Heat won't really effect mercury vapor in a negative way unless you are talking like 150+ degrees. Cold increase the warm up time. Again, the colder it gets the longer it will take. 50-40 degrees is when you will really start to notice an added length of time. Below freezing will start to take huge amounts of time to warm up.
The temperature is not a safety issue, it is merely a time to turn on to full issue. Unless you are doing something really stupid, like having an 8k Fresnel pointed right at the unit point blank range causing it to cook, temperature won't actually cause a problem.