Adiabatic lapse rate

[Leitwolf]

The adiabatic lapse rate is the key component of the GHE. Regrettably I do not seem to fully understand it and so I need to ask for help.

The average lapse rate on Earth is about 6.5K/1km. The "dry" lapse rate however is said to be 9.8K/km. So vapor (and the latent heat it transports) reduces the lapse rate significantly, in this way it also serves as a kind of anti-GHG. I mean if we understand the GHE as a combination of an elevated emission layer (photosphere) where we have 255K and the adiabatic lapse rate, which produces higher temperatures at the surface. Anyhow, the exact impact of vapor on the lapse rate is a key question.

Now I found this sightly confusion chart on the subject on wikipedia. There are some things I understand, and some things I don't. For instance we have lines for altitude (scale on the right) which are sloped. That is to be understood relative to the left scale of pressure, meaning with cold air pressure will decrease faster with altitude, as it has a higher density.

https://en.wikipedia.org/wiki/Lapse_rate#/media/File:Emagram.GIF

Now if I look at the bold line for the dry adiabat, for instance the one starting at 20°C, it intersects the 5km line at roughly -24°C. That are 44K for 5km, or 10.8K/km, significantly more than the 9.8K quoted before. Assuming the chart is right, what causes this difference? Is it because the troposphere is naturally unstable and heated at the surface, so to say?

Then the "wet" lapse rate starting at +15°C intersects the 5km line at about -14.5°C, meaning a delta of 29.5K or 5.9K/km. +15°C roughly corresponds to the actual surface temperatures on Earth, yet 5.9K/km is significantly less than the quoted 6.5K. Why is that? Is it representing a more theoretical perfectly wet atmosphere with a 100% H2O saturation??

[lookingnorth]

On 12/13/2020 at 10:46 AM, Leitwolf said:

The adiabatic lapse rate is the key component of the GHE. Regrettably I do not seem to fully understand it and so I need to ask for help.

The average lapse rate on Earth is about 6.5K/1km. The "dry" lapse rate however is said to be 9.8K/km. So vapor (and the latent heat it transports) reduces the lapse rate significantly, in this way it also serves as a kind of anti-GHG. I mean if we understand the GHE as a combination of an elevated emission layer (photosphere) where we have 255K and the adiabatic lapse rate, which produces higher temperatures at the surface. Anyhow, the exact impact of vapor on the lapse rate is a key question.

Now I found this sightly confusion chart on the subject on wikipedia. There are some things I understand, and some things I don't. For instance we have lines for altitude (scale on the right) which are sloped. That is to be understood relative to the left scale of pressure, meaning with cold air pressure will decrease faster with altitude, as it has a higher density.

https://en.wikipedia.org/wiki/Lapse_rate#/media/File:Emagram.GIF

Now if I look at the bold line for the dry adiabat, for instance the one starting at 20°C, it intersects the 5km line at roughly -24°C. That are 44K for 5km, or 10.8K/km, significantly more than the 9.8K quoted before. Assuming the chart is right, what causes this difference? Is it because the troposphere is naturally unstable and heated at the surface, so to say?

Then the "wet" lapse rate starting at +15°C intersects the 5km line at about -14.5°C, meaning a delta of 29.5K or 5.9K/km. +15°C roughly corresponds to the actual surface temperatures on Earth, yet 5.9K/km is significantly less than the quoted 6.5K. Why is that? Is it representing a more theoretical perfectly wet atmosphere with a 100% H2O saturation??

I have no idea why the dry adiabatic lapse rate is so great in the chart. I've only ever used Skew-T log-P charts, not emagrams. However, the moist adiabatic lapse rate varies significantly based on temperature, and 5.9K/km is well within the range of possibility. The average environmental lapse rate is not the same as the average moist lapse rate because sometimes the atmosphere will be dry adiabatic, sometimes there will be an inversion, etc. But since the atmosphere is often moist adiabatic, the two happen to be similar.

[Moderately Unstable]

On 12/13/2020 at 10:46 AM, Leitwolf said:

The adiabatic lapse rate is the key component of the GHE. Regrettably I do not seem to fully understand it and so I need to ask for help.

The average lapse rate on Earth is about 6.5K/1km. The "dry" lapse rate however is said to be 9.8K/km. So vapor (and the latent heat it transports) reduces the lapse rate significantly, in this way it also serves as a kind of anti-GHG. I mean if we understand the GHE as a combination of an elevated emission layer (photosphere) where we have 255K and the adiabatic lapse rate, which produces higher temperatures at the surface. Anyhow, the exact impact of vapor on the lapse rate is a key question.

Now I found this sightly confusion chart on the subject on wikipedia. There are some things I understand, and some things I don't. For instance we have lines for altitude (scale on the right) which are sloped. That is to be understood relative to the left scale of pressure, meaning with cold air pressure will decrease faster with altitude, as it has a higher density.

https://en.wikipedia.org/wiki/Lapse_rate#/media/File:Emagram.GIF

Now if I look at the bold line for the dry adiabat, for instance the one starting at 20°C, it intersects the 5km line at roughly -24°C. That are 44K for 5km, or 10.8K/km, significantly more than the 9.8K quoted before. Assuming the chart is right, what causes this difference? Is it because the troposphere is naturally unstable and heated at the surface, so to say?

Then the "wet" lapse rate starting at +15°C intersects the 5km line at about -14.5°C, meaning a delta of 29.5K or 5.9K/km. +15°C roughly corresponds to the actual surface temperatures on Earth, yet 5.9K/km is significantly less than the quoted 6.5K. Why is that? Is it representing a more theoretical perfectly wet atmosphere with a 100% H2O saturation??

So, first, water vapor is actually a ghg, in the sense that it blocks outgoing lw radiation. In fact, water vapor is our MOST important ghg. Look it up on Google. The reason we talk about CO2 & CH4 is because we produce those en masse and they have a long lifetime. 

The reason earth's avg temperature is 287 K instead of 255 isn't due to the lapse rate. The lapse rate defines how temperature changes but doesn't cause the changes. The dry adiabatic lapse rate is a conceptualized rate of temperature change in which heat is not added or released to an air parcel. Using the ideal gas law, changes to the parcel temperature depend on changes to volume and pressure. Convection, the process of warm air with higher internal energy rising, expanding, and cooling, is responsible for the dry adiabatic lapse rate. As air rises and expands, it does work (1st law of thermo), and cools as it loses internal energy U. The IGL has a different constant for dry air vs moist air which basically is to say, moist air has more interal energy and therefore requires a moist parcel do more work to cool, hence the lower moist adiabatic lr.

 

Ok so onto the skew t. Earth's sfc T avg is 287.15 kelvins. For both of these measures it seems you are off mostly because of a calculation error. For the dry adiabat to me it looks like around -34, perhaps you followed the wrong line...the 30 degree line perhapsas that appears to intersect at -24? If you plug that in, we have a delta of 47 (14.15 sfc, ~-34 @5km). 47/5=9.6. Close enough to 9.8 that I expect the plot likely is showing a 9.8 lapse rate and I just can't see that fine grained of a resolution. 

Let me know if this helps.