Lifted condensation level

The lifted condensation level or lifting condensation level (LCL) is formally defined as the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to liquid water when it is cooled by dry adiabatic lifting. The RH of air increases when it is cooled, since the amount of water vapor in the air (i. e., its specific humidity) remains constant, while the saturation vapor pressure decreases almost exponentially with decreasing temperature. If the air parcel is lifting further beyond the LCL, water vapor in the air parcel will begin condensing, forming cloud droplets. (In the real atmosphere, it is usually necessary for air to be slightly supersaturated, normally by around 0.5%, before condensation occurs; this translates into about 10 meters or so of additional lifting above the LCL.) The LCL is a good approximation of the height of the cloud base which will be observed on days when air is lifted mechanically from the surface to the cloud base (e. g., due to convergence of airmasses).

There are also many different ways to approximate the LCL, to various degrees of accuracy. The most well known and widely used among these is Espy's equation, which James Espy formulated already in the early 19th century. His equation makes use of the relationship between the LCL and dew point temperature discussed above. In the Earth's atmosphere near the surface, the lapse rate for dry adiabatic lifting is about 9.8 K/km, and the lapse rate of the dew point is about 1.8 K/km (it varies from about 1.6 1.9 K/km). This gives the slopes of the curves shown in the diagram. The altitude where they intersect can be computed as the ratio between the difference in the initial temperature and initial dew point temperature to the difference in the slopes of the two curves. Since the slopes are the two lapse rates, their difference is about 8 K/km. Inverting this gives 0.125 km/K, or 125 m/K. Recognizing this, Espy pointed out that the LCL can be approximated as:

where is height of the LCL (in meters), is temperature in degrees Celsius (or kelvins), and is dew point temperature (likewise in degrees Celsius or kelvins, whichever is used for T). This formula is accurate to within about 1% for the LCL height under normal atmospheric conditions, but requires knowing the dew point temperature.

The convective condensation level (CCL) results when strong surface heating causes buoyant lifting of surface air and subsequent mixing of the planetary boundary layer, so that the layer near the surface ends up with a dry adiabatic lapse rate. As the mixing becomes deeper, it will get to the point where the LCL of an air parcel starting at the surface is at the top of the mixed region. When this occurs, then any further solar heating of the surface will cause a cloud to form topping the well mixed boundary layer, and the level at which this occurs is called the CCL. If the boundary layer starts off with a stable temperature profile (that is, with a lapse rate less than the dry adiabatic lapse rate), then the CCL will be higher than the LCL. In nature, the actual cloud base is often initially somewhere between the LCL and the CCL. If a thunderstorm forms, then as it grows and matures, processes such as increased saturation at lower levels from precipitation and lower surface pressure usually lead to a lowering of the cloud base. Finally, the LCL can also be considered in relation to the level of free convection (LFC). A smaller difference between the LCL and LFC (LCL LFC) is conducive to the rapid formation of thunderstorms. 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), after which deep, moist convection ensues and air parcels buoyantly rise in the positive area of a sounding, accumulating convective available potential energy (CAPE) until reaching the equilibrium level (EL).

Bohren, C. F., and B. Albrecht, Atmospheric Thermodynamics, Oxford University Press, 1998. M K Yau and R. R. Rogers, Short Course in Cloud Physics, Third Edition, published by Butterworth Heinemann, January 1, 1989, 304 pages.