Figure 5.2 illustrates an important property of the tephigram. A line along a dry adiabat (6) through the dry-bulb temperature of the surface air (TA), an isopleth of saturation mixing ratio (xs) through the dew-point (Td), and a saturated adiabat (6w) through the wet-bulb temperature (Tw), all intersect at a point corresponding to saturation for the airmass. This relationship, known
as Normand's theorem, is used to estimate the lifting condensation level (see Figure 5.3). For example, with an air temperature of 20°C and a dew-point of 10°C at 1000 mb surface pressure (Figure 5.1), the lifting condensation level is at 860 mb with a temperature of 8°C. The height of this 'characteristic point' is approximately h (m) = 120(T- Td)
where T = air temperature and Td = dew-point temperature at the surface in °C.
The lifting condensation level (LCL) formulation does not take account of vertical mixing. A modified calculation defines a convective condensation level (CCL). In the near-ground layer surface heating may establish a superadiabatic lapse rate, but convection modifies this to the DALR profile. Daytime heating steadily raises the surface air temperature from T0to Tv T2 and T3 (Figure 5.4). Convection also equalizes the humidity mixing ratio, assumed equal to the value for the initial temperature. The CCL is located at the intersection of the environment temperature curve with a saturation mixing ratio line corresponding to the average mixing ratio in the surface layer (1000 to 1500 m). Expressed in another way, the surface air temperature is the minimum that will allow cloud to form as a result of free convection. Because the air near the surface is often well mixed, the CCL and LCL, in practice, are commonly nearly identical.
Experimentation with a tephigram shows that both the convective and the lifting condensation levels rise as the surface temperature increases, with little change of dew-point. This is commonly observed in the early afternoon, when the base of cumulus clouds tends to be at higher levels.
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