Granted, we live in the age of electronic ballasts, but there are still alot of magnetic ballasts still out there, so I think this topic is still interesting.

The most well known nemesis of flourescents is temperature. But what I never hear about, and drives me the craziest, is lamps that won't start because of humidity. 

At my work are lots of two lamp 40w fixtures which all work fine until it rains the night before and the next day is warm and humid. I'd say as many as one third of those lights won't come on the next day if the weather is just right.  I get up on the ladder and run my hand across the lamps to make them turn on.

In my bathroom, I have a 40w fixture. Brand new, works great, but try turning it on after the shower is used. When I replaced it with an electronic ballast, problem was solved.

Now, I think I know my rules for wiring. Black to black, white to white, and all metal enclosures with a solid ground. Lamps mounted within 1/2 inch of metal reflecter. Line volts are a solid 120, ballast open circuit for a 2-40w r.s. is 280v, etc-etc.

I guess I'm wondering if there's anyone else's take on this because this  can be aggravating and I wonder if this was a design weakness from the get-go in the industry. Do you think it's in the ballasts or in the lamps?
Truly, Don

Something to do with moisture being slightly conductive and killing the capacitance coupling the tube needs to start.

My kitchen light which has the U shaped T-12 bulbs used to have problems starting sometimes....sometimes it wouldn't go on at all, sometimes only one bulb would glow dimmly, sometimes it would start fine.....

it has 40 watts bulbs....

So I opened it up....cleaned the lightbulbs (they were not really that dirty at all) and stripped some paint out (the whole reflector is painted white) in the grounding area so the grounding wire and the metal of the fixture gets better contacts...  and now it works perfectly fine!!!

before those bulbs...it had two 34 watters...it had the same problem....I tried changing the ballast...didn't work...but changing the bulbs to 40 watts works...but I still have the 34 watt bulbs in the house.....stored away since they still work......

(I put the old ballast back in and returned the new one by the way)

It is because rapid start systems were flawed from day one. When tubes are dirty  or during high humidity, the lamps won't start. Aside from low grade shop lights and 14-20 watt trigger start ballasts, they are the most cheesy and inefficient fluorescent systems I have ever seen. The old preheat fixtures from the 1940s and early 1950s actually work bettter than any rapid start. The starter just takes care of starting the lamp, no worries about weather, bad ground, dirty tubes, subtle flicker,  etc. The ballast losses in preheat ballasts are actually lower than rapid start ballasts. My recommendation is to go to T-8 (or T-12 electronic) if you can afford it. T-8s and T-5s are good systems too.

I like the T5's myself although I dont have any

I have one T5 fixture I built out of a F32T8 fixture with a bad ballast.  I have had the most trouble with rapid start fixtures as compared to preheat, instant start and electronic.  Having said that, rapid starts do work pretty well as long as they are grounded.

my house was built in 1987 so most every fluorescent fixture is a F40T12 rapid start. the ballast brand is Advance. the 8ft ones in my basement and garage are magnetic instant start and always work well even with philips alto, which is now the only non-specialty T12 you can get. alto's work okay with rapid start but sometimes when they are new they sputter when first started. i have had problems for YEARS with a F20T12 rapid start in my basement that you have to hit to come on. it isnt the ground wire i already fixed that, i am either gong to convert it to pre-heat or put an Electronic T12 in.

I noticed a strange thing with my trigger start ballast: It starts up F14s and F20s easily, but has hard time to start a F15! Touching the tube and/or grounding the ballast changes nothing at all.

Is Advance a good ballast brand?

Advance ballasts are O.K.  G.E. and Universal ballasts are better, in my opinion.  Universal has been making ballasts since the preheat days.

In the basement of where I work, there are F40T12 strip lights that were installed when the building was built in 1960 (all rapid start), and half of them don't work.  The ones that do work light up sometimes, and sometimes they just flicker because for whatever reason (age, probably) the ballasts don't give enough of an inductive kick to get the lamps running.

Usually, needing to touch a lamp or fixture to get the bulb to light indicates a bad ground.  But I also think that rapid start ballasts can be fussy with no rhyme or reason.  Sometimes I notice that a rapid start light won't light up when turned on, but if you flip the switch on and off quickly several times, the light will light up. (Dunno why that happens).  But I still think that for fluorescent lights, preheat is probably the most reliable for longevity and reliable starting purposes.

By the way, @ lightman, I think preheat is the way to go for that F20T12.  You'll never have a problem with a preheat circuit for that fixture.  I've converted several programmed start F20T12 electronic fixtures to preheat, I run them for hours and hours on end...they always light up fine, and the lamps show no signs of aging after several months of usage...

Something to do with moisture being slightly conductive and killing the capacitance coupling the tube needs to start.

You could try Philips MCFE tubes - I read somewhere that these have a special external coating of silicone that prevents surface moisture from forming.

My experience:
In buses were installed electronic F20T12, lamp circuity being RS-style (at about 1kHz; design from 60's). And when tubes were changed to F18T8 (as F20T12 were not available anymore), these start to have troubles starting when cold or humid. What helped was a wire loop around each end (at about the filament position, or ~inch towards the center), connected to ground. This seemed to solve the problem entirely - no troubles recorded since.

I found the problem! The ballast says: "For use with preheat lamps on a rapid start circut". I wasn't using preheat lamps! It now works fine. BTW the ballast is an Adavnce, for use with F15T8's, F15T12's or F20T12's...

In an addition we built onto my parents' house we installed two FB40T12 2-tube rapid start fixtures in it.  We had nothing but trouble with them.  We finally took them down and went with R30 flood lights.  I'd never had that much trouble with linear or circline RS fixtures has I had with those U-tubes.

I found the problem! The ballast says: "For use with preheat lamps on a rapid start circut". I wasn't using preheat lamps! It now works fine. BTW the ballast is an Adavnce, for use with F15T8's, F15T12's or F20T12's...

Actually, all F14 through F20T12 are designated 'preheat'. The ballast merely adapts existing preheat lamps into a rapid start circuit. As for those sizes, I have found that the high power factor ballasts work better than the normal power factor. Also, the 'cold temperature' version...which are no longer made in magnetic, are the best if you can find them. The open voltage is the highest so proper starting is easier.

......In the basement of where I work, there are F40T12 strip lights that were installed when the building was built in 1960 (all rapid start), and half of them don't work.  The ones that do work light up sometimes, and sometimes they just flicker because for whatever reason (age, probably) the ballasts don't give enough of an inductive kick to get the lamps running.....

......  Sometimes I notice that a rapid start light won't light up when turned on, but if you flip the switch on and off quickly several times, the light will light up. (Dunno why that happens)..... 

If both lamps glow dim and they aren't burned out yet, it is because one of the sockets has a bad connection and the filament isn't getting the heating voltage. Also, flipping the switch quickly causes high voltage inductive kicks that can help start a stubborn lamp.

@icefoglights....I have found the U-tube lamps to be terrible for starting. We have a sandblaster at work that has those and they had that problem until I replaced the ballast with T12 electronic and now they start fine every time.