Initial conditions

Air temperature

At the starting surface.

-4050

Must not exceed air temperature.

-8050

Equivalent: RH = 53.80 %

Environmental lapse rate

Temperature decrease with height; a negative value indicates a temperature inversion.

°C/km

-512

Results

LCL height

1250

m AGL

4101 ft AGL

Temperature at level: 12.75 °C

CCL height

2128

m AGL

6981 ft AGL

Temperature at level: 11.17 °C

Dew-point depression

10.00

°C

Convective temperature

32.02

°C

Condensation levels

The clouds mark the theoretical LCL and CCL heights. Their positions are schematic and do not represent observed cloud cover.

Show temperature–height profile

Environment · Γenv
Dew-point reference · Γdp
Initial parcel · Γd
Parcel from Tc · Γd

LCL is the intersection of the initial parcel and the dew-point reference; CCL is the intersection of the environment and that reference. Dry adiabatic paths stop at saturation.

Fundamentals & Explanation

What LCL, CCL and Tc represent

An air parcel is a small volume whose changes in temperature and humidity are considered during ascent. The LCL (lifting condensation level) estimates where an initially unsaturated parcel reaches saturation as it rises and cools without prior heating.

The CCL (convective condensation level) is associated with ascent driven by surface heating. It is obtained by intersecting the environmental temperature profile with a constant mixing-ratio line starting at the surface dew point. Convective temperature Tc is the initial temperature whose dry adiabatic path reaches that intersection.

In the linear approximation, dew point decreases at a constant Γ_dp = 1.8 °C/km. This straight line simplifies the constant mixing-ratio line; it represents a parcel humidity reference rather than measured environmental dew point.

Dew-point depression is the difference T − Td. At a fixed air temperature, a smaller difference corresponds to higher relative humidity.

Equations and units

T and Td are the initial temperatures in °C. Lapse rates are in °C/km; z is in km in these equations. Results are converted to metres and feet above the starting surface (AGL), without adding elevation above sea level.

T_p(z)=T-\Gamma_d z,\qquad T_{dp}(z)=T_d-\Gamma_{dp}z

z_{LCL}=\frac{T-T_d}{\Gamma_d-\Gamma_{dp}}

T_{env}(z)=T-\Gamma_{env}z,\qquad z_{CCL}=\frac{T-T_d}{\Gamma_{env}-\Gamma_{dp}}

Γ_d = 9.8 °C/km. For unsaturated air, the CCL equation gives an intersection above the surface only if Γ_env > 1.8 °C/km. When T = Td, both levels are 0 m AGL for every admitted lapse rate: the air is already saturated at the surface, including when the lines coincide.

T_{CCL}=T-\Gamma_{env}z_{CCL},\qquad T_c=T_{CCL}+\Gamma_d z_{CCL}

When Γ_env = Γ_d, LCL = CCL and Tc = T. A superadiabatic lapse rate can yield Tc < T in the linear approximation. In that regime, Tc − T is a temperature difference rather than required heating. Actual convection also depends on atmospheric stability and lifting mechanisms.

Relative humidity and dew point

Relative humidity (RH) compares vapor pressure with saturation vapor pressure at the air temperature. Bolton's (1980) liquid-water fit, equation 10, uses constants 17.67 and 243.5 °C. RH is expressed as a percentage and α is an intermediate coefficient:

RH=100\exp\left(\frac{17.67T_d}{243.5+T_d}-\frac{17.67T}{243.5+T}\right)

\alpha=\ln(RH/100)+\frac{17.67T}{243.5+T},\qquad T_d=\frac{243.5\alpha}{17.67-\alpha}

RH = 0 % has no finite dew point in this fit. Small positive humidities are admissible while the equivalent dew point remains within the domain. Below 0 °C, Td still refers to liquid water and differs from frost point, which uses an ice reference. When T or Td falls outside −35 < T < 35 °C, the interval documented by NCAR for the vapor-pressure fit, the conversion is an extrapolation.

Reading the visualizations

The scene places LCL and CCL on a height scale. The clouds mark theoretical saturation levels rather than observed cloud cover.

The optional temperature–height profile shows the lines of the linear approximation: temperature on the horizontal axis and AGL height on the vertical axis.

The environmental line starts at T and decreases according to Γ_env; a negative value means temperature increases with height.
The blue line represents the initial parcel cooling at Γ_d, up to LCL.
The dew-point reference uses Γ_dp; its intersection with the initial parcel determines LCL and with the environment determines CCL.
The dashed orange line starts at Tc and ends at CCL. Subsequent saturated ascent is not modeled.
The vertical scales adjust up to 12 km AGL. A higher intersection lies outside the column considered.

Domain and limitations

Parameter	Allowed range
Initial T	−40 to 50 °C
Initial or derived Td	−80 to 50 °C, Td ≤ T
RH	0 < RH ≤ 100 %
Environmental Γ	−5 to 12 °C/km
Calculation column	0 to 12 km AGL

These ranges bound the calculation but do not establish the approximation's physical validity throughout the column. Lapse rates are constant; pressure, incorporation of surrounding air (entrainment) and latent heat after saturation are omitted. The 12 km limit does not necessarily correspond to the tropopause.

A condensation temperature below 0 °C does not by itself identify water phase; a low LCL does not confirm fog. Obtaining CCL from a sounding requires vertical profiles and a pressure-dependent mixing-ratio line.

Example and references

For T = 25 °C, Td = 15 °C and Γ_env = 6.5 °C/km: LCL = 1250 m AGL, CCL ≈ 2128 m AGL and Tc ≈ 32.02 °C. This is an example of the linear model.

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Condensation level · LCL and CCL

Initial conditions

Results

Condensation levels

Fundamentals & Explanation

What LCL, CCL and Tc represent

Equations and units

Relative humidity and dew point

Reading the visualizations

Domain and limitations

Example and references

What LCL, CCL and Tc represent

Equations and units

Relative humidity and dew point

Reading the visualizations

Domain and limitations

Example and references