[NASA's Earth Observing System][1]NASA's Earth Observing System 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. [1]: http://eospso.nasa.gov/
