You should aquint yourself with some signal analysis tools.

Best,

[citation]
….Think about 100 years ago. We are comparing all the data today against data from a time when istrumentation is poor, documentation was poor, no computers existed to collect or trck data, etc. I can imagine the temperatures reported from Arizona back then. Some cowboy read the thermometer for some official, didn’t write it down, by the time he got back after riding his horse, he forget and made up a number….
[/citation]

Ray, I somehow can’t agree:
“at that time instrumentation is poor” – nowadays it’s sometimes poor, too – and very often in poor locations.
“documentation was poor” – nowadays documentation – sometimes – poor, too.
“Some cowboy read….” – I’m not from U.S., I’m from Western Europe, but I did a work on the live
of the real cowboys – not the ones in the movies – in my last year of school (114 pages). I did more on that in the following 4 to 5 years – it was an amazing theme.
- Cowboys had usually a very high working ethic.
- Cowboys were often very astute in weather forecasting. Weather was something which could influence
success/failure of their work, could even make their live dangerous. Don’t expect that these guys did be sloppy with documentation about weather. In fact, I do know of ranches in the west, taking notes on nearly everything, like the logbooks on ships. The remembering of cowboys was exptremely well. Compared to them, we would look like Alzheimers.
- Funny: A plain cowboy did know of and did use nearly 4 times as much different knots than an ordinary seaman on a sailing vessel. (my personal knowledge from different knots – only workink knots, not the fancy knots: from seamen (about 40), from cowboys (over 150)

http://www.woodfortrees.org/plot/hadcrut3vgl/from:1979/trend/offset:-0.15/plot/rss/scale:0.8333333/trend/plot/gistemp/from:1979/trend/offset:-0.24/plot/uah/from:1979/scale:0.8333333/trend

“If the readings are recorded daily at say 5:00pm, an intermediate temperature period of the day, then TOO adjustments are not required. Is the actual time of minimum or maximum important? Getting an accurate record is important.”

Again, I recommend the links I posted earlier. But consider your example. It would not be unusual for the high of the day to occur around 5 o’clock. Say it’s a very warm day, 94 deg F. Our dutibound temperature recorder trudges out to the weather station, takes his/her reading and resets the thermometer. The next day is not as warm, but guess what the the 24 hour high is…94 deg F. Now let’s say the day is unusually cool…68 deg F. If the next day is warmer, the next day’s temperature will correctly be recorded as the next day’s high (assuming the temperature doesn’t keep rising after 5 o’clock).

If the routine is to reset the temperature every day at 5 o’clock, there will be many days where an erroniously high temperature reading will be recorded for the next day. There will be virtually none where an erroniously low reading will be recorde. If your example was 10 PM or 10 AM, you would be closer to the “intermediate” temperature you posited, but 5 PM is too close to most daily highs to be considered “intermediate”.

As I said earlier, the effect is real. Quantifying it is very difficult.

It appears your words have fallen on deaf ears. (eyes?) This instrumentation issue seems to nearly impossible to get across.
It is wrong technically to attempt to find a long term measured value trend smaller than the accuracy of the instruments doing the measurement. Our current scientific weather instruments have a typical calibration accuracy of plus or minus one half degree. That means, in this case, that if we take the instrument back to the calibration lab within some reasonable time frame (six months? a year?) it should be within that range. It may have drifted up or down half a degree or more during that time span and still be within specs.
It is also wrong technically to assume that instrument calibration accuracy carries through to the final measurement. How and where the instrument connects to the process to be monitored matters even more. This is a critical engineering issue in facilities such and power plants and manufacturing plants. Careful location of sensors (and their thermal wells if needed) to make sure they measure what is desired and not in some non-representative area. It would be unlikely to expect a process (air temperature) measurement accuracy under ideal circumstances to be more accurate than about plus or minus one degree for our weather instruments.
What about modern signal analysis programs? Can’t they find extremely weak radio signals buried in tons of noise? Isn’t the analysis of weather temperatures the same thing? Nope, not by a big measure. First of all, instrument bias and drift cancel out in analyzing weak radio signals. Radio signals are sampled over many, many repeated cycles looking for self correlation from cycle to cycle. 20th and 21st century weather analysis, on the other hand is looking for trends much longer than the measurement periods involved, not repeated cycles of signal.
What does this all mean from a technical perspective. No long term trend should be claimed unless it is at least three times the instrument accuracy or in this case, plus or minus three degrees. Preferably, the trend should be ten times the instrument accuracy. Trends claimed below these values are indistinguishable from instrument bias and drift.
For those folks who believe otherwise, please go visit the folks at a calibration lab and have them explain what is needed to make accurate measurements.

“When Bill Clinton took office in 1993, global warming was a slow-moving environmental problem that was easy to ignore. Now it is a ticking time bomb that President-elect Barack Obama can’t avoid.

Since Clinton’s inauguration, summer Arctic sea ice has lost the equivalent of Alaska, California and Texas. The 10 hottest years on record have occurred since Clinton’s second inauguration. Global warming is accelerating. Time is close to running out, and Obama knows it.”

The whole thing is a jaw-dropper.