For the purpose of this question, let's assume that relamping HID fixtures is taking place.

Lets say you have an HID fixture of relatively high wattage up in a utility pole, and that it has been day burning for a while. The utility workers come up to fix it, and it's raining. When removing the still hot lamp, does it happen to have rain hit it and shatter it ? Or maybe the slightest rain means a no go to repairing streetlights ? Just curious.

We do not have any day burning street lights but it happens from time to time that I’m replacing older HID bulbs in the nights in rainy weather. I often use the cardboard sleeve from a new lamp or a cloth to remove the old hot bulb and I put in the box im my bucket van. If it’s raining then the droplets just evaporates as soon they land on the bulb’s surface without any damage. One exception I have is with old MV 125W. They were often made of soft glass and if the bulb is hot then there are some cracks forming and as soon as I touch the glass it just falls apart without explosion.

About 20 years ago I had to replace an old blackened 1000W incandescent bulb which was on. As soon as I tried to screw it out it made a loud pop and then the bulbs hot envelope fell on my head.   

Most HID lamps have the outer envelope made of borosilicate glass which is more resistant to thermal shock compared to the soda-lime glass used in incandescent and fluorescent lamps.

I believe older HID lamps also had the outer envelope made of soda-lime glass but as early as 1956 Westinghouse advertised their Weather Duty mercury lamps which they claimed could operate unprotected under rain, snow, or sleet.

Personally, I find it dangerous to relamp any fixture in the rain because there is the possibility of electrocuting yourself or damaging the fixture from rain droplets that might short circuit internal components.

Rain doesn't prevent us from relamping.

The equipment is durable and can survive being relamped in the rain.

Generally, relamping is done with an aerial vehicle, which may be a fiberglass bucket truck or an older metal platform truck. The tires of the truck isolate you enough to reduce risk of electrocution to almost nil. The fiberglass buckets are even safer. Just don't touch anything with more than one hand.