CWOP Information for CW1129 (C1129) in Chelmsford, MA, US

The following information is known about CW1129. If you think that any information is incorrect, then please follow the instructions for correcting that data item.

It is possible that this page will report a problem where no problem exists. If you believe that this has happened, then please contact me (address at the bottom of this page) and explain clearly why you think this page is in error. Note that any changes that you make may take a few days to show up here, so please give it a week before commenting.

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Registered Location

Latitude: 42° 36' 7" N (deg min sec), 42.6018° (decimal), 4236.11N (LORAN)
Longitude: 71° 24' 5" W (deg min sec), -71.4015° (decimal), 07124.09W (LORAN)
Elevation: 102 metres (335 feet) -- validated against 107 metres (351 feet) from Geonames
Location: Chelmsford, MA, US
County: Middlesex, MA
Forecast Office: Boston (BOX)
Weather station Manager/Operator: Jeremy
Station type/software: Weewx version 4.1.1 / Vantage

The site location should be at the mark shown -- or very close. Clicking within the map area (or you can drag the 'CW1129' marker) will display the latitude/longitude of that point.

If the displayed location (or elevation) of the weather observing location is incorrect, please drag the marker to the right location (or click in the right location), and then send the updated information. Please note that we are trying to spot either the Stevenson Screen (or equivalent) or the location where the barometric readings are taken.

NOTE: The Registered Location is the location that the station owner provided when they signed up with CWOP. Show location history.

There are no photographs of the station. If you are the site operator and have pictures of this location, then please email them to

You can try to find photographs from the area surrounding the site marker by clicking on Find nearby photographs.

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APRS Location

APRS Location recorded at 2020-Sep-25 00:25 UTC
Latitude: 42° 35' 34" N (deg min sec), 42.5927° (decimal), 4235.56N (LORAN)
Longitude: 71° 23' 12" W (deg min sec), -71.3868° (decimal), 07123.21W (LORAN)

The location reported in the weather reports (via APRS) is significantly different to the registered location above. In particular, the difference is about 1.0 miles. If you have changed your location (or software) since the time above, then this can be a false alarm. Check the location currently being reported at

Please check which location is correct. If the registered location is incorrect, then you can correct it by dragging the marker above and using the 'send the updated information link'. If the APRS location is incorrect, then you must enter the correct coordinates into your software which generates the weather report (APRS) messages. Note that the various different types of software take the location in different formats, so be sure to use the correct format when entering the latitude and longitude.

Data Status

Over the last 28 days, no data was seen on the following dates: 2020-09-09 to 2020-09-19.

The last 24 hours of weather data can be viewed at Findu. This page also provides access to the raw APRS packets and various other APRS related tools.

Recent observations from this weather station can be downloaded as a CSV file suitable for importing into various spreadsheet programs: last 7 days, last 14 days, last 28 days, last 56 days. This data can also be linked directly from Excel using a Web Query. The main link goes to the page with the data on it, and the IQY link is the Web Query that will access it: last 7 days (IQY: last 7 days), last 14 days (IQY: last 14 days), last 28 days (IQY: last 28 days), last 56 days (IQY: last 56 days).

All the means and standard deviations shown below are from the last 28 days. Change analysis period to Last 3 days  Last 7 days  Last 14 days  Last 8 weeks  Last 13 weeks  Last 26 weeks  Last 39 weeks  Last 52 weeks  

The inconsistency noted above in your location information means that the analyzed readings for your location may be incorrect. For example, if your location is incorrect, then the analysis values will be for a location other than your station. This will give different analysis results, probably leading to errors being shown below. If the elevation is incorrect, then the comparison of temperature readings will be in error.

A recently received weather report from this station was


Average barometer error: 3.5 milliBars
Error standard deviation: 0.4 milliBars
Worst daily average barometer error: 3.9 milliBars
Location problem
MADIS rating 0% QC OK
Sensor fails following checks:
L3 Spatial Consistency

Latest daily graph:

Latest daily graph

There is a significant mean error in your barometer readings. You may need to calibrate your barometer. Since your readings are low, it is possible that your software is not adjusting the reading for elevation, or that your station altitude has been entered incorrectly into your software.


  24 hours  Day time  Night time
Average temperature error-1.3 °F-0.0 °F-1.6 °F
Error standard deviation1.6 °F1.1 °F1.6 °F

NOTE: If the error above is POSITIVE, then it means that the analysis temperature is HIGHER than the reported temperature. This means that your sensor is reading COLDER than expected.

MADIS rating 100% QC OK

Latest daily graph:

Latest daily graph

These values are within the acceptable range. This probably means that your sensor is sited correctly and is calibrated correctly.

Dewpoint -- or switch to Relative Humidity

  24 hours  Day time  Night time
Average dewpoint error2.9 °F2.0 °F2.9 °F
Error standard deviation1.5 °F1.7 °F1.5 °F
Worst average dewpoint error3.3 °F4.1 °F3.3 °F
Worst standard deviation2.3 °F0.9 °F2.2 °F

NOTE: If the error above is POSITIVE, then it means that the analysis temperature is HIGHER than the reported temperature. This means that your sensor is reading COLDER than expected.

