Environmental Heat & Humidity Adjustments

In tropical races, heat and humidity are the biggest outside factors on race day. A pace that feels easy at 15°C can be exhausting at 28°C with sticky air. This lesson gives you a simple tool, the Stress Score, to judge how much the conditions should change the group's expectations.

Why humidity matters as much as heat#

Your body cools itself mainly by sweating. Sweat only cools you when it evaporates from the skin. In humid air, evaporation is slow, so sweat drips off without cooling you. Heart rate and effort rise, even at the same pace.

The best measure of humidity for runners is the dew point: the temperature at which the air would become saturated and moisture would condense. The higher the dew point, the more moisture is in the air and the stickier it feels. In Vietnam, early-morning dew points in the low to mid 20s °C are common.

The Stress Score#

Add the air temperature and the dew point, both in degrees Fahrenheit:

Stress Score (SS) = temperature (°F) + dew point (°F)

To convert Celsius: °F = °C × 9/5 + 32.

Then slow the target pace by the percentage for that band:

Within a band, the slowdown rises evenly from the lower to the upper value. The Weather compensator does the maths for you.

Worked example (°F)#

Temperature 85°F, dew point 75°F, target pace 6:00/km (360 s).

A typical Vietnam morning (°C)#

Temperature 27°C, dew point 24°C, 4:00 marathon group at 5:41/km.

• 27°C × 9/5 + 32 = 80.6°F
• 24°C × 9/5 + 32 = 75.2°F
• SS = 80.6 + 75.2 = 155.8, in the 141–160 band, about 4.7% slower
• 341 s × 1.047 ≈ 357 s, so about 5:57/km, a finish of about 4:11

Who decides, and how to brief the group#

The Stress Score helps you understand the conditions. It does not give you permission to change the plan on your own. As the Support Guide says, if the weather requires an adjustment, the pacer coordinator decides and tells all teams, so every group follows the same approach.

When an adjustment is agreed, brief your group at the start corral in plain words:

• What: "It's humid today. We will run about 15 seconds per kilometre slower than the usual 4:00 pace."
• Why: "This keeps the effort the same as a cool day, so you still have energy at 35 km."
• What it means for them: "Expect to finish a few minutes later than 4:00. That is a good result today."
• Drinking: remind runners to drink at every station. In hot conditions the Fuel planner raises fluid from 600 to 800 ml per hour.

Remember that conditions get worse as the sun rises. Check the forecast for the end of your race, not just the start.

Key takeaways#

• Stress Score = temperature (°F) + dew point (°F); convert with °F = °C × 9/5 + 32.
• Up to 100 needs no change; 141–160 means 3.5–5% slower; above 180 is unsafe to pace for time.
• 85°F + 75°F = 160 → 5% slower → 6:00/km becomes 6:18/km.
• The coordinator decides on adjustments; the pacer explains them clearly to the group.