Cloud Base Calculator (Lifted Condensation Level)
Enter your surface temperature and dew point (or relative humidity) to get the estimated cloud base height - the altitude at which rising air cools enough for water vapor to condense into visible cloud. Results are shown in metres and feet above ground level (AGL) and above mean sea level (AMSL), plus the temperature at the cloud base. Switch freely between metric and imperial units.
Formula
Worked example
Surface temperature 25 degC, dew point 12 degC: spread = 13 degC. LCL height = 125 x 13 = 1,625 m (5,331 ft) AGL. Cloud temperature = 25 - 0.98 x (1625 / 100) = 25 - 15.9 = 9.1 degC.
What is cloud base and how is it calculated?
Cloud base (more precisely, the Lifted Condensation Level or LCL) is the lowest altitude at which rising surface air becomes saturated and visible cloud droplets form. As a parcel of air rises it cools at the dry adiabatic lapse rate (DALR) of about 9.8 degC per kilometre. Its dew point drops more slowly, at about 1.8 degC per kilometre. The two temperatures converge at a rate of 8 degC per kilometre (roughly 125 m per degC of spread), so the LCL height in metres equals 125 times the temperature-dew point spread at the surface. This is Espy's approximation, first published in the 1830s, and it is accurate to within about 1 percent under typical mid-latitude conditions. For imperial units, the equivalent rule is to divide the Fahrenheit spread by 4.4 and multiply by 1,000 to get feet above ground level.
Why cloud base matters in aviation
The cloud base defines the ceiling, the lowest broken or overcast cloud layer above an aerodrome. Visual Flight Rules (VFR) typically require a ceiling of at least 1,000 ft (300 m) AGL and 3 statute miles visibility. When the ceiling falls below that, pilots must file Instrument Flight Rules (IFR) flight plans and rely on approved approaches that may have minima as low as 200 ft. Pilots planning cross-country VFR flights use the temperature-dew point spread reported in METARs - a spread less than 2 to 3 degC signals that a low ceiling or fog is imminent, especially near dawn when temperatures fall toward the dew point. This calculator gives a quick pre-flight estimate, but always verify with current METARs and TAFs from your departure and destination stations.
Temperature, dew point, and relative humidity
The dew point is the temperature to which air must cool, at constant pressure, for saturation to occur. A tight spread between temperature and dew point means the air is nearly saturated (high relative humidity) and a cloud base is close to the surface. A wide spread means drier air and a higher cloud base. If you know relative humidity instead of dew point, you can estimate the dew point using the Bolton (1980) approximation: dew point (degC) = temperature (degC) - (100 - RH) / 5. At 100 percent RH the spread is zero and the cloud base is at ground level (fog). For thunderstorm forecasting, a useful rule is that convective cloud bases tend to be lower in humid tropical air (spread 2-5 degC) and higher in drier continental interiors (spread 10-20 degC or more).
Limitations and how to get better data
Espy's approximation assumes a well-mixed boundary layer with a uniform surface observation, no temperature inversion, and standard lapse rates. Real cloud bases can vary by hundreds of metres from this estimate because of terrain effects, inversions, frontal boundaries, and spatial variation in dew point. For the best accuracy, use measured ceilometer data from an ASOS or AWOS station published in METARs (field RR in the cloud group). The METAR format reports cloud layers as FEW, SCT (scattered), BKN (broken) or OVC (overcast) followed by height in hundreds of feet AGL, for example BKN015 means a broken ceiling at 1,500 ft. Radiosonde soundings from the nearest upper-air station give the most complete vertical picture of the atmosphere.
Cloud ceiling categories and aviation significance
| Cloud base AGL | Category | Aviation impact |
|---|---|---|
| 0 - 150 m (0 - 500 ft) | Very low / fog | IFR mandatory - severe restrictions |
| 150 - 300 m (500 - 1000 ft) | Low ceiling | IFR likely - marginal VFR |
| 300 - 600 m (1000 - 2000 ft) | Marginal VFR | VFR minimums just met at most airports |
| 600 - 2000 m (2000 - 6500 ft) | Low cloud base | VFR flight possible - some restrictions |
| 2000 - 6000 m (6500 - 20000 ft) | Mid-level | Good VFR - typical cumulus range |
| Above 6000 m (above 20000 ft) | High cloud | Cirrus - no ceiling issue for most aircraft |
Standard AGL ceiling categories used in aviation and meteorological reporting. VFR = Visual Flight Rules, IFR = Instrument Flight Rules.
Frequently asked questions
What is the difference between cloud base AGL and AMSL?
AGL (above ground level) is the height of the cloud base above the surface directly below it. AMSL (above mean sea level) adds the station elevation to AGL to give the cloud altitude on a standard reference datum - the number used by air traffic control and on altimeters. A cloud base of 1,000 m AGL above a mountain plateau at 2,000 m elevation has an AMSL altitude of 3,000 m.
How accurate is the temperature-dew point spread rule?
Espy's approximation (125 m per degC of spread, or 1,000 ft per 4.4 degF of spread in imperial) is accurate to within about 1% for a well-mixed boundary layer under typical mid-latitude conditions. Accuracy degrades near temperature inversions, over complex terrain, in strongly stratified air, and when the boundary layer is very shallow (early morning) or very deep (hot afternoon).
Can I use relative humidity instead of dew point?
Yes. The Bolton (1980) approximation gives dew point from temperature and relative humidity as: Td = T - (100 - RH) / 5, where temperatures are in degC and RH is in percent. This is accurate to about 1 degC for RH above 50%. Substituting into Espy's rule gives cloud base height in metres = 25 x (100 - RH). This calculator performs that conversion automatically when you select the relative humidity input mode.
What does a low temperature-dew point spread mean?
A spread below 3 degC means the air is nearly saturated. Cloud or fog will form with very little lift or cooling. In aviation weather, a spread of 2 degC or less is a reliable indicator of imminent low ceiling or fog, particularly overnight and in the early morning when surface temperatures cool toward the dew point. Pilots monitor the trend in spread over several hours, not just a single snapshot.
Why is the cloud base height different from what the METAR shows?
METARs report ceiling heights measured by a ceilometer (laser or light-beam instrument) at a specific point on the aerodrome surface. The calculator gives a theoretical estimate based on surface temperature and dew point. Differences arise from spatial variation in dew point across the area, terrain effects, multiple cloud layers, and the fact that the surface observation may not fully represent the local boundary layer. Always use METAR data for operational aviation decisions.
What is the Lifted Condensation Level (LCL)?
The Lifted Condensation Level is the altitude at which a parcel of surface air, lifted dry-adiabatically, becomes saturated. It closely approximates the base of convective cumulus clouds when the boundary layer is well-mixed. The LCL is lower than the Level of Free Convection (LFC), which is where the parcel becomes buoyant relative to the surrounding environment and can continue rising freely to form cumulonimbus or thunderstorm towers.
How do I convert the cloud base from metres to feet?
1 metre = 3.28084 feet, so multiply metres by 3.281 to get feet, or divide feet by 3.281 to get metres. Aviation convention reports cloud layers in hundreds of feet AGL in METARs. This calculator shows both units simultaneously so no manual conversion is needed.