In article , Edwin
Pawlowski wrote:

"Don Klipstein" wrote in message
It is the bulbs rather than the ballasts that are
impaired by low temperatures.

- Don Klipstein )

If that is true, why do low temp fixtures have different ballasts and use
the same bulbs?

Because the bulbs need extra voltage to start at lower temperatures.
The concentration of mercury vapor in fluorescent bulbs has significant
effect on their operation at room temperature and even at freezing
temperatures, and varies all too significantly with temperature.

As for how significant is mercury in the workings of fluorescent bulbs?
At ideal temperature, the mercury vapor is at most a few tenths of 1% and
sometimes less than 1/10% of the gas/vapor mixture - yet is responsible
for nearly all ultraviolet produced by the diffuse arc that causes the
fluorescent phosphor coating to glow. Even when the mercury vapor content
is as low as .01% of the gas/vapor mix, it is a significant active
ingredient.

Some more details in:

http://www.misty.com/~don/dschtech.html

Yet, the essential mixture of an inert "majority gas" plus the minority
but active ingredient of mercury vapor has a significant sensitivity to
temperature. And the mercury vapor does affect significantly the
electrical characteristics of fluorescent bulbs, especially including
starting through the "Penning effect".
As a result, reliable starting at temperatures significantly lower than
usual requires more voltage. This is the job of the ballast, especially
when the ballast has more than 2 leads.

A lot more info with a bit relevant is in:

http://www.misty.com/~don/f-lamp.html

- Don Klipstein )