body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 1200px;
margin: 0 auto;
padding: 20px;
}
.calc-container {
background: #f8f9fa;
border: 1px solid #e9ecef;
border-radius: 8px;
padding: 30px;
max-width: 600px;
margin: 20px auto;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.calc-title {
text-align: center;
color: #2c3e50;
margin-bottom: 25px;
font-size: 24px;
font-weight: 700;
}
.form-group {
margin-bottom: 20px;
}
.form-label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #495057;
}
.form-input {
width: 100%;
padding: 12px;
border: 2px solid #ced4da;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
transition: border-color 0.2s;
}
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border-color: #3498db;
outline: none;
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.form-help {
font-size: 12px;
color: #6c757d;
margin-top: 5px;
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.calc-btn {
display: block;
width: 100%;
padding: 14px;
background-color: #3498db;
color: white;
border: none;
border-radius: 4px;
font-size: 18px;
font-weight: 600;
cursor: pointer;
transition: background-color 0.2s;
margin-top: 20px;
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.calc-btn:hover {
background-color: #2980b9;
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.result-box {
margin-top: 30px;
padding: 20px;
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border-left: 5px solid #2ecc71;
border-radius: 4px;
display: none;
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.result-row {
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justify-content: space-between;
margin-bottom: 10px;
padding-bottom: 10px;
border-bottom: 1px solid #eee;
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.result-row:last-child {
border-bottom: none;
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.result-label {
font-weight: 600;
color: #555;
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.result-value {
font-weight: 700;
color: #2c3e50;
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.article-content {
margin-top: 50px;
background: #fff;
padding: 20px;
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.article-content h2 {
color: #2c3e50;
border-bottom: 2px solid #3498db;
padding-bottom: 10px;
margin-top: 30px;
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.article-content h3 {
color: #34495e;
margin-top: 25px;
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.article-content ul {
padding-left: 20px;
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.article-content li {
margin-bottom: 10px;
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.formula-box {
background: #f1f8ff;
padding: 15px;
border-radius: 5px;
font-family: "Courier New", monospace;
margin: 15px 0;
border: 1px solid #d1e8ff;
}
function calculateEvaporation() {
// Get inputs
var Tw = parseFloat(document.getElementById('waterTemp').value);
var Ta = parseFloat(document.getElementById('airTemp').value);
var RH = parseFloat(document.getElementById('relHumidity').value);
var V = parseFloat(document.getElementById('airVelocity').value);
var A = parseFloat(document.getElementById('surfaceArea').value);
// Validation
if (isNaN(Tw) || isNaN(Ta) || isNaN(RH) || isNaN(V) || isNaN(A)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if (RH 100) {
alert("Relative Humidity must be between 0 and 100.");
return;
}
// Constants
var Patm = 101.325; // Atmospheric pressure in kPa at sea level
// 1. Calculate Saturation Vapor Pressure (kPa)
// Using Magnus formula: P = 0.61094 * exp( (17.625 * T) / (T + 243.04) )
var Ps_water = 0.61094 * Math.exp((17.625 * Tw) / (Tw + 243.04));
var Ps_air = 0.61094 * Math.exp((17.625 * Ta) / (Ta + 243.04));
// 2. Calculate Actual Vapor Pressure of Air (kPa)
var Pv_air = Ps_air * (RH / 100);
// 3. Calculate Humidity Ratios (kg water / kg dry air)
// x = 0.622 * p / (Patm – p)
var x_s = 0.622 * Ps_water / (Patm – Ps_water); // Humidity ratio at saturation (water interface)
var x_a = 0.622 * Pv_air / (Patm – Pv_air); // Humidity ratio of ambient air
// Ensure we don't get negative evaporation (condensation) if air is warmer and humid than water
// For simple evaporation rate logic, if x_a > x_s, condensation occurs.
// We will display the signed value (negative means condensation).
