The Free Convection Level (FCL), also known as the Level of Free Convection (LFC), is a crucial concept in meteorology, particularly in understanding atmospheric instability and the potential for convective thunderstorm development. It represents the altitude at which a rising parcel of air, if lifted adiabatically from the surface or a specific level, becomes warmer than its surrounding environment. Above this level, the air parcel will continue to rise on its own buoyancy, driving convection.
Understanding the FCL is vital for weather forecasters to predict the likelihood, intensity, and potential height of thunderstorms. A lower FCL often indicates a greater potential for strong convection and severe weather. It is determined by analyzing atmospheric soundings (vertical profiles of temperature and humidity) and comparing the temperature of a lifted parcel with the ambient temperature at various altitudes.
How to Calculate the Free Convection Level
The calculation of the Free Convection Level involves tracing the thermodynamic path of a hypothetical air parcel as it rises and cools adiabatically, and comparing its temperature to the environmental temperature at each level. Here's a simplified approach for manual calculation, which this calculator automates:
Identify the Surface (or Lifting) Level: This is typically the ground level, but can also be a specific altitude where air is forced to rise (e.g., orographic lift).
Determine Surface Temperature and Dew Point: Obtain these values for the starting level.
Calculate the Parcel's Temperature and Dew Point at Altitude:
Dry Adiabatic Lapse Rate (DALR): For unsaturated air, the temperature decreases at approximately 9.8°C per kilometer.
Moist Adiabatic Lapse Rate (MALR): For saturated air, the temperature decreases at a slower rate (varying from about 4°C/km in very cold conditions to 7°C/km in warm, moist conditions) because latent heat is released during condensation.
Dew Point Lapse Rate: The dew point also decreases with altitude, but at a slower rate than temperature, as water vapor is removed by condensation.
Determine Saturation: As the parcel rises and cools, its dew point will approach the ambient temperature. The level at which the parcel's dew point equals the ambient temperature is the Lifting Condensation Level (LCL). Above the LCL, the air parcel is saturated and cools at the MALR.
Find the Free Convection Level (FCL): Continue to raise the parcel (first dry adiabatically to the LCL, then moist adiabatically above it) and compare its temperature to the ambient environmental temperature at each successive level. The FCL is the altitude where the parcel's temperature becomes equal to the environmental temperature. Above this point, the parcel will be warmer than the environment and will continue to rise freely due to positive buoyancy.
This calculator requires you to input the surface temperature, surface dew point, and several environmental temperatures at increasing altitudes. The calculator will then compute the FCL.
Free Convection Level Calculator Inputs
var DALR = 9.8; // Dry Adiabatic Lapse Rate in °C/km
var MALR_avg = 6.5; // Average Moist Adiabatic Lapse Rate in °C/km (simplified for this example)
function calculateFCL() {
var surfaceTemp = parseFloat(document.getElementById("surfaceTemp").value);
var surfaceDewPoint = parseFloat(document.getElementById("surfaceDewPoint").value);
var envTemps = [
parseFloat(document.getElementById("envTemp1").value),
parseFloat(document.getElementById("envTemp2").value),
parseFloat(document.getElementById("envTemp3").value),
parseFloat(document.getElementById("envTemp4").value),
parseFloat(document.getElementById("envTemp5").value)
];
var altitudes = [1, 2, 3, 4, 5]; // km
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (isNaN(surfaceTemp) || isNaN(surfaceDewPoint) || envTemps.some(isNaN)) {
resultDiv.innerHTML = "Please enter valid numerical values for all fields.";
return;
}
// 1. Calculate Lifting Condensation Level (LCL) – Simplified approximation
// This is a complex calculation in itself. For simplicity, we'll use a common approximation.
// A more precise calculation involves iterative steps and saturation vapor pressure formulas.
// For this simplified calculator, we will assume the LCL is roughly between 500m and 1500m
// and proceed with calculating parcel temperature from surface to that level dry-adiabatically.
// A more accurate LCL calculation involves iteratively finding the level where
// parcel temperature equals dew point temperature when cooled dry-adiabatically.
