This calculator helps estimate certain properties of atmospheric fog based on key meteorological and atmospheric physics parameters. Fog is essentially a cloud at ground level, composed of tiny water droplets or ice crystals suspended in the air. Its formation, density, and impact on visibility are crucial factors in transportation, aviation, and various industrial processes.
How it Works: The Science Behind the Calculation
The calculator uses several inputs to provide an estimation:
Temperature (°C): The current air temperature. This is a primary driver for atmospheric processes.
Dew Point (°C): The temperature to which air must be cooled to become saturated with water vapor. The difference between temperature and dew point (the "dew point depression") is a key indicator of how close the air is to saturation. When temperature equals dew point, the air is saturated.
Visibility Limit (km): This represents a baseline or desired visibility distance (e.g., for aviation safety or road conditions). While not directly used in the core fog density calculation, it provides context for how the calculated fog properties might affect real-world visibility.
Average Particle Size (micrometers): Fog droplets are typically very small, ranging from 1 to 100 micrometers in diameter. This input helps approximate the scattering effect on light.
Particle Concentration (particles/m³): The number of fog particles suspended in a cubic meter of air. Higher concentrations lead to denser fog.
The Calculation Logic
While a complete, highly accurate fog model is complex and involves dynamic atmospheric conditions, this calculator provides a simplified estimation of fog density and its effect on visibility, often expressed by a "visual range" or an index of obscurity.
1. Fog Formation Check (Saturation):
Fog is most likely to form when the air temperature is close to or at the dew point. The difference between the temperature and the dew point (dewPointDepression = temperature - dewPoint) indicates the air's relative humidity. When this difference is small (typically less than 2°C), the air is highly saturated, favoring fog formation.
2. Estimated Fog Density Index:
A simplified index for fog density can be approximated by considering the saturation level and particle concentration. A common simplified approach relates the difference between temperature and dew point to the likelihood and density of fog. We can create an index by combining these factors.
fogDensityIndex = (1 / (1 + Math.exp(-(temperature - dewPoint - 5)))) * (particleConcentration / 100000);
The exponential term approximates a logistic function, where a small dew point depression leads to a high value, indicating saturation. The particle concentration scales this effect. The constants are empirically derived for illustrative purposes.
3. Estimated Visibility Reduction:
Visibility is significantly reduced by light scattering from fog particles. The Koschmieder's Law provides a basis, stating that visibility is limited by the amount of light attenuation. A simplified relationship can be derived where visibility decreases exponentially with increasing particle concentration and size.
attenuationFactor = 0.000003 * particleConcentration * particleSize;estimatedVisibilityKm = visibilityLimit * Math.exp(-attenuationFactor);
This formula suggests that as the attenuation factor (dependent on particle size and concentration) increases, the estimated visibility decreases exponentially from the initial visibility limit. The constant 0.000003 is a simplification factor.
Use Cases:
This calculator is useful for:
Meteorology Students: To understand the basic relationship between temperature, dew point, and fog formation.
Pilots and Air Traffic Controllers: To get a quick estimate of potential visibility impacts in foggy conditions.
Transportation Planners: To assess the potential hazards related to reduced visibility on roads and railways.
General Interest: For anyone curious about the atmospheric conditions that create fog.
Disclaimer: This calculator provides a simplified estimation for educational and informational purposes only. Actual fog conditions are influenced by many dynamic factors (wind, humidity advection, terrain, aerosol composition) and require sophisticated meteorological models for precise prediction.
function calculateFog() {
var temperature = parseFloat(document.getElementById("temperature").value);
var dewPoint = parseFloat(document.getElementById("dewPoint").value);
var visibilityLimit = parseFloat(document.getElementById("visibilityLimit").value);
var particleSize = parseFloat(document.getElementById("particleSize").value);
var particleConcentration = parseFloat(document.getElementById("particleConcentration").value);
var resultTextElement = document.getElementById("resultText");
var resultDetailsElement = document.getElementById("resultDetails");
var resultElement = document.getElementById("result");
// Clear previous results and styles
resultTextElement.innerHTML = "";
resultDetailsElement.innerHTML = "";
resultElement.style.backgroundColor = "var(–success-green)";
resultElement.style.borderColor = "#1e7e34";
// Input validation
if (isNaN(temperature) || isNaN(dewPoint) || isNaN(visibilityLimit) || isNaN(particleSize) || isNaN(particleConcentration)) {
resultTextElement.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (visibilityLimit <= 0 || particleSize <= 0 || particleConcentration <= 0) {
resultTextElement.innerHTML = "Visibility Limit, Particle Size, and Particle Concentration must be positive.";
return;
}
// — Calculations —
// 1. Dew Point Depression & Saturation Indicator
var dewPointDepression = temperature – dewPoint;
var saturationStatus = "";
if (dewPointDepression < 0) {
saturationStatus = "Air is supersaturated (temperature below dew point).";
} else if (dewPointDepression < 2) {
saturationStatus = "Air is highly saturated, favorable for fog formation.";
} else if (dewPointDepression 0.8) {
densityDescription = "Very Dense Fog";
} else if (fogDensityIndex > 0.5) {
densityDescription = "Dense Fog";
} else if (fogDensityIndex > 0.2) {
densityDescription = "Moderate Fog";
} else if (fogDensityIndex > 0.05) {
densityDescription = "Light Fog/Mist";
} else {
densityDescription = "No significant fog";
}
// 3. Estimated Visibility Reduction (based on attenuation)
// Simplified attenuation model: more particles and larger particles mean less visibility.
var attenuationFactor = 0.000003 * particleConcentration * particleSize;
var estimatedVisibilityKm = visibilityLimit * Math.exp(-attenuationFactor);
// Ensure estimated visibility is not negative
if (estimatedVisibilityKm < 0) {
estimatedVisibilityKm = 0;
}
// — Display Results —
resultTextElement.innerHTML = "Fog Properties Estimated";
var detailsHtml = `
Dew Point Depression: ${dewPointDepression.toFixed(1)} °C
Saturation Status: ${saturationStatus}
Estimated Fog Density: ${densityDescription} (Index: ${fogDensityIndex.toFixed(2)})
Estimated Visibility: ${estimatedVisibilityKm.toFixed(2)} km (Reduced from ${visibilityLimit.toFixed(1)} km limit)
`;
resultDetailsElement.innerHTML = detailsHtml;
// Adjust background color for result based on density
if (fogDensityIndex > 0.5) { // Dense or Very Dense Fog
resultElement.style.backgroundColor = "#d9534f"; // Reddish for severe conditions
resultElement.style.borderColor = "#b03a35";
} else if (fogDensityIndex > 0.2) { // Moderate Fog
resultElement.style.backgroundColor = "#f0ad4e"; // Orange for moderate
resultElement.style.borderColor = "#d99432";
} else { // Light Fog or No Fog
resultElement.style.backgroundColor = "var(–success-green)"; // Green for better conditions
resultElement.style.borderColor = "#1e7e34";
}
}