In our blog of July 11, we introduced the concept of moist enthalpy (see also Pielke, R.A. Sr., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211. ). This is an important climate change metric, since it illustrates why surface air temperature alone is inadequate to monitor trends of surface heating and cooling. Heat is measured in units of Joules. Degrees Celsius is an incomplete metric of heat.
Surface air moist enthalpy does capture the proper measure of heat. It is defined as CpT + Lq where Cp is the heat capacity of air at constant pressure, T is air temperature, L is the latent heat of phase change of water vapor, and q is the specific humidity of air. T is what we measure with a thermometer, while q is derived by measuring the wet bulb temperature (or, alternatively, dewpoint temperature).
To illustrate how important it is to use moist enthalpy, we can refer to the current heat wave in the southwest United States. The temperatures in Yuma, Arizona, for example, have reached 110°F (43.3°C), but with dewpoint temperatures around 32°F (0°C). In terms of moist enthalpy, if the temperature falls to 95°F (35°C) but the dewpoint temperature rises to 48°F, the moist enthalpy is the same. Temperature by itself, of course, is critically important for many applications. However, when we want to quantify heat in the surface air in its proper units in physics, we must use moist enthalpy.
In terms of assessing trends in globally-averaged surface air temperature as a metric to diagnose the radiative equilibrium of the Earth, the neglect of using moist enthalpy, therefore, necessarily produces an inaccurate metric, since the water vapor content of the surface air will generally have different temporal variability and trends than the air temperature.
There are quite a few other issues with using the global-averaged surface temperature to characterize climate change (see NRC 2005, Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties). The realization that temperature is an incomplete measure of heat adds another problem to its use.
Could you give a breakdown of the contribution to the heat capacity of air by its constituients of O2, N2, H2O, and CO2? Also, which is larger the energy due to heat capacity or the energy of latent heat? These simple calculations should give a good rule of thumb about how much CO2 can really contribute to greenhouse warming.
Comment by Paul — July 18, 2005 @ 12:57 pm
#1, A great question. Please make the answer easy for an average person to understand. We need to keep this as simple as possible, so the maximum number of people get the real picture of CO2 contribution, verses the other gasses.
Comment by Russ — July 18, 2005 @ 5:02 pm
Thanks Paul and Russ for your questions. The use of moist enthalpy permits the heat associated with the phase change of liquid (or solid) water to water vapor to be included in the heat content of the air. A radiative imbalance can cause the air temperature to increase, but also can evaporate (or sublimate) water. Both forms of heat must be accounted for. However, the contribution of latent heat associated witht phase changes of water is not in the current observational assessments of surface air “global warming”. The other gases (O2, N2 and CO2) do not involve phase changes.
Comment by Roger Pielke Sr. — July 20, 2005 @ 7:49 am
But what indications are there of a change in atmospheric moisture content such that this effect is significant?
Comment by Steve Bloom — July 21, 2005 @ 2:53 am
Let me try to be a little more specific: Is there anything like a worldwide moist enthalpy index being maintained? If not, is enough data being collected to do so? Regardless, is there any reason to expect that globally-averaged moist enthalpy has changed over a period of time such that the effect could be considered meaningful? Short of that, are there regional changes that could be significant?
Comment by Steve Bloom — July 22, 2005 @ 1:09 am
This is an excellent question Steve. I will discuss in a later posting the large scale trends based on analyses summarized by one of my student’s in my class this past spring (there ins’t a significant globally averaged trend according to satellite data). However, with respect to the surface moist enthalpy, even very local changes in the microclimate of observing sites can result in different non-spatially representative temporal trends in temperature and atmospheric moisture content. If we just measure temperature, we only see part of this effect. If a location became drier over time, for example, because nearby trees were cut down, we can have an increase of temperature, but the moist enthalpy might not change or even decrease. These local changes in microclimate may not be representative of a larger area yet the analyses used to contruct large scale averages could use this spatially non-representative temperature information, thus resulting in a bias. We show that there are very substantial issues with the local microclimate exposure of surface observing sites in eastern Colorado which we have concluded is likely an issue world-wide (see http://blue.atmos.colostate.edu/publications/pdf/R-274.pdf.
Comment by Roger Pielke Sr. — July 22, 2005 @ 6:36 am
Really cool stuff I llvoe goingto this college!!
Comment by Jessica — August 9, 2005 @ 3:40 pm
I am not sure if this answers some of the questions, but the table below shows the percentage of enthalpy from water content at various temperatures and reletive humidities. You will note that at higher temperatures and humidities the more than half the content is in the form of water vapour.
Ratio of heat content of Air due to water vapour
Temp °C RH
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
-10 1% 3% 4% 6% 7% 8% 10% 11% 12% 13%
0 3% 6% 9% 11% 13% 16% 18% 20% 22% 24%
5 4% 8% 12% 15% 18% 21% 23% 26% 28% 30%
10 6% 11% 15% 19% 23% 27% 30% 33% 35% 38%
15 8% 14% 20% 25% 29% 33% 37% 40% 43% 45%
20 10% 18% 25% 31% 36% 40% 44% 47% 50% 53%
25 13% 23% 31% 37% 43% 47% 51% 55% 58% 60%
30 16% 28% 37% 44% 50% 54% 58% 62% 65% 67%
37 22% 36% 46% 54% 59% 64% 67% 70% 73% 75%
40 25% 40% 50% 57% 63% 67% 71% 74% 76% 78%
Comment by Sean Williams — August 31, 2005 @ 1:47 pm
Thanks Sean- this a very useful table!
Comment by Roger Pielke Sr. — September 1, 2005 @ 11:23 am
ice Site. Could use more of these instead of the many trash blogs on the web.
Comment by jeffrye — July 17, 2006 @ 12:50 pm