function updateNLRInputs() {
var system = document.getElementById("unitSystem").value;
var tempLabel = document.getElementById("tempUnitLabel");
var altLabel1 = document.getElementById("altUnitLabel1");
var altLabel2 = document.getElementById("altUnitLabel2");
var rateLabel = document.getElementById("rateUnitLabel");
var rateInput = document.getElementById("lapseRateValue");
var baseTemp = document.getElementById("baseTemperature");
if (system === "metric") {
tempLabel.innerText = "°C";
altLabel1.innerText = "m";
altLabel2.innerText = "m";
rateLabel.innerText = "°C / 1000m";
// Set default Normal Lapse Rate for Metric
rateInput.value = "6.5";
baseTemp.placeholder = "e.g. 15";
} else {
tempLabel.innerText = "°F";
altLabel1.innerText = "ft";
altLabel2.innerText = "ft";
rateLabel.innerText = "°F / 1000ft";
// Set default Normal Lapse Rate for Imperial (approx 3.566°F per 1000ft)
rateInput.value = "3.57";
baseTemp.placeholder = "e.g. 59";
}
}
function calculateLapseRate() {
// Inputs
var t1 = parseFloat(document.getElementById("baseTemperature").value);
var a1 = parseFloat(document.getElementById("baseAltitude").value);
var a2 = parseFloat(document.getElementById("targetAltitude").value);
var rate = parseFloat(document.getElementById("lapseRateValue").value);
var system = document.getElementById("unitSystem").value;
// Validation
if (isNaN(t1) || isNaN(a1) || isNaN(a2) || isNaN(rate)) {
alert("Please enter valid numbers for all fields.");
return;
}
// Logic
// The formula for temperature change based on lapse rate is:
// T_final = T_initial – (LapseRate * (Altitude_final – Altitude_initial) / 1000)
// Note: Lapse rate is positive for cooling with height.
var altitudeDiff = a2 – a1;
var tempChange = (rate * altitudeDiff) / 1000;
var finalTemp = t1 – tempChange;
// Formatting
var unitSymbol = (system === "metric") ? "°C" : "°F";
var altSymbol = (system === "metric") ? "m" : "ft";
// Display Results
var resultDiv = document.getElementById("result");
var finalValueDiv = document.getElementById("finalTempValue");
var detailsDiv = document.getElementById("calculationDetails");
resultDiv.style.display = "block";
finalValueDiv.innerHTML = finalTemp.toFixed(2) + " " + unitSymbol;
var direction = (altitudeDiff > 0) ? "ascent" : "descent";
var tempDirection = (tempChange > 0) ? "decrease" : "increase";
// Handle negative temp change display (absolute value for text description)
var absTempChange = Math.abs(tempChange);
detailsDiv.innerHTML =
"Analysis:" +
"Altitude Change: " + altitudeDiff + " " + altSymbol + " (" + direction + ")" +
"Total Temperature Change: " + (tempChange * -1).toFixed(2) + " " + unitSymbol + "" +
"Using a lapse rate of " + rate + " " + unitSymbol + "/1000" + altSymbol + ".";
}
How to Calculate Normal Lapse Rate
The Normal Lapse Rate refers to the average rate at which temperature decreases with an increase in altitude within the Earth's troposphere. Understanding this calculation is vital for meteorologists, pilots, mountaineers, and environmental scientists to predict temperature changes at different elevations.
What is the Formula?
To calculate the temperature at a specific altitude using the lapse rate, or to determine the lapse rate between two known points, the following fundamental linear equation is used:
T = T₀ - L × (z - z₀)
T = Final Temperature at target altitude
T₀ = Initial Temperature at base altitude
L = Lapse Rate (degree of temperature change per unit of height)
z = Target Altitude
z₀ = Base Altitude
Standard Normal Lapse Rate Values
While the actual Environmental Lapse Rate (ELR) fluctuates based on humidity, wind, and local weather conditions, the accepted global averages for the Normal Lapse Rate are:
Measurement System
Standard Rate
Metric (SI)
6.5 °C per 1,000 meters (0.65 °C per 100 meters)
Imperial (US)
Approx. 3.5 °F per 1,000 feet
Calculation Example
Let's assume you are standing at sea level (0 meters) where the temperature is 20°C. You plan to hike a mountain to an elevation of 2,000 meters. Using the standard normal lapse rate of 6.5°C/1,000m, what will the temperature be at the summit?
Identify Values: T₀ = 20°C, z = 2000m, z₀ = 0m, L = 6.5°C/1000m.