The Physiology of Summer Running: Why Heat is the "Poor Man’s Altitude"

By Coach Johnny Crain | 4x Olympic Trials Qualifier & Professional Distance Coach

If you’ve ever stepped outside in July for a routine easy run and found yourself gasping for air at a pace that is usually a jog, you are not alone. And more importantly, you are not losing your fitness.

As a coach to NCAA All-Americans at the University of Oklahoma, Olympic Trials qualifiers, and World Team athletes, I spend every spring having the exact same conversation with my runners: You cannot out-tough the heat; you have to out-smart it.

To train effectively in the summer, you have to stop looking at your GPS watch and start understanding the biological war happening inside your body. Here is the exact science behind why summer running feels so hard, and how we use it to actually get faster.

1. The Physics of Sweat: Why Relative Humidity is a Trap

The biggest mistake runners make is looking at the "Relative Humidity" percentage on their weather app. Relative humidity is just a ratio of how much moisture the air holds compared to its temperature. It doesn’t tell you how your body will actually react.

For that, we look at the Dew Point.

Dew point is the absolute measurement of moisture in the air. The human body cools itself through the evaporation of sweat, not the sweating itself. When the dew point climbs above 65°F (18°C), the air is so saturated with moisture that your sweat has nowhere to go. It drips off you instead of evaporating.

When sweat doesn't evaporate, your body cannot dump the heat it is generating. This sets off a physiological chain reaction.

2. Thermoregulation and the 102°F Threshold

When you run, your working muscles generate a massive amount of heat. In perfect, 50-degree weather, that heat easily radiates off your body. In the summer, that heat gets trapped.

As your core temperature begins to climb toward 102°F (39°C), your central nervous system enters a protective state. To prevent your organs from overheating, your brain commands your cardiovascular system to divert oxygen-rich blood away from your working leg muscles and push it out toward the capillaries in your skin to try and cool you down.

The Result: Cardiovascular Drift

Because you now have less blood delivering oxygen to your legs, your heart has to beat significantly faster to maintain the same pace. This is called cardiovascular drift. A 7:30/mile pace that normally requires a heart rate of 140 BPM might suddenly require 165 BPM.

If you try to force your normal paces during this physiological shift, you push your body past its lactate threshold. What was supposed to be an aerobic base run accidentally becomes a grueling, anaerobic threshold workout. You aren't building fitness—you are digging a hole of overtraining.

To prevent this, we use the Combined Score Method (Air Temperature + Dew Point) to gauge true environmental stress.

(This is exactly why I built the Heat Pacing Calculator—to help you find the equivalent physiological effort and avoid this trap.)

