Reason for my post was to highlight the T0 T-12 concept for those that would not be aware.

My pedantics re Average, Mean, and Median, stem from a mathmatics lecturer hammering the point home.]]>

Allthough you already know this, for the sake of our readers, the calculation of MSLP ( "barometer" in WeeWX terms) requires the average/mean temperature 12 hours ago and the current temperature.

I guess I deferred to Dr. David Burch's explanation of why not to use current temperature for SLP calculations:

"Namely the proper air temperature to use is not what you read at the time you read the barometer, but rather we should use the average of the air temperature at the time of observation T(0h) and the value from 12 hours earlier T(-12h):

T (avg) = [ T(0h) + T(-12h) ] ÷ 2

This 12-hr time average is not an obvious choice. It is also not the result of a mathematical theory, but we can expect that some average is called for. We know the virtual temperature of this virtual air column affects the pressure, and it is reasonable to assume that the the pressure at the base of this column cannot respond instantly to a change in temperature at the top, so we are led to averaging over some time period to reach equilibrium. In other words, the air temperature at the moment is not what we should use to project this pressure down to the sea level, but rather some average of what it has been in the recent past."

Going back to the usage of average vs mean, in a dataset of 2 values, I suspect the difference is largely semantical after reading all too many "hits" from searching "difference between average and mean".

Technically, depending on the context, one can argue that "mean" could be more appropriate in a statistical sense but "average" might be more appropriate in a mathematical sense.

Anyways, statistics was never my favourite course

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Statistically.

Average is defined as the sum of values in a dataset divided by the number of values in the dataset.

Mean is defined as the average of the minimum value and maximum value in a dataset.

Median requires sorting a dataset in order and picking the value in the middle.

So, the 3 terms give different results.

A simple analogy.

Take a 12 hour car journey, with a weewx 60 sec archive period.

You start at a standstill (0mph), you finish at a standstill (0mph), apart from acceleration and braking, we’ll assume the rest of the journey can be travelled at 50mph.

The average speed of our hypothetical journey is 49.86mph.

The mean speed of our hypothetical journey is 25mph.

The median speed of our hypothetical journey (in this case) is also 25mph.

However, if we use the meteorological/weewx method we get 0mph.

This is because it is the average of the value now, and the value from 12hrs ago (both are at a standstill).

Historically, I used the Keisan calculator, but, I changed the default temperature to 20C, as being more typical for my location.

Later, I became aware of the “12 hour average”, and later still, how it was actually derived.

However, I have not been able to find the reasoning behind, or a justification, for this statistically erroneous methodology?????

I’ve attached an extract of an excel analysis of my weewx database, showing the difference between the SLP average, true average and mean values.

Now, it has to be said, that at my lowly altitude, the difference equates to only 0.2Hpa, but increases for higher altitudes.

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