Saturday, July 21, 2012

Breaking Down How Temperatures Are Recorded

There was recently a local television meteorologist who decided to surmise that the National Weather Service was driving an agenda with daily high temperatures and how they were recorded here in Philadelphia.  In the interests of protecting this individual, since they removed the post from their Facebook page on Friday morning, I won't post who said it (although you can figure it out if you caught it on their page) but I do think it's worth a bit of conversation.   The post was in reference to Thursday's high temperature in Philly, which was 85 degrees.

To be fair, three degree temperature spikes within an hour is a bit high compared to what normally can happen.  It has, does, and occasionally will happen however.  Thursday was an example of it.  Sun breaking out over the course of an hour for a number of minutes, localized factors, and the fact that the Airport's temperature sensor is located near a runway and near the river don't exactly help as there are a number of little things at play that can nudge temperatures up or down a degree or three over the course of an hour.   Thursday did feature a couple of hours of relative brighter weather where the sun did get out and temperatures did spike -- in fact the 10-12 timeframe was the warmest of the day and many locations locally measured their daily highs above their "in between hour" readings.

Trenton's high was 80, just after 10 AM, with their 10 AM and 11 AM readings both at 78.  In Philly, during the 2 PM hour on Thursday, the temp spiked from 78.8 to 80.6 in 17 minutes.  Temperature fluctuations during the course of an hour are completely normal.  Three degrees might be a bit high but it has happened a few times over the years.

Does the National Weather Service manually record the high and low of the day?  Not unless there's a known sensor problem at their temperature observation sites.  Philadelphia's temperature is measured at the southeast corner of the Airport, just below the top of the twin runways.  It's a couple of hundred yards away from the river...and its position (while not perfect) is probably one of the "better" spots at the Airport for temperatures to be measured.   More important, the weather information that's gathered here each hour is gathered on an automated basis.  High, low, wind, wind gust is all recorded on a constant basis and specific "special" reports are provided when there's a marked change in wind, sky cover, or precipitation.  That isn't the case for temperature but information on high and low temperature over a six hour basis is submitted through reports the station sends to the NWS every six hours (2 and 8 AM & PM in daylight time, 1 & 7 AM & PM in standard time).  There are quality control procedures in place that can help spot fishy data...and occasionally when a trend of fishy data crops up those observation stations are fixed to improve the issue.  Such a trend was noted last year in Pottstown and this year in Trenton, both sites that ran consistently warmer with lower dew points than anyone in the region.  That's not the case with Philadelphia as their temperature and dew point are generally consistent with other sites nearby.  They may run a degree or two warmer than everyone else locally but its location (Airport in a major urban center) accounts in part for that warmth.

As long-time readers here know, I'm not a fan of having a singular "official" temperature record at the Airport because I don't think it's representative of the city's temperature or precipitation (I've argued for a second site in Fairmount Park a few times over the years here and informally on other places...similar to that of Central Park in New York).

Regarding the comment asserting that the National Weather Service was jacking temperatures up to 90 degrees last week in order to prolong a heat wave and that Philly was the only 90 between Boston and Richmond...that day had a couple of other 90 degree readings in areas between Boston and Richmond and also featured temperatures for two consecutive hourly observations in Philadelphia at 89 degrees.  Again, that one degree nudge is certainly possible in between hours. Given how dry the area around the Airport has been, dry soil would certainly provide an efficient boost to help temperatures warm an extra couple of degrees compared to other locations.

Computers that record temperature data aren't agenda-driven...and the National Weather Service isn't simply nudging temperature data higher or lower in order to drive a heat wave longer or try to push a global warming or climate change agenda down anyone's throat.