I was watching a storm yesterday and just wondering. Can water leak inside a NEMA head where the top attaches to the reflector? Also how does water not short on the photocell receptacle? I noticed the power company removes the photocell and leaves the bulb installed on NEMAs when the customer doesn't pay for it. So you would think the water would short the photocell socket.
I know sometimes bulbs can break if rain hits when it's on. But I see a lot of yardblasters missing the refractor but still working so it might be fine.
Water always leaks everywhere. The question is, whether it will be "pushed" out before it may do any harm. So the key is not that much to prevent the water from entering (that is impossible when you intend to rely on it), but most important is to prevent the water from collecting inside. And for that the easiest way is a hole in the bottom anb a good ventilation.
If something act as a roof, it protects against direct splashes pretty well, while the otherwise open design allows it to flow/evaporate away. But if you attempt to seal it, the result would just be a pool of water inside (an evening storm after a hot day means rapid cooling of the air inside of the bowl, what sucks in the water from the outside, unless it has other means to equalize the air pressure). Then the distillation "makes sure" to transport the water to the components that can easily be damaged by it (mainly when the fixture is off and the sun is heating the water pool through the bowl).
The photocell socket is shaped so the photocell itself act as the "roof", when the photocell itself is missing, the socket plastic shape makes almost impossible for the water to connect two pins with different voltage levels on it. However when the cell is missing, nothing protect the contact metal surface from becoming directly contaminated by the air pollution and then watered during the rain (the combination uses to be quite corrosive mixture on all metals used for connectors), so such socket would be ruined anyway when kept like that for a longer time.
For the running lamp and rain:
Most quality lamps use hard glass for the outer. Compare to the soda-lime material, the hard glass expands only very little with temperature, so even when it gets some thermal shocks from the droplets, the resulting stresses are not that large compare to the soft soda-lime material (that is the same, why the glass is "hard" - there is less stress after cool down during manufacture, so more remains as the resulting strength, therefore that material is then harder). And the lower operating temperature helps too.
The general recommendation to protect the lamps against that water takes into account the presence of defects, which may become a crack starting points. Because these defects are practically impossible to completely eliminate and/or screen out at the end of line tests, lamp maker usage recommendation and the fixture design should take their existence into account at least in safety measures.
