Wednesday, December 19, 2007

Surface Air Temperature

The Goddard Institute For Space Studies explains the meaning of surface air temperatures.

Q. What exactly do we mean by SAT [Surface Air Temperature - ed.]?

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.
So. We do not know the real surface air temperature, but we can use the number we get from weather stations to determine climate. OK.

It gets better.
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.
So there is no no standard of measurement, but we can use the non-standard numbers we have to determine climate. OK.
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.
So. It is practically impossible to measure surface air temperatures and from this number we get from the weather stations can derive local and regional and global climate? OK.
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 70F), 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.
Ah. So the temperature really has no meaning except as a rough guide as to whether you can go out in a T shirt or need a heavy overcoat. Or should you take a jacket with you on a warm afternoon. And from this meaningless temperature we can derive climate. OK.
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 that the inaccuracies of the model become 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.
Well at least he is hopeful about climate guesses. That is something. If I were placing trillion dollar bets with people's lives at stake (lack of energy leads to premature death) I'd want to be hopeful too. If aircraft were designed with hope, I think very few people would fly.

Now what NASA Official was responsible for the above text? None other than James "coal trains are death trains" E. Hansen.

1 comment:

papertiger said...

Sure - now you tell me - two days after. ;)


Seriously, I need to make a hard copy of this one. It's a keeper.