Latent Heat Flux Calculator

Formula Used

Evaporation approach

LE = Lv × E

• LE is latent heat flux in W/m².
• Lv is latent heat of vaporization in J/kg.
• E is evaporation in kg/m²/s (or mm/s).

Bulk aerodynamic approach

LE = ρ × Lv × Ce × U × Δq

• ρ air density (kg/m³), Ce transfer coefficient (−).
• U wind speed (m/s), Δq specific humidity difference (kg/kg).
• This gives a practical estimate for turbulent moisture transport.

How to Use This Calculator

1. Choose a method that matches your measurements.
2. Pick whether to enter Lv or compute it from temperature.
3. Enter all required inputs and select appropriate units.
4. Press Calculate to show results above the form.
5. Use Download CSV or Download PDF for records.

Example Data Table

Values are illustrative and depend on conditions.

Notes on Interpretation

• Positive LE usually means energy supports evaporation.
• Check units carefully; tiny mistakes change results a lot.
• For aerodynamic estimates, Ce depends on stability and roughness.

Latent Heat Flux: Technical Overview

1) What latent heat flux represents

Latent heat flux (LE) is the energy rate used for phase change of water, mainly evaporation and transpiration. Unlike sensible heat, it does not directly raise temperature; it moves energy with water vapor. In surface energy budgets, LE often dominates during wet, warm, and windy conditions.

2) Common units and conversions

LE is reported in watts per square meter (W/m²). Evaporation is frequently measured as mm/day or mm/hour. Because 1 mm of liquid water over 1 m² equals 1 kg/m², converting E to kg/m²/s is straightforward. Multiplying by Lv (J/kg) yields W/m².

3) Typical values and useful benchmarks

Over irrigated crops or open water, daytime LE commonly ranges from 50 to 300 W/m², depending on radiation and wind. In dry, water-limited terrain, values may drop below 50 W/m². A quick reasonableness check is evaporation equivalent; 100 W/m² is roughly a few mm/day for water.

4) Evaporation-rate method

If you have measured evaporation or evapotranspiration, the direct relation LE = Lv·E provides a clean estimate. This approach is often used with lysimeters, pan evaporation, or converted remote-sensing ET products. The calculator also returns mm/day and mm/hour equivalents to support reporting and field comparisons.

5) Bulk aerodynamic method

When evaporation is not measured, LE can be approximated from turbulence using LE = ρ·Lv·Ce·U·Δq. Here, wind speed and humidity difference drive vapor transport. The coefficient Ce summarizes roughness and atmospheric stability effects, so site-specific calibration can improve accuracy.

6) Choosing input ranges

For near-surface air, ρ is often close to 1.1–1.3 kg/m³. Ce frequently falls between 0.0005 and 0.0025 for bulk estimates. Δq in g/kg can vary from near 0 in humid air to several g/kg in dry air over moist surfaces. Use consistent heights for U and humidity.

7) Interpreting results carefully

Large LE values usually indicate strong evaporation or transpiration, which can cool the surface and moisten the boundary layer. Negative values are uncommon in simple bulk estimates and may indicate sign conventions or input mismatch. Always confirm unit selections and the definition of Δq used by your data source.

8) Practical applications

Latent heat flux supports irrigation scheduling, drought monitoring, and watershed hydrology. In meteorology, it helps diagnose boundary-layer growth and convective potential. In climate studies, LE links surface moisture availability to temperature extremes. Exported tables from this calculator are useful for audits, reports, and scenario tracking.

FAQs

1) What is the difference between LE and evaporation rate?

Evaporation rate describes mass loss of water per area per time. LE describes the energy per area per time required to support that mass transfer. They are related by LE = Lv × E after unit conversion.

2) Why does the calculator offer Lv from temperature?

Latent heat of vaporization decreases as temperature rises. Using temperature-based Lv improves consistency across seasons or experiments. It is especially helpful when you compare LE at different temperatures using the same evaporation measurements.

3) What does Δq mean in the aerodynamic method?

Δq is the specific humidity difference between the surface air and the overlying air. A larger difference increases evaporation potential. Use the same units and reference conditions for both values to avoid sign and scaling errors.

4) How do I pick a reasonable Ce value?

Ce depends on roughness and stability. For rough, windy surfaces it can be higher; for smooth, stable conditions it can be lower. If you have site measurements, calibrate Ce so modeled LE matches observed evaporation or flux data.

5) Can LE be converted back to mm/day?

Yes. The calculator reports an evaporation equivalent using E = LE / Lv, then converts to mm/day and mm/hour. This is useful for hydrology and agronomy reports that prefer depth units instead of energy units.

6) Why might my LE look too high?

Common causes include using mm/day as mm/hour, entering Lv in MJ/kg but selecting J/kg, or using an unrealistically large Δq. Recheck units, magnitudes, and whether your evaporation data already includes scaling factors.

7) Does this calculator replace eddy-covariance measurements?

No. Eddy-covariance directly measures turbulent fluxes and captures rapid variability. This calculator provides an estimate based on simplified relationships. It is best for screening, reporting, and scenario comparisons when full flux data are unavailable.