var parcelTempAtLCL = surfaceTemp – (0.0098 * 1000); // Assuming LCL is around 1km for simplicity in this approximation
var parcelDewPointAtLCL = surfaceDewPoint – (0.002 * 1000); // Dew point lapse rate is roughly 2°C/km
// Find LCL using a more iterative approach for better accuracy
var LCL_altitude = 0;
var temp_dalr = surfaceTemp;
var dp_dalr = surfaceDewPoint;
var step = 100; // meters
var foundLCL = false;
for (var alt_m = 0; alt_m = temp_dalr) { // Simplified saturation check
LCL_altitude = alt_m;
parcelTempAtLCL = temp_dalr; // The temperature of the parcel at LCL
foundLCL = true;
break;
}
}
if (!foundLCL) {
resultDiv.innerHTML = "Could not determine LCL within the search altitude. Please check input values.";
return;
}
// 2. Calculate Free Convection Level (FCL)
var fclAltitude = -1; // Initialize FCL altitude
var parcelTemp = parcelTempAtLCL; // Start with parcel temp at LCL
var currentAltitude = LCL_altitude; // Start at LCL altitude
var envIndex = 0;
if (currentAltitude > altitudes[0] * 1000) {
// Find the correct starting environmental temperature index if LCL is above 1km
while (envIndex altitudes[envIndex] * 1000) {
envIndex++;
}
if (envIndex > 0) envIndex–; // Use the temperature from the previous known level
}
while (envIndex = nextAltitudeTarget) {
// If the parcel is still warmer than environment, continue
if (parcelTemp > currentEnvTemp) {
envIndex++; // Move to the next environmental level
continue;
} else {
// Parcel is not warmer, FCL is not at or above this level
break;
}
}
// Interpolate environmental temperature if our target altitude is between known points
var interpolatedEnvTemp = currentEnvTemp;
if (envIndex > 0 && currentAltitude altitudes[envIndex-1] * 1000) {
var lowerAlt = altitudes[envIndex-1] * 1000;
var upperAlt = altitudes[envIndex] * 1000;
var lowerTemp = envTemps[envIndex-1];
var upperTemp = envTemps[envIndex];
interpolatedEnvTemp = lowerTemp + (upperTemp – lowerTemp) * ((currentAltitude – lowerAlt) / (upperAlt – lowerAlt));
} else if (envIndex == 0 && currentAltitude < altitudes[0] * 1000) {
// Interpolate between surface and 1km if LCL is below 1km
var surfaceEnvTemp = surfaceTemp; // Assuming surface environmental temp is surface temp for this context
interpolatedEnvTemp = surfaceEnvTemp + (envTemps[0] – surfaceEnvTemp) * (currentAltitude / (altitudes[0] * 1000));
}
// Raise parcel moist-adiabatically until it reaches the next target environmental level
var altitudeStep = 100; // meters
while (currentAltitude < nextAltitudeTarget && currentAltitude 0 && currentAltitude altitudes[envIndex-1] * 1000) {
var lowerAlt = altitudes[envIndex-1] * 1000;
var upperAlt = altitudes[envIndex] * 1000;
var lowerTemp = envTemps[envIndex-1];
var upperTemp = envTemps[envIndex];
interpolatedEnvTemp = lowerTemp + (upperTemp – lowerTemp) * ((currentAltitude – lowerAlt) / (upperAlt – lowerAlt));
} else if (envIndex == 0 && currentAltitude interpolatedEnvTemp) {
fclAltitude = currentAltitude;
break; // Found FCL
}
}
if (fclAltitude !== -1) {
break; // Exit outer loop if FCL found
}
// If we reached the target altitude without finding FCL, update for the next iteration
currentAltitude = nextAltitudeTarget;
parcelTemp = currentEnvTemp – MALR_avg * ((nextAltitudeTarget – currentAltitude) / 1000); // Parcel cools to environment then continues MALR
// Correction: Parcel temp should be calculated based on its actual temperature at nextAltitudeTarget.
// The above line is incorrect. It should be the parcel temp continuing from the last point.
// Recalculate parcel temp at nextAltitudeTarget correctly
var temp_at_next_alt = parcelTempAtLCL – MALR_avg * ((nextAltitudeTarget – LCL_altitude) / 1000);
// This requires careful tracking of parcel temperature as it rises.
// Let's simplify the loop logic to directly compare parcel temp vs env temp at discrete levels.
envIndex++; // Move to the next environmental level
}
// Simplified iterative check at discrete altitudes
parcelTemp = parcelTempAtLCL; // Reset parcel temp to LCL temp
currentAltitude = LCL_altitude;
// If LCL is below 1km, check between LCL and 1km
if (LCL_altitude < altitudes[0] * 1000) {
var temp_step_m = 100; // meters
for (var h_m = LCL_altitude; h_m interpolatedEnv) {
fclAltitude = h_m;
break;
}
}
}
// Check at defined environmental levels
if (fclAltitude === -1) { // Only proceed if FCL not found yet
var prev_alt_km = LCL_altitude / 1000;
var prev_parcel_temp = parcelTempAtLCL;
for (var i = 0; i < envTemps.length; i++) {
var current_alt_km = altitudes[i];
var current_env_temp = envTemps[i];
// If current altitude is lower than LCL, skip
if (current_alt_km * 1000 current_env_temp) {
// The parcel became warmer than the environment between prev_alt_km and current_alt_km
// We need to find the exact altitude. Perform a finer search.
var finer_step_m = 50; // meters
var start_search_alt_km = prev_alt_km;
var end_search_alt_km = current_alt_km;
for (var h_km = start_search_alt_km; h_km interpolated_env_fine) {
fclAltitude = h_km * 1000;
break;
}
}
if (fclAltitude !== -1) break;
}
// Update for the next iteration
prev_alt_km = current_alt_km;
prev_parcel_temp = parcel_temp_at_current_alt; // This is the temperature of the parcel at current_alt_km
}
}
if (fclAltitude !== -1) {
var fclKm = (fclAltitude / 1000).toFixed(2);
resultDiv.innerHTML = "The calculated Free Convection Level (FCL) is approximately " + fclKm + " km (or " + fclAltitude.toFixed(0) + " meters).";
resultDiv.innerHTML += "This is the altitude at which a lifted air parcel becomes warmer than its environment and will continue to rise freely.";
} else {
resultDiv.innerHTML = "Could not determine Free Convection Level with the provided data. It may be above the highest input altitude or convection is suppressed.";
}
}
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