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
adiabaticcooling (caused by expansion) and condense into cloud. It approximates the height of cloud basewhen there is mechanical forcing.
Method of finding
Meteorologists determine the LCL on
thermodynamic diagrams, such as a Skew-T log-P diagramor the Tephigram, as follows:
# Start at the initial
temperature(T) and pressure of the parcel and follow upward the dry adiabatic lapse rateline if the air is not saturated. Otherwise, the parcel is already at or above LCL.
# From the
dew pointtemperature (Td) of the parcel, follow upward the mixing ratioline at that point.
# At the intersection of the two lines is the LCL.
potential temperatureof the parcel remains the same, as it is done adiabatically (no exchange of heatwith 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 parcelrises, its temperature decreases while its moisturecontent remains constant, eventually reaching the point of saturation. It is the point where the temperature and dew point are equivalent, where relative humidityis 100%.
The LCL is the level where a parcel rising dry adiabatically from the surface (the
mixed layerand boundary layer) intersects the saturation mixing ratioline 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 buoyantlifting spontaneously to the point of saturation when the convective temperatureis 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 lift, since they'll reach their LFC more quickly and easily.
* 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
* [http://theweatherprediction.com/habyhints/299/ LCL tutorial]
* [http://theweatherprediction.com/habyhints/307/ SKEW-T: A LOOK AT SBLCL ]
* [http://amsglossary.allenpress.com/glossary/search?id=lifting-condensation-level1 Lifting condensation level (LCL)] (Glossary of Meteorology)
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