Lifting Condensation Level (LCL)

Now, consider that same parcel of air we just described above with just one deviation. Instead of dry air, the air now contains moisture. The amount of moisture in the air is expressed by the dew point temperature of the air. If we begin to lift (which is equivalent to expand) that moist parcel of air adiabatically, it will cool as it expands. As the parcel expands, the dew point of the parcel also changes in response to the pressure change. When the temperature of the air parcel and the dew point temperature are equal, condensation will occur, and a cloud will form. The dew point temperature represents the temperature at which an air parcel will be saturated with water vapor and condensation will occur. The height at which a parcel is lifted adiabatically and cools to the dew point temperature is known as the Lifting Condensation Level or the LCL. The LCL can also be obtained very easily from an adiabatic chart using the temperature, dew point temperature, and pressure. In the atmosphere, air is lifted and cools adiabatically when moist air is lifted up over a mountain or when moist air lifts up over dry air because the moist air is less dense, as in the case of an advancing cold front.

In the summer, the sky is often dominated by puffy cumulus clouds. While cumulus clouds often form as a result of adiabatic lifting, as in the case of air moving up a mountain side, they can also form from the convection of warm air heated at the surface of the earth. The term "lifting condensation level" is used in reference to this type of lifting as well. Once a parcel leaves the heat source, which is the ground, heat is no longer added, and the process is considered adiabatic. So, when you look up in the sky, the base of the lower forming cumulous clouds is considered the LCL.

The LCL will help us to evaluate atmospheric stability in Session 6. In fact, most of these parameters we are discussing are pieces of the process of determining atmospheric stability. In terms of developing storm systems, stability is the issue in which we are most concerned because it helps us predict the intensity of a storm . Atmospheric stability is also important in the study of air quality. The stability of the atmosphere influences the transport and dispersion and, ultimately, the concentrations of pollutants in the atmosphere.

Calculating the LCL

This link will provide you with a "pop-up" Javascript calculator that will calculate the properties of the lifting condensation level for a parcel lifted from the surface. Because this calculator is written in Javascript and uses frames, it requires Netscape 2.02 (or a more recent version).

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