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require_once "../../config/settings.inc.php";
require_once "../../include/myview.php";
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$t->title = "Note about ASOS Precipitation Data";

$t->content = <<<EOM
<h3>Notes about ASOS Precipitation Reports</h3>

<p>The Automated Surface Observation System <a href="/ASOS/">ASOS</a> is the
primary automated weather observation system in the country.  The network is
maintained by the Federal Aviation Administration and National Weather Service.
The IEM processes, archives, and <a href="/request/download.phtml">makes available data</a> from this network.  The
purpose of this page is to document some interesting aspects of the precipitation
data from this network.

<p><div class="warning">Please feel free to 
<a href="/info/contacts.php">report any doubts or concerns</a> you
have with this documentation.  This information has been pieced together based
on conversations with the NWS and reviewing the 
<a href="https://www.weather.gov/media/asos/aum-toc.pdf">ASOS Users Manual</a>.</div>

<h4>Regarding Time Zones...</h4>

<p>Depending on the report type, observations from this network are reported in
either local standard time or UTC (sometimes called GMT) time.  The internal 
clocks on the ASOS sites are not updated for daylight time.  This is an important
consideration when using its local daily precipitation report found in the 
Daily Summary Message (DSM).  This 24 hour period is between local midnight in
standard time.  So when the local calendar is in daylight time, the period will
represent the time between 1 AM and 1 AM.  This 24 hour is not exactly as stated
and leads to the next section.

<h4>Regarding Exact Timing...</h4>

<p>The ASOS reports time in hours and minutes without seconds.  This creates some
ambiguity when attempting to ascertain exactly when a reported precipitation rate
occurred.  When the time hits 00 seconds, the ASOS starts processing its memory of 
recently saved data.  This processing ends promptly at 23 seconds after and various
displays are updated, products disseminated, and one minute interval data archived.

<p>The one minute precipitation data is thus some nearly 60 second period between
when the previous minute's processing got to totaling the precipitation data
to this minute's processing.  So the one minute total is strictly not from a 
period between 00 seconds of the previous minute and 00 seconds of the current
period.  Instead this is approximately the 60 second period between 24 seconds
after the previous minute to 23 seconds of the current minute.  For example:

<p>For one minute data, the 12:45 UTC observation approximately represents
a period between 12:44:24 and 12:45:23 UTC.  

<p>For the hourly METAR precipitation, the 12:54 UTC observation represents a
period between 11:54:24 and 12:54:23 UTC.

<p>For the daily summary message, the observation represents a period between
11:59:24 PM LST of the previous day to 11:59:23 PM LST of the current day.

<p>For the hourly data in the daily summary message, the observation is a 
period from (HH-1):59:24 to HH:59:23 LST.

<h4>Heavy Precipitation Rates...</h4>

<p>The ASOS uses a tipping bucket method to measure precipitation.  This means
that a single tip of the bucket records 0.01 inch of precipitation with the data
logger.  Under intense rainfall rates, various physical things happen causing
the tipping mechanism to not be able to keep up with the flowing water rate. The <a href="https://www.weather.gov/media/asos/aum-toc.pdf">ASOS Users Manual</a>
notes in section 3.4.2 that a correction is applied to the measured
accumulation on a minute by minute basis.
The equation shown in the manual is as follows:

<br /><blockquote><pre>C = A(1 + 0.60A)</pre><br />where C is the reported
accumulation and A is the measured accumulation from the tipping bucket.</blockquote>

<p><img src="eqn342.png" />

<p>This means that when the ASOS reports a one minute accumulation of 0.25 
inches, the actual measured value was around 0.22 inches.  <b>Please note that
it is not our intention to claim this correction is wrong, but just to document that
it is there.</b>  Extremely intense rainfall rates such as these are rare, so the 
typical adjustment is practically zero. 

<h4>In Summary</h4>

<p>The timing and precipitation totals are not exact, but they are conservative
over time when summed. Please let us know of any corrections or clarifications
you would like to see made.

EOM;
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