Chicago NWS demonstrates why climate math is hard in their May 10th summary, which I reproduce in entirety below from http://www.crh.noaa.gov/product.php?site=LOT&issuedby=ORD&product=CLI&format=CI&version=7&glossary=0
See if you can spot the error, and the answer follows. (h/t to Joe D’Aleo)
000
CDUS43 KLOT 110638
CLIORD
CLIMATE REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
135 AM CDT FRI MAY 11 2012
...................................
...THE CHICAGO-OHARE CLIMATE SUMMARY FOR MAY 10 2012...
CLIMATE NORMAL PERIOD 1981 TO 2010
CLIMATE RECORD PERIOD 1871 TO 2012
WEATHER ITEM OBSERVED TIME RECORD YEAR NORMAL DEPARTURE LAST
VALUE (LST) VALUE VALUE FROM YEAR
NORMAL
..................................................................
TEMPERATURE (F)
YESTERDAY
MAXIMUM 67 402 PM 90 2011 68 -1 90
MINIMUM 46 433 AM 28 1983 47 -1 62
AVERAGE 57 57 0 76
PRECIPITATION (IN)
YESTERDAY 0.00 2.84 1951 0.12 -0.12 0.00
MONTH TO DATE 3.04 1.16 1.88 0.06
SINCE MAR 1 7.37 7.04 0.33 7.58
SINCE JAN 1 10.87 10.56 0.31 12.02
SNOWFALL (IN)
YESTERDAY 0.0 T 1945 0.0 0.0 0.0
1907
1902
MONTH TO DATE 0.0 0.0 0.0 0.0
SINCE MAR 1 0.3 6.8 -6.5 1.6
SINCE JUL 1 19.8 36.7 -16.9 57.9
SNOW DEPTH 0
DEGREE DAYS
HEATING
YESTERDAY 8 9 -1 0
MONTH TO DATE 52 98 -46 116
SINCE MAR 1 869 1431 -562 1512
SINCE JUL 1 4842 6165 -1323 6326
COOLING
YESTERDAY 0 1 -1 11
MONTH TO DATE 15 8 7 11
SINCE MAR 1 58 18 40 16
SINCE JAN 1 58 18 40 16
..................................................................
WIND (MPH)
HIGHEST WIND SPEED 14 HIGHEST WIND DIRECTION NE (50)
HIGHEST GUST SPEED 28 HIGHEST GUST DIRECTION NE (50)
AVERAGE WIND SPEED 7.5
SKY COVER
POSSIBLE SUNSHINE MM
AVERAGE SKY COVER 0.0
WEATHER CONDITIONS
THE FOLLOWING WEATHER WAS RECORDED YESTERDAY.
NO SIGNIFICANT WEATHER WAS OBSERVED.
RELATIVE HUMIDITY (PERCENT)
HIGHEST 86 400 AM
LOWEST 22 400 PM
AVERAGE 54
..........................................................
THE CHICAGO-OHARE CLIMATE NORMALS FOR TODAY
NORMAL RECORD YEAR
MAXIMUM TEMPERATURE (F) 69 89 1982
MINIMUM TEMPERATURE (F) 47 33 1981
SUNRISE AND SUNSET
MAY 11 2012...........SUNRISE 535 AM CDT SUNSET 802 PM CDT
MAY 12 2012...........SUNRISE 534 AM CDT SUNSET 803 PM CDT
- INDICATES NEGATIVE NUMBERS.
R INDICATES RECORD WAS SET OR TIED.
MM INDICATES DATA IS MISSING.
T INDICATES TRACE AMOUNT.$$
Did you spot the error? It is pretty blatant, and I’m not sure if it is a manual calculation error or an automatic algorithm gone awry. But again, why are all the errors we spot in the warm direction?
Of course it is likely a rounding error up from 56.5°F compared to the round down from 57.5°F due to NOAA throwing out decimal values…except of course when calculating century scale trends for public consumption.
The answer is here.
============================================================
UPDATE: A lot of people didn’t get what I was pointing to, and it is simply this. The average departure comes out zero, but we have two -1’s listed in the “departure from normal” column. This is an artifact of rounding to the nearest integer.
