Let us assume that during the afternnon the surface warms sufficiently to produce convection cells that reach to the marine layer's inversion base (nominally at 500 meters).  If the air below the inversion base is thoroughy mixed, the water vapor mixing ratio will be constant throughout.  Hence, the dew point temperature will be constant throughout.  

As the sun sets, the surface will begin to cool due to thermal radiation escaping to space.  Also, because the marine layer contains water vapor, it also will radiatively cool.   Thus, the temperature profile chnages in two ways:  1) the surface-based superadiabatic layer disappears and is replaced by a ground-based inversion, and 2) the marine layer above the ground-based inversion will cool at an approximate uniform rate.  This is shown in the following figure, indicated by the "6P" and "8P" traces.

Figure 1.  Stratus formation temperature profile sequence.

Later, suppose at 10 PM, the top of the marine layer cools to below the dew point temperature (see trace "10P" in Fig. 1).  Since the air cannot "contain" more water vapor than the "saturation vapor density" assoicated with its temperature, whenever air cools below its dew point temperature water changes phase from vapor to liquid by an amount that maintains the vapor densiity at its saturation vaolue.  For example, at time "10P" in Fig. 1, the air at 500 meters (or just below the inversion base) is at 12.4 C whereas its dew point temperature when condensation began was 12.9 C.  The saturation vapor densities at 12.4 C and 12.9 C are 11.01 and 11.37 [g/m3].  The difference is 0.34 [g/m3].  Thus, at this altitude vapor density is 11.01 [g/m3] and liquid density is 0.34 [g/m3], and the sum of the two water phases equals 11.37 [g/m3], the value it had prior to the onset of condensation.

Furthermore, at 10 PM the stratus has descended to an altitude of 300 meters, where liquid density, or "Liquid Water Concentration" LWC, is just changing from zero to positive values.

At 2 AM the stratus layer base has descended to 70 meters, and at the stratus top the LWC value is now 0.77 [g/m3].  This value is obtained by differencing 11.37 [g/m3] and the saturation vapor density corresponding to the layer's temperature of 11.8 C.  At 11.8 C the air is able to contain only 10.60 [g/m3], so 11.37 [g/m3] - 10.60 [g/m3] = 0.76 [g/m3].  
 
 

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