Combined Stress Score (Air Temp + Dew Point)
Dew Point ↓ | Temp →	50°	55°	60°	65°	70°	75°	80°	85°	90°	95°	100°
50°	100	105	110	115	120	125	130	135	140	145	150
55°		110	115	120	125	130	135	140	145	150	155
60°			120	125	130	135	140	145	150	155	160
65°				130	135	140	145	150	155	160	165
70°					140	145	150	155	160	165	170
75°						150	155	160	165	170	175
80°							160	165	170	175	180
85°						How to Calculate Your Combined Stress Score To find your true environmental stress, we use the Combined Score Method. This is simply your local Air Temperature plus your local Dew Point. Real-World Example: Let's say you are running on a humid morning in North Carolina. Air Temperature: 80°F Dew Point: 70°F Combined Score: 150 Once you have your Combined Score, look at the matrix below to see what "Stress Zone" you are in, and then use this exact scale to determine your required percentage slowdown: 100 or less: No pace adjustment needed 101 to 110: 0% to 0.5% pace adjustment 111 to 120: 0.5% to 1.0% pace adjustment 121 to 130: 1.0% to 2.0% pace adjustment 131 to 140: 2.0% to 3.0% pace adjustment 141 to 150: 3.0% to 4.5% pace adjustment 151 to 160: 4.5% to 6.0% pace adjustment 161 to 170: 6.0% to 8.0% pace adjustment 171 to 180: 8.0% to 10.0% pace adjustment Above 180: Hard running is not recommended. Move your workout to a treadmill or focus solely on easy effort. Pace Adjustments by Percentage Slowdown Penalty % 4:00 4:30 5:00 5:30 6:00 6:30 7:00 7:30 8:00 8:30 9:00 9:30 10:00 10:30 1% 4:02 4:33 5:03 5:33 6:04 6:34 7:04 7:34 8:05 8:35 9:05 9:36 10:06 10:36 2% 4:05 4:35 5:06 5:37 6:07 6:38 7:08 7:39 8:10 8:40 9:11 9:41 10:12 10:43 3% 4:07 4:38 5:09 5:40 6:11 6:42 7:13 7:44 8:14 8:45 9:16 9:47 10:18 10:49 4% 4:10 4:41 5:12 5:43 6:14 6:46 7:17 7:48 8:19 8:50 9:22 9:53 10:24 10:55 5% 4:12 4:44 5:15 5:47 6:18 6:50 7:21 7:52 8:24 8:56 9:27 9:58 10:30 11:02 6% 4:14 4:46 5:18 5:50 6:22 6:53 7:25 7:57 8:29 9:01 9:32 10:04 10:36 11:08 7% 4:17 4:49 5:21 5:53 6:25 6:57 7:29 8:01 8:34 9:06 9:38 10:10 10:42 11:14 8% 4:19 4:52 5:24 5:56 6:29 7:01 7:34 8:06 8:38 9:11 9:43 10:16 10:48 11:20 9% 4:22 4:54 5:27 6:00 6:32 7:05 7:38 8:10 8:43 9:16 9:49 10:21 10:54 11:27 10% 4:24 4:57 5:30 6:03 6:36 7:09 7:42 8:15 8:48 9:21 9:54 10:27 11:00 11:33 4. The Elite Secret: Plasma Volume Expansion So, if summer running is so destructive, why do elite athletes routinely train in the heat? Because when managed correctly, heat acts like a "poor man's altitude." When you subject your body to consistent, controlled heat stress (by slowing your pace to keep your heart rate in check), it triggers a survival adaptation called Plasma Volume Expansion. The Clinical Takeaway: According to studies published in the Journal of Applied Physiology, athletes subjected to 10–14 days of heat acclimation experience up to a 6.5% increase in blood plasma volume. This expands cardiac output, resulting in a significantly lower heart rate at identical workloads once the weather cools. Within just a few days of heat exposure, your body literally creates more fluid volume to ensure it has enough blood to send to your skin for cooling and to your muscles for running. When the weather finally breaks in October, you still have this expanded blood volume. Suddenly, your heart doesn't have to work nearly as hard to deliver oxygen, and you experience a massive drop in your race times. 5. The 14-Day Heat Acclimation Protocol To get the benefits of plasma volume expansion without ruining your season, you must acclimate strategically. Whether I am prepping an athlete for a World Championship or a local runner for a fall marathon PB, we use a strict protocol: Days 1–5 (The Shock Phase): Run 100% by perceived effort or heart rate. Drop your ego. Use the Heat Pacing Calculatorto find your new target splits. Keep runs short (under 60 minutes). Days 6–10 (The Adaptation Phase): Your sweat rate will naturally increase, and your sweat will become less salty as your body learns to conserve electrolytes. You can slightly increase duration, but maintain the adjusted, slower paces. Days 11–14 (Full Acclimation): Plasma volume has expanded. You will still run slower than you do in the winter, but your heart rate will stabilize, and the "heavy" feeling in your chest will subside. Train With the Science The heat is only your enemy if you try to fight it. If you respect the dew point, slow down, and allow your body to adapt, the summer becomes the ultimate training tool for a massive fall Personal Best. Ready to stop guessing and start training like a pro? Step 1: Bookmark my Interactive Heat Pacing Calculator and use it before every summer run. Step 2: If you want a periodized training schedule that safely builds your aerobic engine through the toughest months of the year, Apply for 1-on-1 Coaching Here References & Further Reading Sawka, M. N., et al. (2011). Heat Acclimation to Improve Athletic Performance. Sports Medicine. This landmark review outlines the timeline of plasma volume expansion and its direct effect on VO2 max preservation in hot environments. Gleeson, M. (1998). Temperature Regulation During Exercise. International Journal of Sports Medicine. Details the 39°C (102°F) core temperature threshold and the onset of central nervous system fatigue. Coach Mark Hadley. Original creator of the combined Air Temperature and Dew Point pace adjustment percentage matrix. Johnny Crain Previous Previous  Strength Training for Runners: Build Speed and Prevent Injury at Home Next Next The Science of Speed: Why The Best Running Plan Demands App Integration RunFitCoach Find Out More Features (Coming soon) How It Works Personal Coaching Pricing (Coming soon) Store Blog About FAQ (Coming Soon) Privacy Policy Terms of Service Contact Us Contact@RunFitCoach.com