Bruce --
Kerosene is a petroleum distillate not unlike gasoline, fuel oil, etc. All these distillates are composed of a range of different length hydrocarbon chains. Shorter chains, lower boiling point, more fumes. Assuming the container isn't perfectly sealed, the light fractions will evaporate leaving a higher percentage of the heavier stuff.
Your old Kero is fine but it may behave a little more like No. 2 Fuel Oil, e.g. a little more stink, little more soot, little harder to light than it really should.
As for the humidity, Mr. Graham is right on. Hydrocarbons are just that -- a bunch of hydrogens bound to carbon atoms. When they burn the H-C and C-C bonds break and form new ones with oxygen. The carbon mostly winds up as CO2 and CO but the hydrogen mostly winds up as water (H2O). There's a lot more H than C hence all the water.
Sorry for the chemistry babble. Short story is that it should be fine for nearly all purposes but may stink a bit.
Pete P.
Harborcreek, Penna.
