1. Outline Further Reading: Chapter 06 of the text book - adiabatic processes - dry and wet adiabatic lapse rates - ascent versus descent Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (1 of 10)

2. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (2 of 10) Introduction Previously, we discussed various quantities associated with gas phase of water in the atmosphere –Specific humidity –Saturation humidity –Relative humidity –Dew Point Important points –Air can only hold a finite amount of water vapor; the amount depends on the temperature of the air –Any change of phase requires or releases energy Now we want to talk about the liquid phase of water in the atmosphere -> this will introduce the subject of clouds and precipitation

3. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (3 of 10) Adiabatic Processes Clouds form when the capacity of the atmosphere to hold water vapor is exceeded –Excess water must go into liquid –Usually does not happen because H2O is added but because the air is cooled As the air cools, saturation humidity goes down so the relative humidity goes up –Primary method of cooling the air? Lifting Adiabatic process: Process in which there is no external source or sink of energy No solar heating No longwave heating There can be latent heating Can allow latent heating because this represent internal energy within the atmosphere Essentially, we are talking about process where the air parcel does not interact with the surrounding air

4. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (4 of 10) Pressure-Temperature Relation –If we raise an air parcel, the pressure of the air around the parcel decreases –The parcel expands and the temperature decreases

5. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (5 of 10) Adiabatic Lapse Rate Adiabatic Lapse Rate: The temperature change of a hypothetical parcel of air as it moves up in the atmosphere Dry adiabatic lapse rate - the temperature change of a parcel of unsaturated air –Dry adiabatic lapse rate represents the temperature change a parcel will experience if it does not interact with the air around it –It is equal to 10-degrees/km –Constant –This is for a hypothetical parcel (i.e. it is not measured)

6. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (6 of 10) Dry Adiabatic Lapse Rate-Examples Example 1 –If T(ground) = 295K and we raise the parcel until T=280K, how high have we raised the parcel? Example 2 If T(2km) = 255K and we move the parcel until T=275K:  Have we moved the parcel up or down?  How far?

7. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (7 of 10) Lifting Condensation Level Level at which clouds will form –Clouds form when T(air) = T(Dew point) –By lifting the air, T(air) decreases –T(dew point) also changes, but not as much –Lapse rate for T(dew point) = 1.8K/1km; –Lapse rate for T(air) = 10K/1km –Therefore, at some point T(air)=T(dew point), i.e. the parcel is saturated If we keep lifting after clouds form, air will continue to cool because of the adiabatic process But now water vapor is also being converted to liquid –Releases latent heat of vaporization –This heats the parcel somewhat

8. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (8 of 10) Moist Adiabatic Lapse Rate-1 Moist adiabatic lapse rate: The temperature change of a parcel of saturated air as it rises –“Adiabatic” because part of the temperature change is due to the same processes as a dry parcel (i.e. temperature changes because the pressure around the parcel changes) –“Moist” because part of the temperature change is due to water going from vapor to liquid

9. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (9 of 10) Moist Adiabatic Lapse Rate-2 300290 1 km 2 km 280 Temperature Height Cooling due to Expansion (dry adiabatic lapse rate Warming due to Latent Heat Release Moist Adiabatic Lapse Rate –The adiabatic process changes the temperature by 10K/1km –Conversion of vapor to liquid releases heat and also leads to a temperature change –The moist adiabatic lapse rate is not constant Depends upon surrounding temperature and pressure Usually approximated as 6K/1km

10. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 13: Lapse-Rates Feb-23-05 (10 of 10) Ascent vs Descent –Let’s follow a parcel as it rises above the LCL then comes back down –Initially the parcel’s temperature changes according to the dry adiabatic lapse rate –Once it hits the LCL however, condensation releases latent heat that warms the parcel somewhat as it rises, hence it follows the moist adiabatic lapse rate and it’s temperature doesn’t change as much with height –As the parcel descends, however, there is no condensation so there is no gain or release of latent heat; hence the temperature change is purely due to pressure effects, i.e. it follows the dry adiabatic lapse rate as it warms –Note, when it reaches the bottom, it is warmer than when it started; this is due to warming as latent heat is released –Also, note that the specific humidity will be lower, as will the relative humidity, hence the parcel will be drier Lifting Condensation Level Movie