<p><strong>Provide a monthly average in every grid cell, and describe how many measurements were used for each cell.</strong></p>

<p>There is no fixed rule for the minimum number of days for reporting a monthly average.  A reasonable threshold will depend on the geophysical quantity of interest <strong>y</strong>, in particular on how much <strong>y</strong> varies on short and long timescales.</p>

<p>If <strong>y</strong> varies from month to month, but does not vary a lot from day to day (example: sea surface temperature), you can probably get away with reporting a monthly average even when only one or two days have cloud-free measurements.  On the other hand, if you're measuring precipitation, your signal will be relatively noisy even if you choose a threshold of 20 days.  And any threshold is going to introduce a bias, because a lack of observation is typically due to clouds.  Although you will get a bias in any case.  In other words, if you only have one day per month you see a gap and in the clouds to observe whatever you want to know, it is a pity to throw it away.  Therefore, personally I choose a minimum of 1 observation.</p>

<p>Critically important though: when releasing the product, add a field that <em>tells</em> the user how many days worth of measurements make up the monthly average in any given cell!  Like that, the <em>user</em> can decide which ones to use and which ones not to use.  More broadly speaking: producers should <em>document</em> everything they do.</p>

<p>For your specific examples: search for the ATBD and/or scientific papers describing the product.  Hopefully, there is a field in the level-3 product that describes the number of measurements per cell.  If the answer is not in there, write to the producers and tell them to redo the product...  You will need this information to accurately calculate a yearly average!</p>

CC BY-SA 3.0
2016-02-02T03:14:56.713Z
When mapping geospatial data across large regions (e.g. thousands of grid-cells), it is unconventional to then map data contribution by location.  I've used whisker-bar charts to show the change in contribution by month, which is effective.  However, data regularly gets published as maps and no such companion chart is available.  I think map-makers should adopt a minimum to go into the map, and exclude those as "no data".  Unfortunately, I see a lot of NASA data reported that doesn't get  scrutinized to that level.
f.thorpe
CC BY-SA 3.0
2016-02-02T10:27:20.513Z
On a single map you can't show such information, but in the actual level-3 data files one should have multiple fields, one for mean, one for count, and probably others showing other statistics.
gerrit
CC BY-SA 3.0
2016-02-01T10:40:51.410Z
2016-02-01T10:40:51.410Z
gerrit
<p><a href="http://eospso.nasa.gov/" rel="nofollow noreferrer">NASA's Earth Observing System</a> produces many maps of air quality and land-surface variables, from instruments like MODIS, AIRS, OMI, VIIRS, etc.  Both NASA and the scientific community produce maps of the retrieved data on a daily basis, but there are often gaps (e.g.  no data) for many locations because of clouds, satellite coverage, or other limitations.  Daily retrievals are often averaged by month or season so that a complete global map (or sub-region) can be made and then presented to the public and put into publications.</p>
<p>Some locations have very few retrievals available for a monthly average (e.g. if there is a predominant cloud mass that sticks around for a long time).  So, researchers must choose some threshold where no data is reported.  As a graduate student, I would report the average if I had at least 4 days in the month, which seemed reasonable to me at the time for some reason.  However, the number of available data points for a monthly average fluctuates so much, that the minimum number of members in the monthly average that I chose affected my temporal trendlines.</p>
<p>What's the minimum number of days needed for satellite-derived maps of monthly averages?  I can't call 3 days of data a &quot;monthly average&quot; in good faith, but is 4 days enough?  For the purposes of this question, assume there is only one possible data retrieval per day per geo-location.</p>

CC BY-SA 3.0
2016-01-31T03:11:39.977Z
there's only 31 days max in a month... sooooo
f.thorpe
CC BY-SA 3.0
2016-01-31T03:15:42.990Z
For the ones I've mentioned, some provide 2 per day, but typically you average those separately because either the day/night retrievals are different (e.g. AIRS) or they are different instruments (e.g. MODIS/Terra and MODIS/Aqua)
f.thorpe
CC BY-SA 3.0
2016-01-31T07:41:27.983Z
Doesn't it depend on what the actual inter-day variation is? So if that's very low for one region, then we need very few observations for that region; and if it's very high for another region, then we need more observations for that region? And can't we *always* report it, just with wider or narrower uncertainties, accordingly?
410 gone
CC BY-SA 3.0
2016-02-01T22:29:39.430Z
Yibo Jiang, a Data Engineer of PODAAC, responded to my inquiry saying "Personally, I think average over 28 to 33 days is considered as monthly average."

