Dew Point vs. Humidity: Why It Matters for Runners
For runners, checking the weather app goes beyond looking at the temperature. While relative humidity gives you a percentage of saturation, dew point is the absolute measure of how much moisture is in the air. It is the gold standard for determining how effectively your body can cool itself through sweat evaporation.
When you run, your muscles generate heat. Your body sweats to release that heat, relying on evaporation to cool the skin. If the dew point is high, the air is already saturated with moisture, preventing sweat from evaporating. This causes your core temperature to rise faster, heart rate to increase (cardiac drift), and perceived effort to skyrocket.
Interpreting Your Dew Point Score
This calculator uses the dew point to determine the "misery index" for your run. Here is a breakdown of the ranges typically used by exercise physiologists and coaches:
Dew Point (°F)
Sensation
Performance Impact
< 55°F
Comfortable
Little to no impact. Ideal for PRs.
55°F – 60°F
Moderate
Noticeable humidity. Hard efforts feel slightly tougher.
60°F – 65°F
Sticky
Performance degrades. Expect pace to slow by 1-3%.
65°F – 70°F
Uncomfortable
Heavy sweating. Pace adjustment of 3-5% recommended.
70°F – 75°F
Oppressive
Very difficult. Modify workouts significantly.
> 75°F
Dangerous
Risk of heat illness. Run easy or indoors.
How to Adjust Your Training
Using the output from the calculator above, adjust your expectations for your workout:
Hydration: In high dew point conditions (60°F+), pre-hydration is critical. Electrolyte intake should be increased as sweat rates rise without the benefit of cooling.
Pace vs. Effort: Ignore your GPS watch pace. Run by Heart Rate (HR) or Rate of Perceived Exertion (RPE). If you try to maintain your standard 8:00/mile pace in a 72°F dew point, your heart rate might spike into zone 4/5, turning an aerobic run into a threshold workout.
Acclimatization: It takes the human body approximately 10-14 days to acclimatize to heat and humidity. During this period, be extra conservative with pace adjustments.
Frequently Asked Questions
Why is 100% humidity at 40°F comfortable, but 50% humidity at 90°F difficult?
This is why Dew Point is superior. At 40°F with 100% humidity, the dew point is 40°F—very dry in absolute terms. At 90°F with 50% humidity, the dew point is roughly 69°F—very humid and oppressive.
function calculateRunningConditions() {
// 1. Get Inputs
var tempVal = document.getElementById("temperature").value;
var humidityVal = document.getElementById("humidity").value;
var unit = document.getElementById("tempUnit").value;
var resultsDiv = document.getElementById("results");
// 2. Validate Inputs
if (tempVal === "" || humidityVal === "") {
alert("Please enter both temperature and relative humidity.");
resultsDiv.style.display = "none";
return;
}
var temp = parseFloat(tempVal);
var humidity = parseFloat(humidityVal);
if (humidity 100) {
alert("Humidity must be between 0 and 100.");
return;
}
// 3. Convert to Celsius for Calculation (Magnus Formula)
var tempC = temp;
if (unit === "F") {
tempC = (temp – 32) * (5/9);
}
// 4. Calculate Dew Point (Magnus Formula)
// constants
var a = 17.27;
var b = 237.7;
// calculation
var alpha = ((a * tempC) / (b + tempC)) + Math.log(humidity / 100.0);
var dewPointC = (b * alpha) / (a – alpha);
// Convert Dew Point back to F for standardization in running metrics
var dewPointF = (dewPointC * 9/5) + 32;
// 5. Determine Logic/Rating based on Dew Point F
var rating = "";
var desc = "";
var adjustment = "";
var detail = "";
var boxClass = "result-box"; // Default
var colorClass = "";
if (dewPointF = 55 && dewPointF = 60 && dewPointF = 65 && dewPointF = 70 && dewPointF < 75) {
rating = "Oppressive";
desc = "Very dangerous for hard efforts.";
adjustment = "Major (7-10%+)";
detail = "Add 60+ seconds per mile. Consider shortening the run.";
colorClass = "condition-red";
} else {
rating = "Misery / Dangerous";
desc = "Extremely inefficient cooling. Heat illness risk high.";
adjustment = "Survival Mode";
detail = "Run very easy or move indoors. Walk breaks recommended.";
colorClass = "condition-red";
}
// 6. Update UI
resultsDiv.style.display = "block";
// Format Dew Point Display
var displayDp = "";
if (unit === "F") {
displayDp = dewPointF.toFixed(1) + " °F";
} else {
displayDp = dewPointC.toFixed(1) + " °C";
}
document.getElementById("dewPointDisplay").innerText = displayDp;
document.getElementById("conditionRating").innerText = rating;
document.getElementById("conditionDescription").innerText = desc;
document.getElementById("paceAdjustment").innerText = adjustment;
document.getElementById("adjustmentDetail").innerText = detail;
// Apply dynamic styling
var dpBox = document.getElementById("dp-box");
var feelBox = document.getElementById("feel-box");
// Reset classes
dpBox.className = "result-box " + colorClass;
feelBox.className = "result-box " + colorClass;
}