A lapse rate is the change of temperature with unit rise of elevation, and the rate at any particular level is given by the temperature profile (or 'sounding'), measured by radiosonde (Section 1.6). Typical profiles near the ground in the valley of the Parramatta River near Sydney are shown in Figure 7.1. At about 8.40 a.m., for instance, there is a fall of temperature with increased elevation within the lowest 100 metres (i.e. a positive lapse rate), then an inversion layer up to about 180 metres, surmounted by a roughly isothermal layer, i.e. one of equal temperature. The slope of the measured profile at any level in Figure 7.1 is the environmental lapse rate (ELR) of temperature. A corresponding dewpoint lapse rate (DLR) at that level can be derived from the measured profile of dewpoint temperatures (Note 6.I).
We use the ELR value for comparison with two theoretical lapse rates, in order to assess the stability of an atmospheric layer. The first of these is the dry adiabatic lapse rate (DALR), the rate of cooling of a parcel of air lifted adiabatically, i.e. fast enough for there to be no time for heat to flow in or out of the parcel. (By a 'parcel' of air, we mean an amount like that inside a limp balloon, it being assumed that no air enters or leaves the parcel.) The DALR depends solely on the physical properties of air, as it expands on rising to levels of lower pressure. It is nearly 10 K/km (Note 7.B). This figure is used in designing conditions inside large aircraft.
The DALR allows one to calculate the potential temperature of air at any elevation z (km). This is the temperature dry air would have if it were lowered adiabatically to the level at which the atmospheric pressure is 1,000 hPa, i.e. to about sea-level. The potential temperature is calculated by adding 10 z degrees to the air temperature measured at the height z.
It can be inferred that the DALR line joins points representing the same potential temperature, and the potential temperature of dry air does not change when it rises or falls
adiabatically. In other words, this property of air is conservative, i.e. it is one of those unchanging features which simplify what otherwise is a complicated subject. Other such features include the mixing ratio (Section 6.2), provided there is no evaporation or condensation. The potential temperature is a measure of the inherent heat content of dry air, with the height factor removed.
The following example illustrates how we use the concept of potential temperature. Imagine dry air at a temperature of 10°C at 1 km height on the upwind side of a mountain range, and air with a temperature of 20°C at sea-level in the lee. One might think that the air on the windward side is becoming colder, so that people at the coast should brace themselves for chilly weather. But both air masses have the same potential temperature (20°C), so the difference of temperature is not due to a change of weather, but simply to the different elevations.
An even more conservative property is the equivalent potential temperature, which is the equivalent temperature (Section 4.3) of air brought down to sea level adiabatically (Note 7.C).
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