Normal value average calc 68+47/2 = 57.5
Observed value average calc 67+46/2= 56.5
By normal rounding rules, 56.5 would become 57 and 57.5 would become 58, leaving a average departure of -1. But in this case, 57.5 is rounded down to 57, leaving a departure of zero.
There’s this reference to it in Wikipedia on Rounding, something I’ve been aware of for some time from my work in instrumentation:
In a guideline issued in mid-1966,[18] the U.S.Office of the Federal Coordinator for Meteorology determined that weather data should be rounded to the nearest round number, with the “round half up” tie-breaking rule. For example, 1.5 rounded to integer should become 2, and −1.5 should become −1. Prior to that date, the tie-breaking rule was “round half away from zero”.
18. OFCM, 2005: Federal Meteorological Handbook No. 1
So, it seems to me that NWS Chicago broke that rule by rounding the normal value calc down to 57. There’s more support for this here in the NOAA Cooperative observers handbook from 1989 on page 37:
Record the maximum, minimum, and current temperatures on WS Form E-15. Record to the nearest whole degree, even though the readings are displayed to the nearest tenth degree. If the last digit is a 5 (e.g., 43.5), round the temperature upward to the next higher whole degree (i.e., 44).
If NOAA has another rule contrary to this for dealing with 0.5 in the context of reporting averages, I’m unaware of it.
==============================================================
Regarding measuring climate at O’Hare Airport…
I’ll bet that many of you don’t know that the ICAO ID for O’Hare, is KORD, and FAA uses ORD which is what you see on airline luggage destination tags. “ORD” has nothing to do with the name O’Hare, which came after the airport was established. It has everything to do with the name “Orchard Place/Douglas Field” which is what the airport started out as, which at the time was far more rural than it was now.
Here’s that same view today from Google Earth:
Looking down runway 22 today – click for larger image
Look at O’Hare today, a sprawling megaplex of concrete and terminals surrounded by urbanization:
Click for interactive view
The weather station location above is designated by the orange pushpin. Here’s a closeup view:
Click for larger image
Note that there’s two electronics equipment buildings nearby with industrial sized a/c exhaust vents. While not USHCN, NCDC metadata lists O’Hare as a Class “A” station, which means it does in fact record climate. Data from O’Hare can be used to adjust other stations with missing nearby data.
The point I’m making with all the photos is that airports are far from static, especially since airline deregulation in the 1980′s. The are just as dynamic as the cities they serve. We measure climate at a great many airports worldwide. E.M. Smith reports that the majority of the GHCN record is from airports.
O’Hare airport is an extreme example of land use change around a place where climate has been measured long term. It went from being essentially rural, to a megaplex of aviation cast in concrete, asphalt, and terminals surrounded by suburbia.
You can read about its early history here.
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So, sorry to add to the conspiracy here, but it’s worse than portrayed. Someone may already have said this, but *one* of the standard ways to round numbers ending in “5” is to round to the nearest *even* number. The way that most random number generators work you can only get an average that agrees with the expected average for the generator by using this algorithm (as opposed to always rounding “5” up). For instance, go by the Wolfram Alpha site and round 57.5 and 56.5 to see what Wolfram thinks the answers are.
The alternative explanation is that they hold more digits during the calculation and only report to integer values. That’s a perfectly reasonable possibility. Did anyone ask?
*****
O’Hare airport is an extreme example of land use change around a place where climate has been measured long term.
*****
Given so many airport station locations, it’s prb’ly quite common. But you knew that. 🙂
NWS near my home has the same problem. I checked April and found 7 instances. In May so far there are 5. But I found that it doesn’t effect the monthly departure. I went ahead and replaced the proper daily departure, did the math, and it still came out the same as listed on their monthly reports.
I can give you a good guess about what happened. I’ll bet you dollars to doughnuts it’s computer generated. Probably the real data is in tenths of a degree. The math is done with the maximum precision available. Then, when writing the report, they use a rounding function for the display. One example is:
max = 67.4 -> 67
min = 46.3 -> 46
avg = 56.85 -> 57
That’s what any half-decent engineer (or any competent programmer) would do. You never round until the final display.