About

Accurate cloud base estimation prevents controlled flight into terrain and supports safe VFR decision-making. This calculator implements two independent methods: Hennig's spread rule (T − T_d) × 400 ft/°F and Espy's lifting condensation level formula 125 × (T − T_d) m. Both derive from the differential between dry adiabatic lapse rate (~9.8 °C/km) and dew point lapse rate (~1.8 °C/km). When only relative humidity is available, dew point is computed via the Magnus-Tetens approximation with coefficients a = 17.27 and b = 237.7 °C. Results are approximate: strong inversions, frontal lifting, or orographic effects can shift actual cloud bases significantly from calculated values.

Station elevation matters. The formulas yield height above ground level (AGL). To obtain cloud base in MSL (for METAR comparison), add your field elevation. Note: at very low spreads (1 - 2 °C), fog or ground-level stratus is likely. At spreads exceeding 25 °C, the atmosphere is too dry for cumulus formation and the formula loses practical meaning. Pro tip: compare your result against the reported METAR ceiling to calibrate local accuracy.

Formulas

The primary estimation uses Espy's lifting condensation level equation. Given the dry adiabatic lapse rate Γ_d ≈ 9.8 °C/km and the dew point lapse rate Γ_dew ≈ 1.8 °C/km, the height at which a rising parcel reaches saturation is:

LCL = T − T_dΓ_d − Γ_dew ≈ 125 × (T − T_d) m

The equivalent imperial rule-of-thumb (Hennig's Rule) is:

Cloud Base = T_°F − T_d(°F)4.4 × 1000 ft ≈ (T_°F − T_d(°F)) × 227 ft/°F

When dew point is unknown, it is derived from relative humidity using the Magnus-Tetens approximation:

α = a × Tb + T + ln(RH100)

T_d = b × αa − α

Where T = surface air temperature (°C), T_d = dew point temperature (°C), RH = relative humidity (%), a = 17.27 (dimensionless Magnus coefficient), b = 237.7 °C (Magnus coefficient), LCL = lifting condensation level (cloud base height AGL).

Reference Data

Spread (T − T_d)	Cloud Base (AGL)	Typical Condition	Flight Category	Visibility Risk
0 - 1 °C	0 - 125 m / 0 - 400 ft	Fog / ground stratus	LIFR	Severe (<1 SM)
2 °C	250 m / 800 ft	Low stratus / mist	IFR	Poor (1 - 3 SM)
3 °C	375 m / 1,200 ft	Low overcast	MVFR	Marginal
4 °C	500 m / 1,600 ft	Overcast / broken	MVFR	Moderate
5 °C	625 m / 2,000 ft	Low cumulus	VFR	Good
7 °C	875 m / 2,800 ft	Fair weather cumulus	VFR	Good
10 °C	1,250 m / 4,000 ft	Scattered cumulus	VFR	Excellent
12 °C	1,500 m / 4,800 ft	Typical summer fair	VFR	Excellent
15 °C	1,875 m / 6,000 ft	High cumulus	VFR	Excellent
18 °C	2,250 m / 7,200 ft	Dry continental air	VFR	Unlimited
20 °C	2,500 m / 8,000 ft	Desert / arid climate	VFR	Unlimited
25 °C	3,125 m / 10,000 ft	Very dry, unlikely Cu	VFR	Unlimited
30+ °C	3,750+ m	No convective cloud expected	VFR	Formula unreliable

Frequently Asked Questions

The formula yields height Above Ground Level (AGL). To convert to Mean Sea Level (MSL) altitude - which is what METAR ceilings and altimeter settings reference - add your station/field elevation. For example, if your field is at 1,500 ft MSL and the calculator returns 3,000 ft AGL, the cloud base MSL is 4,500 ft.

METAR ceilings are measured by ceilometers or reported by observers at a specific point. The spread-based formula assumes a well-mixed boundary layer with uniform lifting - conditions that break down during inversions, frontal passages, advection fog, or orographic lift. Additionally, METARs report the lowest broken or overcast layer, not necessarily the convective condensation level. Differences of 500 - 1,500 ft are common in non-convective weather.

Below a spread of about 1 °C, fog or ground-level obscuration is more likely than a distinct cloud base, and the formula loses meaning. Above roughly 25 °C spread, the air is too dry for convective cumulus to form at all, so the calculated "cloud base" is theoretical. The sweet spot for accuracy is a spread between 2 and 20 °C in a well-mixed afternoon boundary layer.

Yes. The calculator uses the Magnus-Tetens approximation to derive dew point from temperature and relative humidity. This method is accurate to within ±0.4 °C for temperatures between −40 and 50 °C and RH above 5%. At extremely low humidity (<5%), the approximation degrades. Use a direct dew point sensor reading when available.

They are the same physical relationship expressed in different unit systems. 125 m/°C converts to approximately 228 ft/°F (since 125 m = 410 ft and 1 °C = 1.8 °F, so 410 ÷ 1.8 ≈ 228). The popular "400 ft per degree" simplification uses a rounded 2.5 °F/1000ft spread rate, which slightly overestimates cloud base height. This calculator provides both the precise and rounded results.

Indirectly, yes. Strong surface heating increases convective mixing depth and can raise the actual cloud base above the calculated LCL. Conversely, a subsidence inversion (common in high-pressure systems) can cap cloud formation below the LCL or prevent it entirely. Wind-driven orographic lift forces air upward on terrain, lowering cloud base on windward slopes - sometimes to ground level regardless of spread. The formula assumes free convection in a well-mixed atmosphere; terrain and synoptic-scale dynamics require additional analysis.