  Location problem
MADIS rating 99% QC OK

Latest daily graph:

Latest daily graph

There is a significant mean error in your dewpoint readings. You may need to calibrate your dewpoint sensor. Your readings are not within an acceptable error range. The automated problem analysis cannot identify the problem. You can signup on the wxqc mailing list and post information about this situation there. This is may be caused by poor siting of the sensor (e.g. under eves, in screen porch, or in thick tree coverage). For information on siting, see CWOP Station Guide.

Wind Vector

The wind vector is the average wind over the relevant period. Note that if the wind blows North for half the time at 2 knots, and then blows South for the other half of the time at 2 knots, then the result is zero -- i.e. North wind and South wind will cancel out. Over time, the direction of the wind and of the analysis should match pretty well. If the lengths of the vector are different, then it could mean either that your anemometer reads high or low, or that you are in a more shielded or more open space.
MADIS rating 93% QC OK
TodayLast 7 days
Wind vector chartWind vector chart for last 7 days

Neighboring Stations

This is a list of stations that are close to this station. Note that this is not the list of stations that are used in the data analysis. That list of stations is known only to MADIS. You can see all the stations at MADIS Data Display. Your station should be in the middle of the display. If you think that the analysis is incorrect, then maybe one of the close stations has failed.

You can look at those for comparison purposes. The average difference between your station and each station below is listed as well. This may allow you to detect local stations which are erroneous. You can also use these numbers to adjust your instruments -- especially if a number of ASOS stations have similar differences to your station. Also, try increasing the analysis period to more than the default 28 days. The difference is only given if both stations report reasonably often during each day. Some stations only report every 6 hours, and this makes the average of less interest. In this case, the average is not displayed. The values reported are the differences between this station and the neighboring station. I.e. if the difference is positive, then this station is reading higher than the neighboring station. Since this station has not been reporting continuously during the analysis period (it missed 11 days out of 28), the offsets below may not be accurate.

You can also subscribe to an RSS feed to tell you when new stations come online in this area. Choose either within 100 miles or 250 miles . For stations covered by your Forecast Office: Boston (BOX) .

FW81032.9 milesSouth Chelmsford, MA, US (lat 42.5628°, long -71.3812°, altitude 64 metres) -6.0mb2.8°F-1.7°F
CW36603.7 milesNortheast Lowell, MA, US (lat 42.6260°, long -71.3370°, altitude 54 metres) -6.5mb1.2°F-1.9°F
EW48963.7 milesSouth Westford, MA, US (lat 42.5490°, long -71.3880°, altitude 66 metres) -4.9mb2.5°F-2.3°F
EW89584.6 milesEast Chelmsford, MA, US (lat 42.6075°, long -71.3108°, altitude 37 metres) -6.1mb3.5°F-1.4°F
EW98084.7 milesWest Westford, MA, US (lat 42.6170°, long -71.4908°, altitude 126 metres) -5.9mb1.0°F-0.6°F
EW01865.6 milesSoutheast Carlisle, MA, US (lat 42.5292°, long -71.3520°, altitude 59 metres) -5.6mb1.6°F-2.4°F
DW12835.9 milesSoutheast Carlisle Public School, MA, US (lat 42.5262°, long -71.3495°, altitude 73 metres) -5.0mb2.4°F-0.5°F
CW32246.5 milesNorthwest Dunstable, MA, US (lat 42.6588°, long -71.5028°, altitude 71 metres) -7.1mb4.6°F0.0°F
EW18987.5 milesNorth Pelham, NH, US (lat 42.7085°, long -71.3807°, altitude 97 metres) -6.6mb2.6°F-1.4°F
EW99688.6 milesSouthwest Littleton, MA, US (lat 42.5000°, long -71.5000°, altitude 78 metres) -6.7mb

Quality Control Notes

This page reports on the quality control checks performed by MADIS. The green check mark and red cross are chosen based on the average performance during the analysis period (which is selectable above). The red cross can appear even if MADIS reports that all observations pass the MADIS QC checks.

The 'MADIS rating' reports on the percentage of observations that pass the MADIS checks. You should aim for two thumbs up!

Location Formats

The location is shown in three different formats. Each of these is used in some software, and is shown on some websites.

Degrees Minutes Seconds: VWS/VWSAPRS, WeatherLink 5.5, WeatherLink (5.2-5.4), WeatherView (Linux)

Decimal degrees: FreeWX, WeatherView32 (PC application)

LORAN: WeatherDisplay, WUHU, Henriksen WServer


CWOP Home: This describes the whole CWOP program and has a large amount of useful information.

CWOP Siting Guide (PDF): This is a guide to siting and operating a CWOP station. This is a must-read document.

Another CWOP Guide (PDF): This is another guide to siting and operating a CWOP station. This is a must-read document.

Urban Siting and Operations (PDF): This is a guide to siting and operating a CWOP station in an urban environment.

APRS WX Message format: This describes the actual format of the messages transmitted that carry WX information.

WXQC Mailing List: This is a mailing list where data quality issues are discussed. Assistance with software configuration can often be found here.

Topo Map Reading Guide: How to read USGS topographic maps, with a good section on reading contours.

Geographically nearby web pages: Use Geourl to find web pages that deal with locations geographically close to this site.

Overall website comments, problems etc to Philip Gladstone
Page layout last modified Sunday, 28 February 2016
Questions for this weather station owner/operator to Jeremy
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