There is this one...
%daysTminyear<0Clg%
and the month one
%daysTmin<0Clg%
In the climatedataout52009.html and similar for the named months table there is data you can setup in there.
Check out [View] - [Ave-Ext NOAA Style Reports] & Detailed Climate tab on RHS and in there is ability to setup No of days <= 0 and then upload those tables to your site and setup drop down menu for them for the months and the years.
climatedatout.html is current month, climatedataoutyear.html is current year.
HTH
Graeme