How to order by then start at first non null value and select a limit for number of values
Step 1:  Write a query that fetches only the outsidetemp values:  Use WHERE outsidetemp IS NOT NULL.  Put the results in a TEMPORARY TABLE.

Step 2:  Tack on an AUTO_INCREMENT

Step 3:  Fetch from it with WHERE (id % 5) = 1 to get every 5th, starting with the first available.

Step 4:  If you want other columns, then JOIN back to the original table USING(datetime).