Wet adiabats

It is important not to confuse this dry adiabatic lapse rate with the rate of change in temperature with height in a Standard Atmosphere. The latter represents average conditions in Earth s atmosphere, where heating, mixing, and wet adiabatic processes also are occurring. 

Wet adiabatic lapse rates can be determined from Fig. 4-7, which is a skew T-log P diagram (or adiabatic chart). On this chart, dry adiabats are lines having a nearly constant slope of 9.8°C/1000 m (5.4°F/1000 ft). The wet adiabats are curved and have slopes that not only vary with the temperature at which the adiabat originates but also change along the length of the adiabats. Note that the wet adiabats tend to approach the slope of the dry adiabats at low temperatures, where the absolute amount of moisture in saturated air is small (see Table 4-3). 

Frequently, both kinds of adiabats must be employed to follow the behavior of a parcel of air. A parcel of dry air may rise by its own buoyancy, or may be pushed up by orographic lifting, which occurs when winds meet mountains and are deflected upward. The temperature of the air parcel follows the dry adiabat until water vapor condensation is incipient with further cooling, condensation can occur. If it is assumed that supercooling (a nonequilibrium situation in which air cools below the dew point without condensation) does not occur, the parcel of air then moves upward following the corresponding wet adiabat. Conditional stability refers to conditions under which dry,... 

FIGURE 4-7 The skew T-log P diagram, or adiabatic chart. On this chart all lines of temperature versus altitude for dry conditions (i.e., no condensation of water vapor) are nearly straight lines with a slope corresponding to the dry adiabatic lapse rate of 9.8°C/1000 m (5.4°F/1000 ft). These are called dry adiabats and slope upward to the left. Wet adiabats have a variable slope and appear as curved lines. Horizontal lines denote altitude lines of constant temperature slope upward to the right. 

On a summer afternoon, air heated over a plowed field 1000 m above sea level has a temperature of 25°C and RH of 80%. At what altitude does the rising air begin to form a cloud What is the air temperature 2000 m above the cloud base (inside the cloud) What is the average wet adiabatic lapse rate in this portion of the cloud ... 

Use Fig. 4-7, beginning at 25°C and 1000 m, and follow the slope of the dry adiabats until the air parcel reaches 21°C, which occurs at an altitude of approximately 1500 m. (Strictly, it must cool a bit more, because the expansion of the rising air causes the partial pressure of water to decrease somewhat.) Thereafter, the air follows a wet adiabat at 3500 m (2000 m above the cloud base) the temperature is approximately 12°C. The average wet adiabatic lapse rate in the bottom 2000 m of cloud is therefore 9°C/2000 m, or 4.5°C/1000 m. 

The rapidity with which temperature decreases with altitude. The normal lapse rate is defined to be 3.6 degrees F per 1000 feet change in altitude. The dry adiabatic lapse rate is about 5.5 degrees F per 1000 feet, and the wet adiabatic lapse rate varies between 2 and 5 degrees F per 1000 feet, latent heat... 

C per 100 m this is called the dry adiabatic lapse rate (DALR), or dTldz. As soon as the air parcel is saturated by water vapor, it partly condenses and is then heated by the released heat. Then, the wet adiabatic temperature gradient (lapse rate) is observed, which lies between 0.4 °C at large temperatures and 1 °C for low temperatures. During adiabatic changes, the potential tew/jeramrc remains constant,... 

Air saturated with water vapor releases heat as it cools, and liquid water eondenses. Thus the adiabatic temperature decrease is greater than that given by Eq. (1). The wet adiabatic temperature gradient ranges from about -0.9°C per... 

A rising parcel of dry air containing water vapor will continue to cool at the dry adiabatic lapse rate until it reaches its condensations temperature, or dew point. At this point, the pressure of the water vapor equals the saturation vapor pressure of the air, and some of the water vapor begins to condense. Condensation releases latent heat in the parcel, and thus the cooling rate of the parcel slows. This new rate is called the wet adiabatic lapse rate. Unlike the dry adiabatic lapse rate, the wet adiabatic lapse rate is not constant but depends on temperature and pressure. In the middle troposphere, however, it is assumed to be approximately -6 to -7°C/1000 m. 

In the formation of the puffy, fair-weather clouds known as cumulus clouds, solar heating of the ground warms the adjacent air, and bubbles of this warmed air rise due to buoyancy, their temperature decreasing with altitude according to the dry adiabatic lapse rate (Fig. 4.8). At the altitude where the dew point is reached and condensation begins, a cloud forms, and the stability of the air decreases as the air begins to follow a wet adiabat. Instability can lead to extensive vertical development of the cloud and strong upward air currents within the cloud. On hot summer days, this can lead to the production of a late-afternoon thunderstorm. 

Wet adiabatic lapse rate 5 °C per km dry adiabatic lapse rate 10 °C per km. 

For a height difierence of 2500 m, the extreme lapse rates are 12° in the case of wet adiabatic, which is apparently not the case and 25° in the case of dry adiabatic, nearer to reality in this semi-arid region. 

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