Understanding the Environmental Lapse Rate
The environmental lapse rate (ELR) is a fundamental concept in meteorology and atmospheric science. It describes the rate at which atmospheric temperature decreases with an increase in altitude. In simpler terms, it's how much colder it gets as you go higher up into the atmosphere.
The ELR is not constant; it varies depending on geographical location, time of day, season, and weather conditions. However, a commonly used average for the ELR near the Earth's surface is approximately 6.5°C per kilometer (or about 3.5°F per 1,000 feet). This average is derived from the observed atmospheric temperature profiles. The dry adiabatic lapse rate (the rate at which unsaturated air cools as it rises) and the moist adiabatic lapse rate (the rate at which saturated air cools as it rises, releasing latent heat) are related but distinct concepts that influence the ELR.
Understanding the lapse rate is crucial for various applications, including:
- Weather Forecasting: It helps predict cloud formation, precipitation, and temperature at different altitudes.
- Aviation: Pilots need to consider temperature changes with altitude for flight planning and aircraft performance.
- Climate Studies: It's a key factor in understanding atmospheric stability and energy transfer within the atmosphere.
- Topography and Agriculture: It explains why higher elevations are colder, influencing vegetation types and agricultural suitability.
The formula used in this calculator is derived from the definition of a rate: the change in temperature divided by the change in altitude. Mathematically, it is expressed as:
Lapse Rate = (Temperature at Altitude 1 – Temperature at Altitude 2) / (Altitude 2 – Altitude 1)
The result will be in degrees Celsius per meter (°C/m).
Example Calculation:
Let's say at an altitude of 500 meters, the temperature is 15°C. At an altitude of 2000 meters, the temperature has dropped to 7.5°C.
- Temperature at Altitude 1: 15°C
- Altitude 1: 500 meters
- Temperature at Altitude 2: 7.5°C
- Altitude 2: 2000 meters
Using the formula:
Lapse Rate = (15°C – 7.5°C) / (2000 m – 500 m)
Lapse Rate = 7.5°C / 1500 m
Lapse Rate = 0.005 °C/m
This is equivalent to 5°C per kilometer (0.005 °C/m * 1000 m/km).
function calculateLapseRate() {
var temp1 = parseFloat(document.getElementById("tempAtAltitude1").value);
var alt1 = parseFloat(document.getElementById("altitude1").value);
var temp2 = parseFloat(document.getElementById("tempAtAltitude2").value);
var alt2 = parseFloat(document.getElementById("altitude2").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(temp1) || isNaN(alt1) || isNaN(temp2) || isNaN(alt2)) {
resultDiv.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (alt1 === alt2) {
resultDiv.innerHTML = "Altitudes cannot be the same.";
return;
}
var deltaTemp = temp1 – temp2;
var deltaAlt = alt2 – alt1;
var lapseRate = deltaTemp / deltaAlt;
// Check for NaN in case of division by zero (already handled by alt1===alt2 check but good practice)
if (isNaN(lapseRate)) {
resultDiv.innerHTML = "Calculation resulted in an invalid number. Please check your inputs.";
return;
}
var lapseRateCPerKm = lapseRate * 1000; // Convert °C/m to °C/km
resultDiv.innerHTML =
"
" +
"Change in Temperature: " + deltaTemp.toFixed(2) + " °C" +
"Change in Altitude: " + deltaAlt.toFixed(0) + " meters" +
"Environmental Lapse Rate:
" +
"(Approximately " + lapseRateCPerKm.toFixed(2) + " °C per kilometer)";
}
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