Update: July 7 2010 pm: The high temperature today at BWI was 101 F with a dew point of 65 F. In terms of heat (Joules per kilogram) today was warmer than yesterday.
The current heat wave in the northeast USA provides an opportunity to illustrate why we need to use surface air moist enthalpy, rather than the dry bulb temperature by itself as a diagnostic of surface air heat changes. The dry bulb temperature trend, of source, is what has been used as the diagnostic climate metric of global warming.
We have already published on this subject in
Pielke, R.A. Sr., K. Wolter, O. Bliss, N. Doesken, and B. McNoldy, 2006: The July 2005 Denver heat wave: How unusual was it? Nat. Wea. Dig., 31, 24-35.
From the excellent summary of the observation site in Baltimore by Watts Up With That, we can examine the heat content at the time of the maximum dry bulb temperature at BWI. As shown on Anthony’s weblog, the 105.1 F (40.6C) dry bulb temperature had an associated dew point temperature of 57.0 F (13.9C). A dew point temperature of 57 F at 1000 hPa (which is the approximate air pressure at BWI) corresponds to ~10 g/kg of water vapor in the air. This is relatively dry air for Baltimore in July.
The formula for the moist enthalpy is
H = Cp T + Lq
where H is the moist enthalpy, T is the dry bulb temperature, L is the latent heat of vaporization and q is the specific humidity.
As shown in,
Pielke Sr., R.A., 2001: Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall. Rev. Geophys., 39, 151-177.
the same moist enthalpy will occur when the dry bulb temperature decreases (increases) for the increase (decrease) in specific humidity as given below.
δT = (L/Cp) δq
For example, just an increase in dew point temperature from 13.9 C (57F) to 15.1 C (59F) at 1000 mbar (which changes q from 10 to 11 g per kg), for example, would need a reduction of 2.5 C (4.5 F) in δT to retain the same amount of moist enthalpy. If the BWI temperature would have been 100.5 F but with a dew point temperature of 59F, the same heat would be present [still hot, of course!).
This relationship between dew point temperature, temperature and moist enthalpy is illustrated clearly in Figure 11 in Pielke et al 2006.
We discussed why the use of moist enthalpy is a better surface metric for global warming than the dry bulb temperature in our papers
Pielke Sr., R.A., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211.
Davey, C.A., R.A. Pielke Sr., and K.P. Gallo, 2006: Differences between near-surface equivalent temperature and temperature trends for the eastern United States – Equivalent temperature as an alternative measure of heat content. Global and Planetary Change, 54, 19–32.
It is also a better metric to characterize heat waves.