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Meteorology (Weather)/Range comparison of thermometers

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QUESTION: As temps cool at night the NWS temps drop to ~-21deg and then almost sit for 4 to 6hrs while my alcohol thermometer continues on down to the -30 range. In the morning, the NWS temp will sit at ~-21 till my thermometer catches up and then the two thermometers track close to the same temps. I've seen this happen 20 or 30 times so it isn't a one time fluke. Any thoughts on what might be going on? Could you recommend a certified Hg thermometer model to purchase so I could calibrate my thermometer?

ANSWER: Hi Les

Generally speaking, mercury thermometers are more accurate than other type thermometers.

http://www.crh.noaa.gov/lot/coop/equipment.php

But:

http://www.nist.gov/pml/mercury.cfm

http://www.nws.noaa.gov/om/coop/Publications/coophandbook2.pdf

http://www.nws.noaa.gov/directives/sym/pd01013015curr.pdf

As to the difference between your readings and NOAA's the cause may be instrument exposure to wind differences or proximity to heat or no heat sources in the area.

http://weather.noaa.gov/weather/MN_cc_us.html

You may want to call the local NOAA office and see if they might know of local differences in the geography that could be a factor.

http://www.crh.noaa.gov/contact.php?sid=mpx

http://www.crh.noaa.gov/mpx/

Some daily reports:

http://www.nws.noaa.gov/climate/index.php?wfo=mpx



---------- FOLLOW-UP ----------

QUESTION: OK, I glean from all this that Hg certified thermometers are no longer sold. Where does one buy a certified digital one. It looks like they all need to be field calibrated and many are not functional below zero. And I certainly do not have access to a Pt thermocouple nor do I suspect the local NWS guys.

Answer
Hi Les

Take a look at:

http://www.amazon.com/Fluke-Thermometer-Rejection-Accuracy-Temperature/dp/B00012

http://www.davis.com/Category/Weather_Instruments/51585

http://www.doityourself.com/stry/how-to-calibrate-a-digital-thermometer  

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Donald Rosenfeld

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Any questions (except private) answered from the 1st grade level on up pertaining to any aspect of Weather. I am a 20 year member of the American Meteorological society and a long time forecaster of eastern United States snow storms and Hurricanes.

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