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:
where h is pressure height of LCL, T is temperature in degrees Fahrenheit, Td is dew point temperature in degrees Fahrenheit.
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.
The impact on simulated storm structure and intensity of variations in the mixed layer and moist layer depths
Jul 01, 2002; ABSTRACT The sensitivities of convective storm structure and intensity to variations in the depths of the prestorm mixed layer,...
Comments on "Seasonal Variation of the Physical Properties of Marine Boundary Layer Clouds off the California Coast"/ Reply to Comments on "Seasonal Variation of the Physical Properties of Marine Boundary Layer Clouds off the California Coast"
Jun 15, 2010; (ProQuest: denotes formulae omitted.) YANGANG LIU Brookhaven National Laboratory, Upton, New York (Manuscript received 2...
A Numerical Study of the Interaction between the Large-Scale Monsoon Circulation and Orographic Precipitation over South and Southeast Asia
Apr 01, 2012; ABSTRACT A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000...