There is an aggregation for the number of days where an observation type is greater than some value. However, you would need an observation type. That is, some way of measuring "sultriness." In that case, it would be something like
$month.sultry.max_ge(40).
This would give you the number of days in the month where "sultry" reached 40 or higher.
However, if it's a conditional on multiple variables, that is, the temperature must be greater than 25 and the humidity must be higher than 59%, then you'd have to write either a search list extension, or an xtype aggregation.
-tk