1. The Varying Value of the Saturated Adiabatic Lapse RateThe dry
adiabatic lapse rate (DALR) has a constant value of 1 °C/100 m. The
saturated adiabatic lapse rate (SALR) is usually about half that value,
because the release of latent heat as saturated air rises partially
offsets the cooling by expansion. But unlike the value of the dry
adiabatic lapse rate, the SALR is not constant. Rather, it depends on
the temperature of the saturated air parcel, with higher temperatures
causing lower lapse rates. Figure 5-6-1 can help us see why this is the
case. Recall that if air is saturated, its actual specific humidity is
equal to the saturation specific humidity. As a rising parcel of
saturated air is cooled adiabatically, the amount of wate vapor that can
exist decreases and the surplus water vapor is removed by condensation
(or deposition).Now observe what happens when the temperature of warm,
saturated air decreases 5 °C (9 °F), from 30 °C to 25 °C. A: the air
cools, its specific humidity changes from 27.7 grams of vapor per
kilogram of air to 20.4—a decrease of 7.3 grams of vapor per kilogram of
air. The 7.3 g of water vapor do not simply vanish; rather, they are
converted to an equal mass of liquid water. Upon condensation, each gram
ofwater releases 2500 joules of latent heat, for a total of 18,250 J.
Compare that to what happens if the temper ature undergoes another 5 °C
drop in temperature, but this time from 5 °C to 0 °C.At this lower
temperature, the 5° of cooling reduces the water vapor content from 5.5
to 3.8 g/kg–only a 17 g decrease–and only 4250 J of energy are released
to the air. Thus, at low temperatures relatively little latent heat is
released to offset the cooling due to expansion, and the SALR is nearly
equal to the DALR. When warm, saturated air is lifted, a greater amount
of latent heat is available to offset the cooling by expansion, and the
SALR assumes a lower value.How does temperature affect the value of the
saturated adiabatic lapse rate?FIGURE 5-6-1 Saturated Adiabatic Lapse
Rate (SLR). Unlike the DALR, the SALR is not a constant value. When
warm, saturated air cools, it causes more condensation (and hence more
latent heat release) than does cold, saturated air. For example, if
saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific
humidity decreases from 27.7 grams of water vapor per kilogram of air to
20.4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific
humidity only 1.7 grams for each kilogram of air. This brings about less
warming to offset the cooling by expansion, as well as a greater
saturated adiabatic lapse rate.... Get solution
2. The Varying Value of the Saturated Adiabatic Lapse RateThe dry adiabatic lapse rate (DALR) has a constant value of 1 °C/100 m. The saturated adiabatic lapse rate (SALR) is usually about half that value, because the release of latent heat as saturated air rises partially offsets the cooling by expansion. But unlike the value of the dry adiabatic lapse rate, the SALR is not constant. Rather, it depends on the temperature of the saturated air parcel, with higher temperatures causing lower lapse rates. Figure 5-6-1 can help us see why this is the case. Recall that if air is saturated, its actual specific humidity is equal to the saturation specific humidity. As a rising parcel of saturated air is cooled adiabatically, the amount of wate vapor that can exist decreases and the surplus water vapor is removed by condensation (or deposition).Now observe what happens when the temperature of warm, saturated air decreases 5 °C (9 °F), from 30 °C to 25 °C. A: the air cools, its specific humidity changes from 27.7 grams of vapor per kilogram of air to 20.4—a decrease of 7.3 grams of vapor per kilogram of air. The 7.3 g of water vapor do not simply vanish; rather, they are converted to an equal mass of liquid water. Upon condensation, each gram ofwater releases 2500 joules of latent heat, for a total of 18,250 J. Compare that to what happens if the temper ature undergoes another 5 °C drop in temperature, but this time from 5 °C to 0 °C.At this lower temperature, the 5° of cooling reduces the water vapor content from 5.5 to 3.8 g/kg–only a 17 g decrease–and only 4250 J of energy are released to the air. Thus, at low temperatures relatively little latent heat is released to offset the cooling due to expansion, and the SALR is nearly equal to the DALR. When warm, saturated air is lifted, a greater amount of latent heat is available to offset the cooling by expansion, and the SALR assumes a lower value.Why isn’t the saturated adiabatic lapse rate constant, like the dry adiabatic lapse rate?FIGURE 5-6-1 Saturated Adiabatic Lapse Rate (SLR). Unlike the DALR, the SALR is not a constant value. When warm, saturated air cools, it causes more condensation (and hence more latent heat release) than does cold, saturated air. For example, if saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific humidity decreases from 27.7 grams of water vapor per kilogram of air to 20.4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific humidity only 1.7 grams for each kilogram of air. This brings about less warming to offset the cooling by expansion, as well as a greater saturated adiabatic lapse rate.... Get solution
2. The Varying Value of the Saturated Adiabatic Lapse RateThe dry adiabatic lapse rate (DALR) has a constant value of 1 °C/100 m. The saturated adiabatic lapse rate (SALR) is usually about half that value, because the release of latent heat as saturated air rises partially offsets the cooling by expansion. But unlike the value of the dry adiabatic lapse rate, the SALR is not constant. Rather, it depends on the temperature of the saturated air parcel, with higher temperatures causing lower lapse rates. Figure 5-6-1 can help us see why this is the case. Recall that if air is saturated, its actual specific humidity is equal to the saturation specific humidity. As a rising parcel of saturated air is cooled adiabatically, the amount of wate vapor that can exist decreases and the surplus water vapor is removed by condensation (or deposition).Now observe what happens when the temperature of warm, saturated air decreases 5 °C (9 °F), from 30 °C to 25 °C. A: the air cools, its specific humidity changes from 27.7 grams of vapor per kilogram of air to 20.4—a decrease of 7.3 grams of vapor per kilogram of air. The 7.3 g of water vapor do not simply vanish; rather, they are converted to an equal mass of liquid water. Upon condensation, each gram ofwater releases 2500 joules of latent heat, for a total of 18,250 J. Compare that to what happens if the temper ature undergoes another 5 °C drop in temperature, but this time from 5 °C to 0 °C.At this lower temperature, the 5° of cooling reduces the water vapor content from 5.5 to 3.8 g/kg–only a 17 g decrease–and only 4250 J of energy are released to the air. Thus, at low temperatures relatively little latent heat is released to offset the cooling due to expansion, and the SALR is nearly equal to the DALR. When warm, saturated air is lifted, a greater amount of latent heat is available to offset the cooling by expansion, and the SALR assumes a lower value.Why isn’t the saturated adiabatic lapse rate constant, like the dry adiabatic lapse rate?FIGURE 5-6-1 Saturated Adiabatic Lapse Rate (SLR). Unlike the DALR, the SALR is not a constant value. When warm, saturated air cools, it causes more condensation (and hence more latent heat release) than does cold, saturated air. For example, if saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific humidity decreases from 27.7 grams of water vapor per kilogram of air to 20.4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific humidity only 1.7 grams for each kilogram of air. This brings about less warming to offset the cooling by expansion, as well as a greater saturated adiabatic lapse rate.... Get solution
