I haven’t had time to delve into this, but there seems to be a general interest in emissivity through several lines of investigation on the talkshop recently. Physics makes definitions of things in ideal conditions. Emissivity is defined as the radiation a body will emit at a specific temperature. This quantity is crucial to our understanding of the way Earth balances its energy budget of incoming solar energy with emission from various parts of the system. However, emissivity at different wavelengths forms a curve, it isn’t the same at all wavelengths for a specific material. Moreover, in a non-vacuum, radiation isn’t the only means by which energy can leave a body. Conduction, latent heat of vaporisation (evaporation) and convection also play a role, and dominate over radiation in the Earth’s troposphere.
The most important material to consider so far as Earthj’s energy balance is concerned is seawater, since it covers 70% of the planet.
Contributor ‘Max’ turned up some interesting plots for the emissivity of seawater (and land) the other day, which seem to contradict each other. Here are two of them:
Why the difference?
Wayne Jackson recently posted an alternative energy budget which finds that the ‘effective’ or ‘operational’ emissivity of the surface is around 0.67.
This seems to show that we need to subtract the energy involved in the latent heat and conduction from the figure given by the (in)famous Trenberth and Keihl energy budget diagram for the long wave radiation going up from the surface. But it is claimed that their figure of 356W/m^2 is measured by radiometers. Many people dispute the accuracy and calibration of these devices, but assuming it is a true average of measurements, how can we reconcile the difference?
Could it be that the long wave radiation being exchanged in the air is simply a transient expression of the energy being moved by convection and latent heat? This would make the long wave energy flux within the troposphere more a ‘symptom’ of its energy content than a cause of its temperature or the lapse rate, since it is constantly cancelling out at each altitude level, apart from the relatively small component of the upward radiation which escapes to space through the ‘atmospheric window’.
Several people commenting on their own and each others investigations in this area have been spread across several threads recently. I’m posting this one to get them all together in one place to discuss these issues. I suggest we make a conscious effort not to get sidetracked by anyone, mentioning no names Tim Folkerts, who tries to obfuscate the issue with energy budget calculations which assume a perfect emissivity of 1 or an arbitrary figure such as 0.9 for any part of the system.
Have at it!