The incredibly elusive absolute surface air temperature

Dr James Hansen

Yesterday, Dr Vincent Gray sent out his Climate Truth Newsletter (no. 310). In it he adverts to an outrageous admission of common sense by James Hansen. Years ago, Hansen admitted on his GISS web page that there’s no agreement among scientists on what constitutes an acceptable surface air temperature.

Sensationally, he also said that it’s IMPOSSIBLE to obtain a scientifically meaningful surface air temperature (SAT).

Now, with Hansen’s resignation from NASA, Gavin Schmidt has rushed in to take charge of these surprising admissions. Curiously, I see that Schmidt’s description is “NASA Official,” where Hansen was the “Responsible NASA Official.” Significant, interesting or irrelevant? Speculation might be endless…

The link above to the previous version of the page at the Wayback Machine is from 15 October, 2008, but that page is marked as last updated on 12 July, 2005. There are three more words in the body text of the current version than on the old page; I conclude they’re essentially identical.

These comments asserting the impossibility of determining the SAT put a disturbing slant on Hansen’s alarmism based on the SAT during the last 20 years of the 20th Century. For if he and his colleagues truly believe it’s impossible to obtain an accurate reading of the surface temperature, on what do they base their predictions of disaster?

Three years ago I sent Dr Hansen an email which concluded:

You say there is no agreed method of measuring surface air temperatures and, in fact, there are numerous practical and theoretical obstacles to ever achieving such a measurement.

There is a very obvious question raised by that discussion. We are interested to know why, if it cannot be done, do you do it?

He never replied.

Notwithstanding these criticisms, the page gives some interesting information about temperature, how we interpret and use them and their general utility. Notably, it says we often instinctively convert temperatures to anomalies.

Here’s the page from GISS (emphasis added):

Q. What exactly do we mean by SAT?
A. I doubt that there is a general agreement how to answer this question. Even at the same location, the temperature near the ground may be very different from the temperature 5 ft above the ground and different again from 10 ft or 50 ft above the ground. Particularly in the presence of vegetation (say in a rain forest), the temperature above the vegetation may be very different from the temperature below the top of the vegetation. A reasonable suggestion might be to use the average temperature of the first 50 ft of air either above ground or above the top of the vegetation. To measure SAT we have to agree on what it is and, as far as I know, no such standard has been suggested or generally adopted. Even if the 50 ft standard were adopted, I cannot imagine that a weather station would build a 50 ft stack of thermometers to be able to find the true SAT at its location.

Q. What do we mean by daily mean SAT?
A. Again, there is no universally accepted correct answer. Should we note the temperature every 6 hours and report the mean, should we do it every 2 hours, hourly, have a machine record it every second, or simply take the average of the highest and lowest temperature of the day ? On some days the various methods may lead to drastically different results.

Q. What SAT do the local media report?
A. The media report the reading of 1 particular thermometer of a nearby weather station. This temperature may be very different from the true SAT even at that location and has certainly nothing to do with the true regional SAT. To measure the true regional SAT, we would have to use many 50 ft stacks of thermometers distributed evenly over the whole region, an obvious practical impossibility.

Q. If the reported SATs are not the true SATs, why are they still useful?
A. The reported temperature is truly meaningful only to a person who happens to visit the weather station at the precise moment when the reported temperature is measured, in other words, to nobody. However, in addition to the SAT the reports usually also mention whether the current temperature is unusually high or unusually low, how much it differs from the normal temperature, and that information (the anomaly) is meaningful for the whole region. Also, if we hear a temperature (say 70°F), we instinctively translate it into hot or cold, but our translation key depends on the season and region, the same temperature may be ‘hot’ in winter and ‘cold’ in July, since by ‘hot’ we always mean ‘hotter than normal’, i.e., we all translate absolute temperatures automatically into anomalies whether we are aware of it or not.

Q. If SATs cannot be measured, how are SAT maps created?
A. This can only be done with the help of computer models, the same models that are used to create the daily weather forecasts. We may start out the model with the few observed data that are available and fill in the rest with guesses (also called extrapolations) and then let the model run long enough so that the initial guesses no longer matter, but not too long in order to avoid the inaccuracies of the model becoming relevant. This may be done starting from conditions from many years, so that the average (called a ‘climatology’) hopefully represents a typical map for the particular month or day of the year.

Q. What do I do if I need absolute SATs, not anomalies?
A. In 99.9% of the cases you’ll find that anomalies are exactly what you need, and not absolute temperatures. In the remaining cases, you have to pick one of the available climatologies and add the anomalies (with respect to the proper base period) to it. For the global mean, the most trusted models produce a value of roughly 14°C, i.e., 57.2°F, but it may easily be anywhere between 56 and 58°F and regionally, let alone locally, the situation is even worse.

One Thought on “The incredibly elusive absolute surface air temperature

  1. Richard C (NZ) on May 11, 2013 at 8:49 pm said:

    >”We are interested to know why, if it cannot be done, do you do it?”

    And why, having done it, do you adjust it the way GISS does?

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