How reliable is weather data from web sources such as the Weather Underground?

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Since I've been recommending for years web sources such as the Weather Underground for
those wanting site-specific weather data , it behooves me to know how reliable are such
data??? It's difficult to answer that question in the absolute sense because there are no
incontrovertible sources against which to compare. However, I did the next best thing,
which is to compare the data against that from a nearby major weather station and then
look to see whether the differences make sense according to general understanding of
terrain factors and? urban microclimate variations.

This comparison was not done in a vacuum, but in response to a client who wanted weather
data for a site in Portland Oregon, with a particular interest on the amount of rainfall.?
The map below shows? the site of interest (red teardrop) and the NSW station at the
Portland Airport 4.5 miles away.? The Weather Underground station PORTL437 is located
about 0.5 miles to the east of the site equidistant on the other side of the MLKing Blvd.

I'll let the plots in my reply to the client speak for themselves, but my general feeling
is that the WU data looked quite consistent with the airport data, and that the
differences are quite understandable, e.g., the reduced wind speeds and the lack of
correlation in the wind direction, since the airport station is in a flat open area next
to the Columbia River, while the WU station is in a urban residential area 4-5 miles away.

I'm still withholding judgement whether the 60% difference in the recorded rainfall is
real, a result of the placement of the WU station, or a defect in the measurement
protocol.? Come to think of it, I don't know how these automated weather stations record
rainfall (liquid precipitation to be technically correct).? I can only think of a rain
gauge that's manually recorded and maintained.


-------- Forwarded Message --------
Subject: Re: Weather data for Portland with monthly updates
Date: Fri, 23 Feb 2018 16:27:07 -0800
From: Joe Huang


Attached are both weather files for the ISD station at the Portland Airport
(OR_PORTLAND-IAP_726980_18) and the Weather Underground site KORPORTL437
(OR_WU_PORTL437_18).? The data are available through Feb. 22 for the ISD station and
through Feb. 23 for the WU station.

Out of curiosity, I compared the data between the two files (see below). The dry-bulb and
dewpoint temperatures are very well correlated, so that I find the hourly variations to be
credible and reflecting microclimate differences.? The wind speeds are about three times
higher at the airport, which is also believable, while there is no correlation in the wind
direction.? The rainfall amount also differs, with the airport reporting? 60 % more than
at the? PORTL437 Weather Underground station.

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at for simulation-ready weather data (o)
(925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"

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