Filter a collection using nightime in Google Earth Engine

Question

I'm working with GOES-16 full disk data, trying to calculate the frequency of low clouds in northern Chile.

I need to filter the collection so it contains only images from September 2017 at night. This means, between 22 UTC and 11 UTC. To filter the year and month I have used ee.Filter.calendarRange but I don't know if it is working with the night hours because the start argument (22) is greater than the end (11).

Here is the code I have been using:

//creation of temporal filters
var rYear = ee.Filter.calendarRange(2017,2017,'year'); //year 2017
var rMonth = ee.Filter.calendarRange(9,9,'month'); //september
var rHour = ee.Filter.calendarRange(22,11,'hour'); // nocturnal hour range in UTC (local time: UTC-3)

//filter collection by temporal range
var night = GOES16.filter(rYear).filter(rMonth).filter(rHour);

Here is what the reference page of ee.Filter.calendarRange says, but I don't understand it:

If end < start then this tests for value >= start OR value <= end, to allow for wrapping.

And here is the complete code: https://code.earthengine.google.com/27d18828f97161552d812cf82aa87477?noload=true

Is there a way to filter the collection only in the desired hour range (22-11 UTC) for a whole month and for a specific year?

Answer 1

Your code works and I cannot think of code that is necessarily more elegant. The GEE documentation is saying that

ee.Filter.calendarRange(22,11,'hour')

is equivalent to

ee.Filter.or(ee.Filter.calendarRange(22,24,'hour'),ee.Filter.calendarRange(0,11,'hour'))

It may be helpful to convince yourself with an example such as the following:

var GOES16p = GOES16.filter(rYear).filter(rMonth)
print(GOES16p.filter(ee.Filter.calendarRange(22,11,'hour')).size()) // 1680 
print(GOES16p.filter(ee.Filter.calendarRange(22,24,'hour')).size()) // 240 
print(GOES16p.filter(ee.Filter.calendarRange(0,11,'hour')).size()) // 1440