Cloud Base Calculator

About the Cloud Base Calculator

The **Cloud Base Calculator** estimates the altitude at which cumulus clouds form — the lifted condensation level (LCL). Enter the surface temperature and dewpoint (or relative humidity), and the calculator applies Espy's empirical rule and the Henning formula to predict the cloud base in feet and metres, both above ground and above sea level. That gives you a quick estimate of where clouds may begin to form in the lower atmosphere. The same spread can be used to compare different weather setups without redoing the rule-of-thumb math each time.

Pilots, glider pilots, skydivers, and paraglider pilots use this calculation before every flight: knowing the cloud base determines whether VFR flight is possible, how high thermals will reach, and whether mountain peaks are clear. Meteorologists use it to forecast cloud cover and precipitation onset. The rule of thumb is simple — every 1°C of temperature-dewpoint spread raises the cloud base by about 125 m (400 ft).

Explore presets for summer, tropical, desert, winter, and gliding conditions, and reference the tables showing spread-vs-altitude and cloud type classifications. That makes it easier to compare several plausible weather setups before you rely on one quick estimate in the field.

When This Page Helps

Use this estimate to turn surface temperature and dewpoint into a quick cloud-base check before flight planning, soaring decisions, or basic weather interpretation. It is a fast way to translate the temperature-dewpoint spread into a practical height estimate without doing the rule-of-thumb math by hand. That makes it easier to compare conditions across different days or locations. It is most helpful as a quick ceiling estimate before you compare it with observed reports.

How to Use the Inputs

1. Select a preset or enter the surface air temperature.
2. Choose to input dewpoint temperature or relative humidity.
3. Enter the dewpoint or RH value.
4. Enter station elevation for MSL altitude.
5. Read cloud base AGL, MSL, spread, and surface RH.
6. Use the reference tables for quick lookups.

Example Calculation

Tips & Best Practices

• The 1°C spread = 400 ft rule works for convective (cumulus) clouds only.
• When the spread is closing (decreasing), expect cloud bases to lower.
• At airports, METAR reports give observed ceiling — compare with this estimate.
• For soaring pilots, cloud base = thermal top; higher base = stronger thermals.
• Spread < 3°C means high chance of mist, fog, or low stratus.

What This Estimate Means

Cloud base calculators are estimating the lifted condensation level from near-surface observations. That is most useful for fair-weather cumulus and for quick operational decisions, not as a full cloud forecast for every cloud type.

Aviation Interpretation

Pilots and glider pilots usually care about the result because it sets the rough ceiling for thermals and indicates whether terrain or traffic patterns may be in cloud. Comparing the estimate with observed METAR ceilings is a good sanity check before treating it as operationally reliable.

Limits Of The Rule

The common 400 ft per degree C rule is an approximation. Local terrain, advection, inversions, and non-convective cloud processes can all produce cloud bases that differ from the simple spread-based estimate.

Frequently Asked Questions

• What is the LCL?

  The lifted condensation level is the height at which a rising air parcel cools enough to reach saturation and condensation begins. In practice, it is the approximate cloud base for fair-weather cumulus.

• How accurate is the 400 ft per °C rule?

  It is a good approximation for convective clouds. The Henning formula (122 m/°C) is slightly more precise, and both assume dry-adiabatic lifting rather than strong local forcing.

• Can I use this for fog?

  Fog forms when the spread is near zero. If the calculator shows a very low cloud base, expect fog or low stratus rather than a high cumulus ceiling.

• What is the spread?

  The temperature-dewpoint spread (or depression) is the difference between the air temperature and the dewpoint temperature. Smaller spread = higher humidity, which means the cloud base is lower.

• Does this work at night?

  The estimate still works, but nighttime cooling can collapse the spread quickly and favor fog or low stratus instead of daytime cumulus development. That is why the same afternoon estimate may not hold after sunset.

• Why do pilots care about cloud base?

  VFR flight requires minimum cloud clearance (typically 1 000 ft below a cloud ceiling). Cloud base determines whether VFR is legal and safe, and it also affects glider and paraglider planning.