Lifted condensation level

The lifted condensation level or lifting condensation level (LCL), represents the height at which an air parcel being lifted dry adiabatically will become saturated because of adiabatic cooling (caused by expansion) and condense into cloud. It approximates the height of cloud base when there is mechanical forcing.  

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Method of finding

Meteorologists determine the LCL on thermodynamic diagrams, such as a Skew-T log-P diagram or the Tephigram, as follows:

1. Start at the initial temperature (T) and pressure of the parcel and follow upward the dry adiabatic lapse rate line if the air is not saturated. Otherwise, the parcel is already at or above LCL.
2. From the dew point temperature (Td) of the parcel, follow upward the mixing ratio line at that point.
3. At the intersection of the two lines is the LCL.

While the potential temperature of the parcel remains the same, as it is done adiabatically (no exchange of heat with the environment), the volume expands due to a lower outside pressure. This leads to a lowering of the parcel temperature to compensate (ideal gas law). Since the air parcel does not lose matter either, the mixing ratio of water vapor to dry air remain the same until the temperature has reached the saturation. Then condensation occurs, and if the lift continues the parcel will form cloud.

More simply, as an air parcel rises, its temperature decreases while its moisture content remains constant, eventually reaching the point of saturation. It is the point where the temperature and dew point are equivalent, where relative humidity is 100%.

The LCL is the level where a parcel rising dry adiabatically from the surface (the mixed layer and boundary layer) intersects the saturation mixing ratio line from the surface dew point. A lesser dew point depression (T-Td) results in a lower LCL. High low-level moisture content and low cloud bases are conducive to tornadogenesis. One can approximate the LCL without a sounding, using surface data, with the following formula:

h_LCL = 120 (T - Td)

where h is pressure height of LCL, T is temperature in degrees Fahrenheit, Td is dew point temperature in degrees Fahrenheit.

Relation with CCL

Without mechanical lift, cloud will form at the convective condensation level (CCL) resultant from surface heating causing buoyant lifting spontaneously to the point of saturation when the convective temperature is reached. The CCL is always higher than the LCL, unless the convective temperature is reached, then the heights are the same. This assumes idealized conditions using parcel theory, in nature, the actual cloud base is usually initially somewhere between the LCL and the CCL. This is partly because often both processes are at work lifting a parcel. As a thunderstorm grows and matures, processes (increased saturation at lower levels from precipitation and lower pressure) usually lead to a lowering of the cloud base.

A lower difference between the LCL and LFC (LCL-LFC) is conducive to thunderstorms and tornadoes. One reason for this is that a parcel requires less work and time to pass through the layer of convective inhibition (CIN) to reach its level of free convection (LFC), where after, deep, moist convection (DMC) ensues and a parcel buoyantly rises in the positive area of the sounding consisting of convective available potential energy (CAPE) until reaching the equilibrium level (EL). A lower LCL-LFC difference also means thunderstorms can initiate sooner, requiring less left, since they'll reach their LFC more quickly and easily.

See also

• Atmospheric thermodynamics

References

• M K Yau and R.R. Rogers, Short Course in Cloud Physics, Third Edition, published by Butterworth-Heinemann, January 1, 1989, 304 pages. EAN 9780750632157 ISBN 0-7506-3215-1