The directories listed above contain GIF files with names of the form:
yymmddhh_sathts_snd_alt.gif
yymmddhh_sathts_500_alt.gif
yymmddhh_sathts_300_alt.gif
where yymmddhh is the UTC date and hour. In particular:
yy is the last two digits of the year;
mm is the month (a two digit number);
dd is the day of the month (a two digit number); and
hh is the hour of the day (a two digit number).