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Author Question: Why is the saturated adiabatic lapse rate less than that of dry adiabatic lapse rate? (Read 1706 times)

Hungry!

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on: Jun 21, 2013
This is a question that is on my Weather and Climate homework.  The teacher wants to know why the saturated adiabatic lapse rate is 6 degrees per 1,000m and why the dry adiabatic rate is 10 degrees per 1,000m...why the saturated one is less than the dry one.  Can someone please explain this to me?


Millan

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Reply #1 on: Jun 21, 2013
The dry adiabatic lapse rate is a near constant of 9.8 C/km, however, the wet adiabatic lapse rate is much less of a constant. The wet adiabatic lapse rate varies from about 4 C/km to nearly 9.8 C/km. The slope of the wet adiabats depend on the moisture content of the air. The more moisture (water vapor) that is in the air, the more latent heat that can be released when condensation takes place (the release of latent heat warms the parcel while an absorption of latent heat cools the parcel). Any warming by latent heat release partially offsets the cooling of rising air. If you look at a Skew-T diagram,  notice that the dry and wet adiabats become nearly parallel in the upper troposphere. This is due to the very cold temperatures aloft (cold air does not have much water vapor and therefore can not release much latent heat) The slope of the wet adiabats is 4 to 5 C/km in very warm and humid air (lifting of this saturated air releases a large amount of latent heat).

Warm and humid air in the PBL contributes to atmospheric instability. These warm and humid parcels, since they only cool slowly with height, have a good chance of remaining warmer than the surrounding environmental air and will thus continue to rise. In fact, planetary boundary layer warm air advection and moisture advection are the number 1 contributions to making the troposphere thermodynamically unstable (High convective air potential energy (CAPE), and negative lifted index).

I hope this helps. If you need a less technical explanation, let me know.


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