{"id":95,"date":"2011-01-17T21:17:54","date_gmt":"2011-01-18T04:17:54","guid":{"rendered":"http:\/\/logicalchaos.org\/blog\/?p=95"},"modified":"2013-04-21T21:43:22","modified_gmt":"2013-04-22T03:43:22","slug":"calculating-wind-chill-with-rrdtool","status":"publish","type":"post","link":"http:\/\/logicalchaos.org\/blog\/2011\/01\/calculating-wind-chill-with-rrdtool\/","title":{"rendered":"Calculating wind chill with RRDTool"},"content":{"rendered":"

In the process of switching over my Gnuplot<\/a> graphs<\/a> to RRD<\/a>, I needed to re-do the SQL function I used for calculating wind chill<\/a> into RPN notation. Quite straight forward in sql:
\n
\n[code lang=”sql”]\u0010
\nCREATE OR REPLACE FUNCTION apparent_temp( float, float, float )
\nRETURNS float AS ‘
\nDECLARE
\ntempf ALIAS FOR $1;
\nrelh ALIAS FOR $2;
\nspeed ALIAS FOR $3;
\naptemp float;
\nBEGIN
\nIF tempf >= 80 AND relh >= 40 THEN
\naptemp = -42.379 + 2.04901523*tempf + 10.14333127*relh
\n– 0.22475541*tempf*relh – 0.00683783*(tempf^2)
\n– 0.05481717*(relh^2) + 0.00122874*(tempf^2)*relh
\n+ .00085282*tempf*(relh^2) – 0.00000199*(tempf^2)*(relh^2);
\nELSIF speed >= 3 AND tempf <= 50 THEN
\naptemp = 35.74 + .6215*tempf – 35.75*(speed^.16)
\n+ .4275*tempf*(speed^.16);
\nELSE
\naptemp = tempf;
\nEND IF;
\nRETURN aptemp;
\nEND;
\n‘ LANGUAGE plpgsql IMMUTABLE STRICT;
\n[\/code]
\nCould not find anything quickly in Google for doing this in RRD, so I had to think on my own. The calculation is not quite as obvious in RRD RPN notation, but doable. It’s complicated by not having the power function in RRD:
\nDEF:ot=weather.rrd:outsidetemp:AVERAGE
\nDEF:ws=weather.rrd:windspeedaverage1:AVERAGE
\nCDEF:chill=ot,50,GE,ot,ws,3,LE,ot,35.74,.6215,ot,*,+,ws,LOG,.16,*,EXP,DUP,ot,*,.4275,*,EXC,35.75,*,-,+,IF,IF<\/span><\/code>
\n\"\"<\/a>
\not<\/strong><\/em> – outside temperature in fahrenheit
\nws<\/strong><\/em> – windspeed in miles per hour
\nThis calculation will present the current temperature if the temperature is > 50 degrees fahrenheit or the windspeed < 3 mph.<\/p>\n

Heat index comes next.<\/p>\n","protected":false},"excerpt":{"rendered":"

How to calculate wind chill using wind speed and temperature in rrd. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[8],"tags":[9,10,11],"_links":{"self":[{"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/posts\/95"}],"collection":[{"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/comments?post=95"}],"version-history":[{"count":18,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/posts\/95\/revisions"}],"predecessor-version":[{"id":107,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/posts\/95\/revisions\/107"}],"wp:attachment":[{"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/media?parent=95"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/categories?post=95"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/logicalchaos.org\/blog\/wp-json\/wp\/v2\/tags?post=95"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}