The lapse rate is the rate at which an atmospheric variable, most commonly temperature, decreases with an increase in altitude. This measurement is crucial for meteorologists to understand atmospheric stability, cloud formation, and potential for severe weather.
The Lapse Rate Formula
Γ = (T1 – T2) / (Alt2 – Alt1)
Where:
T1: Temperature at the lower point.
T2: Temperature at the higher point.
Alt2: Height of the higher point.
Alt1: Height of the lower point.
Types of Lapse Rates
The Environmental Lapse Rate (ELR) is the actual rate of temperature decrease measured in the stationary atmosphere. It is compared against the theoretical Dry Adiabatic Lapse Rate (DALR), which is approximately 9.8°C/km, to determine stability:
Unstable: When ELR > DALR (temperature drops very quickly with height).
Stable: When ELR < DALR (temperature drops slowly or increases).
Neutral: When ELR equals the adiabatic lapse rate.
Practical Example
Imagine you are at the base of a mountain at 500 meters where the temperature is 20°C. You hike to a peak at 2,500 meters where the temperature is 7°C.
Calculation: (20 – 7) / (2500 – 500) = 13 / 2000 = 0.0065 °C per meter. To get the standard rate per 1,000 meters: 0.0065 * 1000 = 6.5°C per 1,000m.
function updateUnits() {
var unit = document.getElementById('unitSystem').value;
var labelT1 = document.getElementById('labelTemp1');
var labelT2 = document.getElementById('labelTemp2');
var labelA1 = document.getElementById('labelAlt1');
var labelA2 = document.getElementById('labelAlt2');
if (unit === 'metric') {
labelT1.innerText = 'Ground Temperature (°C)';
labelT2.innerText = 'Upper Air Temperature (°C)';
labelA1.innerText = 'Ground Altitude (m)';
labelA2.innerText = 'Upper Air Altitude (m)';
} else {
labelT1.innerText = 'Ground Temperature (°F)';
labelT2.innerText = 'Upper Air Temperature (°F)';
labelA1.innerText = 'Ground Altitude (ft)';
labelA2.innerText = 'Upper Air Altitude (ft)';
}
}
function calculateLapseRate() {
var t1 = parseFloat(document.getElementById('temp1').value);
var t2 = parseFloat(document.getElementById('temp2').value);
var a1 = parseFloat(document.getElementById('alt1').value);
var a2 = parseFloat(document.getElementById('alt2').value);
var unit = document.getElementById('unitSystem').value;
var resultBox = document.getElementById('lapseResultBox');
var valueDisplay = document.getElementById('lapseValue');
var descDisplay = document.getElementById('lapseDescription');
if (isNaN(t1) || isNaN(t2) || isNaN(a1) || isNaN(a2)) {
alert('Please enter valid numeric values for all fields.');
return;
}
if (a2 9.8) {
stabilityInfo = "Atmospheric Condition: Absolutely Unstable (Greater than Dry Adiabatic Rate).";
} else if (resultRate > 6.0) {
stabilityInfo = "Atmospheric Condition: Conditionally Unstable (Between Saturated and Dry Rates).";
} else if (resultRate > 0) {
stabilityInfo = "Atmospheric Condition: Stable.";
} else {
stabilityInfo = "Atmospheric Condition: Temperature Inversion (Highly Stable).";
}
} else {
// Rate per 1000 feet
resultRate = ratePerUnit * 1000;
unitString = "°F per 1,000ft";
if (resultRate > 5.38) {
stabilityInfo = "Atmospheric Condition: Absolutely Unstable.";
} else if (resultRate > 3.3) {
stabilityInfo = "Atmospheric Condition: Conditionally Unstable.";
} else if (resultRate > 0) {
stabilityInfo = "Atmospheric Condition: Stable.";
} else {
stabilityInfo = "Atmospheric Condition: Temperature Inversion (Highly Stable).";
}
}
valueDisplay.innerHTML = resultRate.toFixed(2) + " " + unitString;
descDisplay.innerHTML = stabilityInfo;
resultBox.style.display = 'block';
}