// 4. Calculate Evaporation Rate (kg/hr)
// Formula: g = Theta * A * (x_s – x_a)
// Theta (Evaporation coefficient) approx: 25 + 19*V for water surfaces
// This yields kg/hr per unit humidity ratio difference
var theta = 25 + (19 * V);
var evaporationRateKgHr = theta * A * (x_s – x_a);
// If condensation (negative), we label it clearly or keep negative sign
// For this calculator, we display the raw rate.
// 5. Conversions
// Water density approx 1 kg/L
var litersPerHour = evaporationRateKgHr; // 1 kg ~ 1 Liter
var litersPerDay = litersPerHour * 24;
var gallonsPerDay = litersPerDay * 0.264172;
// Display Results
document.getElementById('resKgHr').innerHTML = evaporationRateKgHr.toFixed(3) + " kg/hr";
document.getElementById('resLitersHr').innerHTML = litersPerHour.toFixed(3) + " Liters";
document.getElementById('resLitersDay').innerHTML = litersPerDay.toFixed(2) + " Liters";
document.getElementById('resGallonsDay').innerHTML = gallonsPerDay.toFixed(2) + " Gallons";
// Show result box
document.getElementById('resultBox').style.display = "block";
}
How to Calculate Rate of Evaporation of Water
Calculating the rate of evaporation is essential for managing swimming pools, industrial cooling tanks, reservoirs, and agricultural irrigation. Evaporation is the process where liquid water turns into water vapor, a transition driven by energy and pressure differences between the water surface and the surrounding air.
The Physics Behind the Calculation
The rate at which water evaporates depends primarily on the difference in vapor pressure between the water surface and the air. If the air is dry (low vapor pressure), water evaporates quickly. If the air is humid (high vapor pressure), evaporation slows down. This relationship is often described using Dalton's Law or engineering approximations like the Carrier equation.
The core formula used in many engineering contexts involves the Humidity Ratio difference:
E = (Θ) × A × (xs – x)
Where:
- E = Evaporation Rate (kg/hr)
- Θ = Evaporation Coefficient (depends on wind speed: typically 25 + 19×V)
- A = Surface Area (m²)
- xs = Humidity ratio of saturated air at the water temperature (kg/kg)
- x = Humidity ratio of the ambient air (kg/kg)
Key Factors Affecting Evaporation
1. Water Temperature
Higher water temperature increases the kinetic energy of water molecules, allowing them to escape the surface more easily. Warmer water creates a higher saturation vapor pressure at the surface, driving faster evaporation.
2. Air Temperature and Humidity
Warm air can hold more moisture than cold air. However, the critical factor is Relative Humidity. Even if the air is hot, if it is at 100% humidity, net evaporation will be zero (equilibrium). Lower humidity increases the "thirst" of the air for moisture.
3. Air Velocity (Wind Speed)
Wind plays a crucial role by removing the boundary layer of saturated air sitting just above the water surface. Without wind, the air immediately above the water becomes saturated, and evaporation stops. Wind constantly replaces this moist air with drier air, maintaining a high evaporation rate.
4. Surface Area
Evaporation is a surface phenomenon. The larger the surface area exposed to the air, the higher the total volume of water lost. This is why shallow, wide pools lose water faster than deep, narrow tanks of the same volume.
Real-World Example
Consider a swimming pool with a surface area of 50 m².
- Water Temp: 28°C
- Air Temp: 30°C
- Humidity: 50%
- Wind: 2 m/s (Light breeze)
Using the calculator above, you would find that this pool loses significant water daily. The vapor pressure at the warm water surface is high, and the wind helps strip that vapor away. Monitoring these rates helps in planning for water refills and managing chemical balance in pools.
How to Reduce Evaporation
If your goal is to conserve water, the most effective method is to reduce the surface area exposed to open air or to cover the surface. Pool covers can reduce evaporation by up to 95% by eliminating the wind factor and trapping the humid air directly above the water, equalizing the vapor pressure.