-Yibo Jiang
Data Engineer of PODAAC
arkaia
CC BY-SA 3.0
2016-02-02T03:03:57.317Z
if you have 1 retrieval per day per location, and half are cloud covered, you will never have a monthly average by Yibo's standard.  But NASA and data users typically show "monthly averages" which could have as little as 2 values per month.  33 days in a monthly average sounds ... impossible?  I have a problem inserting data from other months into another's average.
f.thorpe
<land-surface><satellites><data-analysis><air-quality>
What's the minimum number of days needed for reporting a "monthly average" of daily satellite retrievals?
f.thorpe
# What's the minimum number of days needed for reporting a "monthly average" of daily satellite retrievals?

(Asked by: username_0 on 2016-01-31)

[NASA's Earth Observing System](http://eospso.nasa.gov/) produces many maps of air quality and land-surface variables, from instruments like MODIS, AIRS, OMI, VIIRS, etc. Both NASA and the scientific community produce maps of the retrieved data on a daily basis, but there are often gaps (e.g. no data) for many locations because of clouds, satellite coverage, or other limitations. Daily retrievals are often averaged by month or season so that a complete global map (or sub-region) can be made and then presented to the public and put into publications.

Some locations have very few retrievals available for a monthly average (e.g. if there is a predominant cloud mass that sticks around for a long time). So, researchers must choose some threshold where no data is reported. As a graduate student, I would report the average if I had at least 4 days in the month, which seemed reasonable to me at the time for some reason. However, the number of available data points for a monthly average fluctuates so much, that the minimum number of members in the monthly average that I chose affected my temporal trendlines.

What's the minimum number of days needed for satellite-derived maps of monthly averages? I can't call 3 days of data a "monthly average" in good faith, but is 4 days enough? For the purposes of this question, assume there is only one possible data retrieval per day per geo-location.

**Tags:** land-surface, satellites, data-analysis, air-quality

> **Comments:**
> **username_0**: there's only 31 days max in a month... sooooo  
> **username_0**: For the ones I've mentioned, some provide 2 per day, but typically you average those separately because either the day/night retrievals are different (e.g. AIRS) or they are different instruments (e.g. MODIS/Terra and MODIS/Aqua)  
> **username_1**: Doesn't it depend on what the actual inter-day variation is? So if that's very low for one region, then we need very few observations for that region; and if it's very high for another region, then we need more observations for that region? And can't we *always* report it, just with wider or narrower uncertainties, accordingly?  
> **username_2**: Yibo Jiang, a Data Engineer of PODAAC, responded to my inquiry saying "Personally, I think average over 28 to 33 days is considered as monthly average."  -Yibo Jiang Data Engineer of PODAAC  
> **username_0**: if you have 1 retrieval per day per location, and half are cloud covered, you will never have a monthly average by Yibo's standard.  But NASA and data users typically show "monthly averages" which could have as little as 2 values per month.  33 days in a monthly average sounds ... impossible?  I have a problem inserting data from other months into another's average.  

---

## 1 answers

### Answer by username_3 on 2016-02-01. Score: 4

**Provide a monthly average in every grid cell, and describe how many measurements were used for each cell.**

There is no fixed rule for the minimum number of days for reporting a monthly average. A reasonable threshold will depend on the geophysical quantity of interest **y**, in particular on how much **y** varies on short and long timescales.

If **y** varies from month to month, but does not vary a lot from day to day (example: sea surface temperature), you can probably get away with reporting a monthly average even when only one or two days have cloud-free measurements. On the other hand, if you're measuring precipitation, your signal will be relatively noisy even if you choose a threshold of 20 days. And any threshold is going to introduce a bias, because a lack of observation is typically due to clouds. Although you will get a bias in any case. In other words, if you only have one day per month you see a gap and in the clouds to observe whatever you want to know, it is a pity to throw it away. Therefore, personally I choose a minimum of 1 observation.

Critically important though: when releasing the product, add a field that *tells* the user how many days worth of measurements make up the monthly average in any given cell! Like that, the *user* can decide which ones to use and which ones not to use. More broadly speaking: producers should *document* everything they do.

For your specific examples: search for the ATBD and/or scientific papers describing the product. Hopefully, there is a field in the level-3 product that describes the number of measurements per cell. If the answer is not in there, write to the producers and tell them to redo the product... You will need this information to accurately calculate a yearly average!

> **Comments:**
> **username_0**: When mapping geospatial data across large regions (e.g. thousands of grid-cells), it is unconventional to then map data contribution by location.  I've used whisker-bar charts to show the change in contribution by month, which is effective.  However, data regularly gets published as maps and no such companion chart is available.  I think map-makers should adopt a minimum to go into the map, and exclude those as "no data".  Unfortunately, I see a lot of NASA data reported that doesn't get  scrutinized to that level.  
> **username_3**: On a single map you can't show such information, but in the actual level-3 data files one should have multiple fields, one for mean, one for count, and probably others showing other statistics.  

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