Net Radiation Calculator

Formula used

Net radiation is the balance of shortwave and longwave energy at a surface:

• Rn = Rns + Rnl
• Rns = (1 − α) · Rs or Rns = Rs − Rref
• Rnl = Rld − Rl↑
• Rl↑ = ε · σ · T⁴ (when computed from temperature)

Here, α is albedo, ε is emissivity, σ = 5.670374419×10⁻⁸ W·m⁻²·K⁻⁴, and T is surface temperature in kelvin.

How to use this calculator

1. Enter incoming shortwave (Rs) and choose its unit.
2. Select an albedo mode: provide α or reflected shortwave.
3. Enter incoming longwave (Rld) and choose its unit.
4. Choose outgoing longwave mode: compute from Ts and ε, or enter it directly.
5. Pick an output unit, then press Calculate.
6. Use the download buttons to save CSV or a printable PDF.

Net radiation and surface energy exchange

Net radiation (Rn) is the radiant energy gained or lost by a surface. It drives evaporation, transpiration, and surface heating. This calculator combines measured or estimated shortwave and longwave terms into one consistent Rn value. It is widely used in hydrology, agriculture, and climate studies to quantify available energy.

Radiation balance components

The balance is written as Rn = (Rs − Rsu) + (Rld − Rlu). Incoming shortwave (Rs) comes mainly from the Sun, while reflected shortwave (Rsu) depends on surface reflectivity. Incoming longwave (Rld) is emitted by the atmosphere and clouds, and outgoing longwave (Rlu) is emitted by the surface.

Albedo choices and typical ranges

Albedo (α) converts Rs into reflected shortwave using Rsu = α·Rs. Typical values include open water ≈ 0.05–0.10, short grass ≈ 0.20–0.25, dry sand ≈ 0.30–0.45, and fresh snow ≈ 0.70–0.90. If you measured reflected shortwave, use the direct reflected mode to avoid assumptions.

Outgoing longwave and emissivity

Surface emissivity (ε) controls longwave emission efficiency. Vegetation and water are often high (about 0.97–0.99), while many soils and built materials are slightly lower (roughly 0.90–0.97). Realistic emissivity helps prevent bias in Rn, especially under clear skies.

Temperature-based longwave estimate

If you do not measure upward longwave directly, estimate it with Rlu = ε·σ·Ts⁴, where Ts is surface temperature in kelvin. Use radiometric surface temperature when available; air temperature can understate emission over sunlit ground. You can still enter Rld from station observations or gridded products.

Units and daily energy interpretation

Radiation can be reported as instantaneous flux (W/m²) or daily energy (MJ/m²/day). The conversion used here is 1 MJ/m²/day = 11.574 W/m², based on 86,400 seconds per day. Choose an output unit that matches your analysis.

Reading results and expected magnitudes

Daytime Rn is usually positive, while nighttime values can be near zero or negative. Under clear summer conditions, midday net radiation over land often falls around 300–800 W/m², depending on humidity, latitude, season, and surface properties. Cloud cover reduces Rs but can increase Rld, shifting the balance.

Practical workflow for field and model use

Start with measured inputs, then use mode toggles only for missing terms. Verify temperature units, emissivity, and albedo. Export results to CSV for analysis, or print to PDF for reporting and records.

FAQs

1) What does negative net radiation indicate?

Negative Rn means the surface loses more radiant energy than it gains. This is common at night, during cold-air outbreaks, or when longwave losses exceed incoming radiation.

2) Should I use air temperature or surface temperature?

Use surface temperature whenever possible because longwave emission depends on the emitting surface. Air temperature can be acceptable for well-mixed conditions, but it often underestimates emission over sunlit ground.

3) How do I pick an albedo value?

Choose an albedo that matches the surface and season. If you are unsure, use measured reflected shortwave instead. Small albedo changes can noticeably affect Rn under strong sunlight.

4) Why is emissivity important?

Emissivity scales emitted longwave radiation. Using an unrealistic ε shifts Rlu and can bias net radiation, especially when Rld is modest and skies are clear.

5) Can I compute net radiation without longwave inputs?

You need either measured longwave components or reasonable estimates for them. If longwave data are missing, use trusted sources for Rld and compute Rlu from temperature and emissivity.

6) Is the MJ/m²/day conversion exact?

For daily averaging, yes. 1 MJ/m²/day equals 11.574 W/m² from 1,000,000 J over 86,400 seconds. Differences only arise if your averaging period is not one day.

7) How can I use the result in water and climate studies?

Net radiation feeds energy-balance and evapotranspiration methods. It helps partition energy into latent and sensible heat, supporting irrigation planning, drought monitoring, and model validation with meteorological data.