cooling strategies like cold towels applied to the neck and forearms or cold water immersion of the hands before exercise in extreme heat, though these are more relevant to competitive athletes than recreational exercisers. For most people, adequate pre-exercise hydration and avoiding exercise in direct sunlight are the most accessible pre-cooling approaches.
Recognize the warning signs. The progression from normal hot-weather fatigue to heat exhaustion and heat stroke follows a recognizable sequence that’s worth knowing for yourself and for others exercising nearby. Heavy sweating and elevated heart rate are normal. Stopping sweating despite continued effort, confusion, difficulty speaking or walking, and collapse are emergencies. When in doubt, stop, seek shade and cooling, and get help.
The Good News: It Gets Better
The genuine encouragement for anyone struggling through early May workouts is that the difficulty is temporary and the adaptation is real. By late May and early June, with two weeks of consistent heat exposure and exercise behind you, the same workouts that felt crushingly hard in early May will feel manageable. By midsummer, you’ll have built heat fitness that allows you to train effectively in conditions that would have stopped you cold in May.
The body adapts to what it’s asked to do. Hot-weather training is asking it to do something genuinely hard and genuinely new each spring, and the first few weeks of that adaptation are the hardest. Running through them — at adjusted pace, with adjusted expectations, with adequate hydration and timing — is how you come out the other side a better hot-weather athlete than you were before.
The May slump is real. It’s also a phase, not a permanent state. Keep going.
