I only got two responses this time, which I attribute to either the close proximity of the
holidays or perhaps reader fatigue about weather data
However,? the answers I did receive, especially the second one, were so much in line with
what I saw on the weather file that it looks copied which I must? affirm was not the case
(see the answers at the bottom of this post).

So, the three things I found unusual about this weather file are, in order of ascending
importance:
? 1. The year is given as 2005 throughout, but the comment says that it's a "typical
year" file made from the time period 1970-2000.
? 2.? Line 8 says that the first day of the weather file, i.e., Jan. 1, is set as a
Sunday, but January 1 2005 was a Saturday.
? 3. The solar radiation appears to be shifted a half-hour ahead, probably because the
original file was created in Europe, which tends to report the solar around the time step,
e.g., -0:30 to +0.30, whereas in North America it's reported for the preceding time step,
i.e., -1:00 to 0:00.? Although that was the only criteria for why I chose this weather
file (Legnica, Poland), I later found other more troubling aspects to the solar radiation
that made me expand the discussion to? QC'ing the solar on weather files in general.

I first became aware of the different conventions of reporting solar back in 1993 when I
was involved in an IEA project on Low-Energy Cooling where one of the first items of
business was? to compare the climate conditions in the participant countries.? When I
tried to work with weather data from European colleagues, I was getting impossibly large
spikes of Direct Normal Irradiance (DNI) for many sunrise hours due to this difference in
convention, since I was using a US simulation program that follows the US convention and
calculates the sun position at the midpoint of the preceding time step.

Although it might seem hard to tell which convention is being used in a weather file, it's
actually quite noticeable if one were to compare the hourly profiles to those calculated
by a Clear Sky Model.? For example,? this plot shows July 1 - 4, where the forward
shifting of the solar on the weather file can be seen, particularly if you look for the
sunrise and sunset hours. What really surprised me, though, was that the Global Horizontal
Irradiance (GHI) and DNI seem switched on the weather file. The GHI profile is generally
more smooth and the DNI more spiky, but here it's the reverse.? Also, the GHI greatly
exceeds the Clear Sky GHI on the morning of Day 3, which simply cannot happen.

The data for Jan. 1 -4 reveals more indications that the GHI and DNI on the weather file
may be switched., with the GHI (?) again more spiky and on Jan. 2? almost three times as
much as the Clear Sky GHI.? The 30-minute shift is less visible owing to the small values,
but can still be detected by looking at the sunrise and sunset hours.

So what does this say about QC'ing weather data?? It seems that more emphasis has gone
into QC'ing excursions in temperature than solar radiation. However, since the solar in
almost all weather files is not measured but calculated, there should be all the more
reason to regard it more carefully.? Luckily, there are several simple, if not
simple-minded,? facts that can be used as reality checks, e.g., (1) the GHI should always
be non-zero when the sun is above the horizon, making it possible to determine the hours
of sunrise and sunset, (2)? the GHI and DNI could never be greater than their Clear Sky
values (don't worry about ground reflectance or atmospheric phenomena like cloud lensing
since we're only dealing with calculated? data),? and (3) the Direct Horizontal ( DHI =
DNI * arcsin of the solar angle) cannot be greater than the GHI.

I hope this answer has provided some insight into the contents of weather files. As in the
previous contest, both respondents, Samuel Letellier-Duchesne and Michael Kummert, are
declared winners and entitled to one free weather file of their choice from the WBT
archive.? Just let me know. (Full disclosure: Samuel appears to be Michael's graduate
student :-))

Holiday greetings to all!

Joe
-------- Forwarded Message --------
Subject: Re: [Bldg-sim] Test your knowledge of simulation weather file formats Part 2:
the EnergyPlus *.epw format
Date: Fri, 15 Dec 2017 13:02:00 -0500
From: Samuel Letellier-Duchesne
To: Joe Huang

Well,?there is clearly something wrong with the sunup/sundown portion of the data as there
is no systematically no transition?for most days of the year. See image for comparison
with us-based weather file. Will try to find more!

Polytechnique Montr?al
Samuel Letellier-Duchesne
Ph.D. Candidate
Mechanical Engineering
2900 Edouard Montpetit Blvd,
Montreal, QC H3T 1J4 - Canada
samuel.letellier-duchesne at polymtl.ca
Tel. +1-514-813-0056