Your 9:00 per mile tempo run turned into a death march at 9:40, and your heart rate was pinned. You aren’t out of shape. You’re running in a hotter engine, and the slowdown is predictable.
In one set of 7 North American marathons, top male finishers ran 1.7%, 2.5%, 3.3%, and 4.5% slower than the course record as the weather went from cool to hot (Ely et al., 2007). Slower runners lost even more. Translation: the heat tax is real, and it hits the mid-pack hardest.
Half Marathon Training in Hot Weather: How Much Slower Will You Run?
WBGT (wet-bulb globe temperature) blends air temperature, humidity, and sun into one heat number. Ely’s quartiles span roughly 5 to 25 C of WBGT. A much larger study of about 1.8 million marathon finishers found a U-shaped curve. Speed peaks at a sweet spot that depends on your ability, then falls as it warms, and dropouts rise (El Helou et al., 2012). Air temperature mattered more than humidity or pressure.
In short: there’s a best temperature for you, and every degree past it costs time.
Another rule of thumb comes from coach Mark Hadley. Add the air temperature (F) to the dew point (F). It’s a coach heuristic, not peer-reviewed, so treat it as a starting guess.
| Temp + dew point (F) | Suggested slowdown | 9:00/mi goal | 10:00/mi goal |
|---|---|---|---|
| 111 to 120 | 0.5% to 1% | about 9:03 to 9:05 | about 10:03 to 10:06 |
| 160 (for example 90F air, 70F dew point) | 4.5% to 6% | about 9:24 to 9:32 | about 10:27 to 10:36 |
Use about half the adjustment for intervals. Then override the number with effort. The watch isn’t lying, but the formula is only a guess.
Why Your Pace Falls Apart: Heat, Dehydration, and Drift
Heat pulls blood two ways. Your skin wants it to cool you. Your muscles want it to run. Sweat then thins your blood volume, so each heartbeat pushes out less.
Think of a car on a hot day with a small coolant leak. The engine works harder, runs hotter, and still delivers less power. Your body is that engine.
In a lab study of 8 trained cyclists riding 2 hours at 33 C, losing 1.1%, 2.3%, 3.4%, and 4.2% of body weight lined up almost perfectly with rising core temperature and heart rate, and falling stroke volume (r = 0.98 to 0.99; Montain and Coyle, 1992). Stroke volume is how much blood your heart pushes per beat. The more you dry out, the harder each beat works and the less it delivers.
Heat lies. Heart rate tells you the truth about your cost.
Hot weather also hits your stomach, because blood heads for your skin instead of your gut. See why your gut shuts down in the heat before you plan gels for race day.
Swap Pace Targets for Effort
On a hot day, a fixed pace gets harder every mile. So change the target. Run your easy days by feel and by a heart rate cap. If your plan says 9:30 and you land at 10:00 at the same heart rate, you did the right workout.
On race day, start 10 to 15 seconds per mile slower than your cool-weather goal. Hold effort steady, not pace. Steady is faster than fast-then-slow. If you need help setting the cool-weather number, read how to set a realistic half marathon goal pace.
A slower summer pace at the same effort is not lost fitness. Say that to yourself out loud if you have to.
Heat Acclimation: A Practical 10 to 14 Day Block
Heat acclimatization is the most effective single tool for performing in the heat. The expert consensus recommends repeated exercise in the heat over 1 to 2 weeks (Racinais et al., 2015). Translation: your body can learn heat, but only if you show up in it several days in a row.
In a two-week heat camp, trained cyclists saw sweat sodium fall from 75 to 52 mmol/L and sweat rate rise about 20% within 5 to 6 days (Stanley et al., 2015). They then flattened out. You sweat sooner and lose less salt. Some gains, like extra blood plasma, also fade fastest once the heat stops.
A simple amateur block looks like this:
| Days | What to do |
|---|---|
| 1 to 3 | 30 minutes easy in the afternoon heat |
| 4 to 8 | 40 to 60 minutes easy, heat 4 to 5 days out of 7 |
| 9 to 14 | Keep 2 to 3 heat runs a week, hard days stay normal |
Keep the runs easy. Heat is the stress. For more on what acclimation does, see heat acclimation, running economy, and VO2max. To fit it into your week, try how to schedule a heat acclimation block. Keep a few heat runs in your taper too. How long to taper for a half marathon shows where they fit.
Run a Home Sweat Test
The ACSM position stand says to prevent more than 2% body-mass loss, and to weigh yourself before and after runs. Sweat rates run from 0.5 to 2.0 liters per hour, and sodium from 10 to 70 mmol/L, across hundreds of athletes (Baker, 2017). Translation: nobody sweats like you, so guessing doesn’t work.
Here’s the test. Weigh yourself in minimal clothing before an hour-long run. Weigh again after, towel dry. Then:
Sweat rate (L/h) = (weight before - weight after + fluid drunk) / hours
One kilogram lost equals about one liter. Day-to-day variation is only 5% to 7%, so one honest test is enough.
Here’s what 2% means for a 70 kg runner, whose limit is 1.4 kg (illustrative math):
| Sweat rate | Time to hit 2% with no drinking |
|---|---|
| 0.5 L/h | about 168 minutes |
| 1.0 L/h | about 84 minutes |
| 1.5 L/h | about 56 minutes |
| 2.0 L/h | about 42 minutes |
A heavy sweater can hit the limit before the finish line of a 2-hour half. A light sweater may never get there.
Drink to Thirst, Not to a Schedule
More is not better. Drinking on a fixed plan can push blood sodium below 135 mmol/L, a condition called hyponatremia. The expert panel’s advice is to let thirst guide you. Early warning signs are nausea, puffiness, dizziness, and weight gain during the run (Hew-Butler et al., 2015).
Use your sweat test to set a ceiling, not a mandate. Start the run well hydrated. Sip at aid stations and take salty fuel if you’re a salty sweater. Cold towels, shade, and ice slurry before the start are worth trying, but the evidence is mostly from longer lab trials, so don’t build your race on them.
Case Study: Dana’s July Reset
Take a runner I’ll call Dana: 41, targeting 1:58 (about 9:00 per mile), 35 miles a week. In July her tempo runs fell to 9:35 and she panicked. She skipped two workouts and wondered if she was losing fitness.
She switched to a heart rate cap and moved key runs to 6 a.m. She added five easy afternoon runs of 45 minutes over 10 days. A home test showed 1.1 L/h of sweat. By week three, the same effort held 9:15, and her heart rate sat lower at the same pace. At the race, 84F and humid, she aimed for 9:20 and finished in 2:03. She hit her heat-adjusted goal with a smooth finish.
How AthleteOS Handles Heat
When hot weeks hit, your AthleteOS training plan adapts and explains the change in the Adaptation Log. You see why a session shifted, so a slow summer run reads as heat, not lost fitness. Sessions can be guided by effort or heart rate rather than a fixed pace. If your goal needs a reset, start with the sub-2 half marathon plan and adjust the target for race-day conditions.