Evaporation Rate Calculator

Estimate the rate of water loss from surfaces using temperature, humidity, and wind speed data based on the ACI 305R-91 formula.

What is an Evaporation Rate Calculator?

An evaporation rate calculator is a specialized technical tool used to determine the speed at which liquid water turns into vapor from a specific surface area over a set period. In professional industries like civil engineering, construction, and hydrology, this measurement is critical for maintaining material integrity and resource management.

Who should use an evaporation rate calculator? Primarily, concrete contractors use it to prevent “plastic shrinkage cracking.” When the evaporation rate exceeds the rate at which bleed water rises to the concrete surface, the surface dries too quickly, leading to structural flaws. Facility managers also use it for swimming pool maintenance to predict water loss and chemical imbalances.

A common misconception is that temperature is the only factor in evaporation. In reality, wind speed and relative humidity often play a more significant role in the total mass transfer of moisture from the surface to the atmosphere.

Evaporation Rate Calculator Formula and Mathematical Explanation

The core logic of our evaporation rate calculator is based on the Menzel Formula, widely adopted by the American Concrete Institute (ACI 305R-91). This empirical formula estimates the evaporation of surface moisture as follows:

E = 5 × ([T_c + 18]^2.5 – r × [T_a + 18]^2.5) × (V + 4) × 10^-6

Variable	Meaning	Unit	Typical Range
E	Evaporation Rate	lb/ft²/hr	0.01 – 0.50
Tc	Concrete/Surface Temp	°F	40 – 110
Ta	Air Temperature	°F	32 – 115
r	Relative Humidity	Decimal (0-1)	0.1 – 0.9
V	Wind Velocity	mph	0 – 30

Practical Examples (Real-World Use Cases)

Example 1: Concrete Pouring on a Hot, Windy Day

Imagine a construction site where the air temperature is 90°F, humidity is 20%, concrete temperature is 95°F, and wind is blowing at 15 mph. Entering these into the evaporation rate calculator yields a result of approximately 0.25 lb/ft²/hr. Since this exceeds the 0.2 threshold, the contractor must implement sunshades, windbreaks, or fogging to prevent cracking.

Example 2: Outdoor Swimming Pool Loss

A pool owner in a dry climate (75°F air, 30% humidity, 80°F water, 5 mph wind) wants to know why they are losing water. The evaporation rate calculator shows a loss of 0.06 lb/ft²/hr. Over a 20×40 foot pool (800 sq ft), this equals 48 lbs of water per hour, or roughly 5.7 gallons per hour, helping the owner distinguish between a leak and natural evaporation.

How to Use This Evaporation Rate Calculator

1. Input Temperatures: Enter the ambient air temperature and the temperature of the surface (concrete or water). Use a non-contact infrared thermometer for accuracy.
2. Set Humidity: Use a hygrometer to find the current relative humidity percentage. Lower humidity significantly increases evaporation.
3. Measure Wind Speed: Input the wind speed near the surface. Note that wind speed at 20 inches height is often different from the speed reported at the airport.
4. Analyze Results: View the primary rate in lb/ft²/hr. Check the color-coded risk indicator to see if immediate action is required for concrete curing.
5. Consult the Chart: Use the dynamic chart below the evaporation rate calculator to see how a potential increase in wind speed would impact your results.

Key Factors That Affect Evaporation Rate Results

• Vapor Pressure Gradient: The difference between the vapor pressure at the surface and the air. A higher gradient drives faster evaporation.
• Thermal Energy: Higher surface temperatures provide the latent heat of vaporization required for water molecules to escape.
• Boundary Layer Turbulence: Wind “scours” the saturated air layer sitting just above the surface, replacing it with drier air.
• Solar Radiation: Direct sunlight can heat a surface far above the ambient air temperature, drastically increasing the results in an evaporation rate calculator.
• Altitude/Atmospheric Pressure: At higher altitudes, lower air pressure allows water molecules to escape more easily.
• Chemical Composition: Dissolved salts or curing compounds in the water can lower the vapor pressure and reduce the evaporation rate.

Frequently Asked Questions (FAQ)

What is a “dangerous” evaporation rate for concrete?

Generally, a rate exceeding 0.2 lb/ft²/hr (1.0 kg/m²/hr) is considered critical. For high-performance concrete or silica fume mixes, the threshold is even lower (0.1 lb/ft²/hr).

Does humidity really matter more than heat?

Yes. A surface in 100°F weather with 90% humidity will often have a lower evaporation rate than a 70°F surface in 5% humidity because the air is already saturated.

Why does the calculator use surface temperature instead of air temperature?

Evaporation happens at the interface. The surface temperature dictates the vapor pressure of the water trying to escape.

How can I reduce the evaporation rate on a job site?

Common methods include using windbreaks, misting/fogging to increase local humidity, or applying evaporation retarders (monomolecular films).

Is this calculator accurate for salt water?

It provides a close estimate, but salt slightly lowers the vapor pressure of water, meaning the actual evaporation rate would be roughly 1-2% lower than the evaporation rate calculator shows.

At what height should wind speed be measured?

For the ACI formula, wind speed should ideally be measured 20 inches (0.5m) above the surface.

What is “bleed water” in concrete?

Bleed water is the excess water that migrates to the surface of freshly placed concrete. If this evaporates faster than it arrives, cracks form.

Can I use this for indoor swimming pools?

Yes, but wind speed will be near zero unless there is a strong HVAC vent blowing directly across the pool surface.

Related Tools and Internal Resources

• Humidity and Drying Guide: Learn how ambient moisture affects structural drying times.
• Wind Speed Measurement Tools: A guide to anemometers and their placement for accurate data.
• Concrete Curing Techniques: Best practices for managing moisture during the first 28 days of concrete life.
• Dew Point Calculator: Calculate the temperature at which condensation will form on surfaces.
• Psychrometric Chart Analysis: A deep dive into the properties of moist air and thermodynamics.
• Vapor Pressure Calculations: Understanding the physics behind liquid-to-gas transitions.