Calculator Inputs
Use either dew point or relative humidity. The responsive grid shows three columns on large screens, two on medium screens, and one on mobile.
Formula Used
Cloud base estimates commonly use the temperature spread between the surface air temperature and the dew point. As the spread grows, the estimated condensation level rises.
1) Aviation quick estimate
Cloud Base (ft AGL) = (Temperature − Dew Point) × 400
This is a practical field rule in Celsius. It gives a fast approximation for low cloud base above ground level.
2) LCL approximation
Cloud Base (m AGL) = (Temperature − Dew Point) × 125
This closely matches the lifting condensation level estimate and is useful in metric workflows.
3) Relative humidity mode
When dew point is unknown, the page first derives it from temperature and relative humidity using the Magnus relation, then applies the selected cloud base method.
4) Mean sea level conversion
Cloud Base (MSL) = Cloud Base (AGL) + Station Elevation
Use the MSL value when comparing the result with terrain or topographic references.
How to Use This Calculator
- Select the input mode: dew point or relative humidity.
- Choose Celsius or Fahrenheit for your weather readings.
- Enter surface temperature and either dew point or humidity.
- Provide station elevation in meters for MSL conversion.
- Pick the aviation rule, LCL approximation, or a custom coefficient.
- Set decimal precision, then press Calculate Cloud Base.
- Review the result panel above the form, comparison table, and chart.
- Export the calculation using the CSV or PDF buttons.
Example Data Table
These examples use Celsius inputs and the aviation quick estimate. Values are approximate.
| Scenario | Air Temp (°C) | Dew Point (°C) | Spread (°C) | Elevation (m) | Cloud Base AGL (ft) | Cloud Base AGL (m) | Cloud Base MSL (m) |
|---|---|---|---|---|---|---|---|
| Cool morning | 18 | 14 | 4 | 250 | 1600 | 487.68 | 737.68 |
| Warm humid afternoon | 30 | 24 | 6 | 120 | 2400 | 731.52 | 851.52 |
| Dry evening | 22 | 10 | 12 | 500 | 4800 | 1463.04 | 1963.04 |
Why the Plot Matters
The chart shows how cloud base changes as the temperature and dew point spread changes. A wider spread raises the estimated cloud base. This is useful for comparing fast operational rules with the metric LCL approximation and your own custom factor.
FAQs
1) What is cloud base?
Cloud base is the approximate height where rising air first reaches saturation and visible cloud begins. In weather work, it is often estimated above ground level using surface temperature and dew point.
2) Why use temperature and dew point?
The difference between air temperature and dew point indicates how far the air is from saturation. Smaller spreads mean the cloud base is usually lower, while larger spreads usually place it higher.
3) Is this exact for every weather situation?
No. It is an approximation. Real cloud formation also depends on mixing, pressure patterns, terrain, wind, and vertical moisture structure. Use observations and forecasts for operational decisions.
4) What does AGL mean?
AGL means above ground level. It tells you how high the cloud base is relative to the local surface at your observation point, not relative to sea level.
5) What does MSL mean?
MSL means mean sea level. The calculator adds station elevation to the AGL estimate so you can compare the cloud base with terrain elevations, maps, and altitude references.
6) When should I use relative humidity mode?
Use it when dew point is unavailable but temperature and humidity are known. The page estimates dew point first, then applies the selected cloud base method.
7) Why are aviation and LCL results slightly different?
They use nearly equivalent quick approximations with different unit conventions. The aviation rule works directly in feet, while the LCL form is usually shown in meters.
8) Can I use a custom coefficient?
Yes. A custom coefficient lets you explore alternative field assumptions or local teaching examples. Enter the coefficient in meters per degree Celsius spread and compare it with the standard methods.