When an air parcel moves to an environment of lower pressure (without heat exchange with surrounding air) its volume increases. Volume increase involves work and the consumption of energy; this reduces the heat available per unit volume and hence the temperature. Such a temperature change, involving no subtraction (or addition) of heat, is termed adiabatic. Vertical displacements of air are the major cause of adiabatic temperature changes.
Near the earth's surface, most temperature changes are non-adiabatic (also termed diabatic) because of energy transfer from the surface and the tendency of air to mix and modify its characteristics by lateral movement and turbulence. When an air parcel moves vertically, the changes that take place are generally adiabatic, because air is fundamentally a poor thermal conductor, and the air parcel tends to retain its own thermal identity, which distinguishes it from the surrounding air. However, in some circumstances, mixing of air with its surroundings must be taken into account.
Consider the changes that occur when an air parcel rises: the decrease of pressure (and density) cause its volume to increase and temperature to decrease (see Chapter 2B). The rate at which temperature decreases in a rising, expanding air parcel is called the adiabatic lapse rate. If the upward movement of air does not produce condensation, then the energy expended by expansion will cause the temperature of the mass to fall at the constant dry adiabatic lapse rate (DALR) (9.8°C/km). However, prolonged cooling of air invariably produces condensation, and when this happens latent heat is liberated, counteracting the dry adiabatic temperature decrease to a certain extent. Therefore, rising and saturated (or precipitating) air cools at a slower rate (the saturated adiabatic lapse rate (SALR)) than air that is unsaturated. Another difference between the dry and saturated adiabatic rates is that whereas the DALR is constant the SALR varies with temperature. This is because air at higher temperatures is able to hold more moisture and therefore on condensation releases a greater quantity of latent heat. At high temperatures, the saturated adiabatic lapse rate may be as low as 4°C/km, but this rate increases with decreasing temperatures, approaching 9°C/km at -40°C. The DALR is reversible (i.e. subsiding air warms at 9.8°C/km); in any descending cloud air, saturation cannot persist because droplets evaporate.
Three different lapse rates must be distinguished, two dynamic and one static. The static, environmental lapse rate (ELR) is the actual temperature decrease with height on any occasion, such as an observer ascending in a balloon would record (see Chapter 2C.1). This is not an adiabatic rate, therefore, and may assume any form depending on the local vertical profile of air temperature. In contrast, the dynamic adiabatic dry and saturated lapse rates (or cooling rates) apply to rising parcels of air moving through their environment. Above a heated surface, the vertical temperature gradient sometimes exceeds the dry adiabatic lapse rate (i.e. it is super-adiabatic). This is common in arid areas in summer. Over most ordinary dry surfaces, the lapse rate approaches the dry adiabatic value at an elevation of 100 m or so.
The changing properties of rising air parcels may be determined by plotting path curves on suitably constructed graphs such as the skew T-logp chart and the tephigram, or T-^-gram, where ^ refers to entropy. A tephigram (Figure 5.1) displays five sets of lines representing properties of the atmosphere:
1 Isotherms - i.e. lines of constant temperature (parallel lines from bottom left to top right).
2 Dry adiabats (parallel lines from bottom right to top left).
3 Isobars - i.e. lines of constant pressure and corresponding height contours (slightly curved nearly horizontal lines).
4 Saturated adiabats (curved lines sloping up from right to left).
5 Saturation mixing ratio lines (at a slight angle to the isotherms).
Air temperature and dew-point temperature, determined from atmospheric soundings, are the variables that
Figure S.I Adiabatic charts such as the tephigram allow the following properties of the atmosphere to be displayed: temperature, pressure, potential temperature, wet-bulb potential temperature and saturation (humidity) mixing ratio.
are commonly plotted on an adiabatic chart. The dry adiabats are also lines of constant potential temperature, 6 (or isentropes). Potential temperature is the temperature of an air parcel brought dry adiabatically to a pressure of 1000 mb. Mathematically,
The relationship between T and 6, also between T and 6w, the wet-bulb potential temperature (where the air parcel is brought to a pressure of 1000 mb by a saturated adiabatic process), is shown schematically in Figure 5.2. Potential temperature provides an important yardstick for airmass characteristics, since if the air is affected only by dry adiabatic processes the potential temperature remains constant. This helps to identify different airmasses and indicates when latent heat has been released through saturation of the airmass or when non-adiabatic temperature changes have occurred.
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