The Elusive Absolute Surface Air Temperature
by Dennis Ambler
With the current claims yet again of “record temperatures” it may be of interest to look again at this Q & A from James Hansen in 2005. It is still on the website.
Here are a few extracts:
The Elusive Absolute Surface Air Temperature (SAT)
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.
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.
- 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.
- 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.
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.
- 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 that the inaccuracies of the model become relevant.
In 2005, the UK Met Office was claiming the year as the “hottest on record”, but with this caveat,
All the temperature values have uncertainties, which arise mainly from gaps in data coverage. The sizes of the uncertainties are such that the global average temperature for 2005 is statistically indistinguishable from, and could be anywhere between, the first and the eighth warmest year in the record.
Similar analyses in the United States rank the year as warmest on record (GISS) and second warmest (NCDC). However, NCDC also note that uncertainties arising from sparse observations or measurement biases make 2005 statistically indistinguishable from the warmest year, 1998, as well as from other recent years such as 2002 and 2003.
James Hansen (NASA) thought 2005 was the hottest year ever.
?A surprising Arctic warm spell is responsible for a 2005 that was likely the warmest year since instrument recordings began in the late 1800s?, added Hansen, who nevertheless admitted that the analysis had to estimate temperatures in the Arctic from nearby weather stations because no direct data were available.
As a result, he said, ?we couldn?t say with 100 percent certainty that it?s the warmest year, but I?m reasonably confident that it was?. Hansen and other researchers wrote in the analysis that ?the inclusion of estimated Arctic temperatures is the primary reason for our rank of 2005 as the warmest year.? (Mercosur News Agency, 27/01/06).