Hourly precipitation calculation in weather.R incorrect

[rmcd1024]

When aggregated, the hourly precipitation numbers in the weather dataframe do not match official NOAA daily totals for the same locations. The calculation of hourly precipitation is somewhat involved. The issue is that hourly cumulative totals reset at 51 minutes (this is not invariably true, but it appears true for the 3 NY airports in 2013). My pull request #26 addresses this.

See this page for an example using ASOS data to match NOAA daily totals.

[rmcd1024]

To summarize the issue, the ASOS weather observations often include multiple observations in an hour. This occurs when the weather situation is changing rapidly, and thus often when there is heavy precipitation, The precipitation totals are cumulated during the hour and reset to zero at between 51 and 57 minutes, depending on the station. To get an accurate per-hour precipitation total, it's necessary to determine the reset time (which is 51 minutes past the hour for the three NYC airports) and to extract the precipitation total each hour from that observation. A possible algorithm is outlined here and the ASOS weather observations are also discussed here.