How to Calculate Lapse Rate
This article will guide you through understanding and calculating the lapse rate, a fundamental concept in atmospheric science and meteorology. The lapse rate refers to the rate at which atmospheric temperature decreases with an increase in altitude. Understanding this phenomenon is crucial for weather forecasting, aviation, and comprehending atmospheric processes.
There are several types of lapse rates, but the most commonly discussed are the environmental lapse rate and the adiabatic lapse rates (dry and saturated).
* **Environmental Lapse Rate (ELR):** This is the actual observed rate of temperature decrease with altitude in the atmosphere at a specific time and location. It is influenced by various factors like solar radiation, cloud cover, and local topography.
* **Dry Adiabatic Lapse Rate (DALR):** This is the rate at which a parcel of unsaturated air cools as it rises and expands due to lower atmospheric pressure. It is a constant value.
* **Saturated (or Moist) Adiabatic Lapse Rate (SALR):** This is the rate at which a parcel of saturated air cools as it rises. It is slower than the DALR because the condensation of water vapor releases latent heat, which warms the air parcel. The SALR is not constant and varies with temperature and pressure.
### How to Calculate the Lapse Rate
The most straightforward way to calculate the lapse rate (specifically the Environmental Lapse Rate) is by measuring the temperature at two different altitudes and then determining the difference.
The formula is:
**Lapse Rate = (Temperature at Lower Altitude – Temperature at Higher Altitude) / (Altitude of Higher Altitude – Altitude of Lower Altitude)**
The result is typically expressed in degrees Celsius per 100 meters (°C/100m) or degrees Celsius per kilometer (°C/km).
Let's create a calculator to help you determine the lapse rate based on your measurements.
function calculateLapseRate() {
var tempLower = parseFloat(document.getElementById("tempLower").value);
var altLower = parseFloat(document.getElementById("altLower").value);
var tempHigher = parseFloat(document.getElementById("tempHigher").value);
var altHigher = parseFloat(document.getElementById("altHigher").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(tempLower) || isNaN(altLower) || isNaN(tempHigher) || isNaN(altHigher)) {
resultDiv.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (altHigher <= altLower) {
resultDiv.innerHTML = "Higher altitude must be greater than lower altitude.";
return;
}
var tempDiff = tempLower – tempHigher;
var altDiff = altHigher – altLower;
var lapseRateCelsiusPerMeter = tempDiff / altDiff;
var lapseRateCelsiusPer100m = lapseRateCelsiusPerMeter * 100;
var lapseRateCelsiusPerKm = lapseRateCelsiusPerMeter * 1000;
resultDiv.innerHTML =
"