You mean lightning, thunder's just sound? Since an aircraft isn't grounded it won't take the full force, and crew/ passengers are safe because a metal aircraft at least forms a Faraday cage. But lightning is capable of blowing some electrical circuits unless properly isolated. High wind is very bad, can cause loss of control, a thunderstorm will reduce visibilty, as will fog, thick cloud, snow etc. Hail is a particular problem since, in extreme conditions, it is capable of actually wrecking an aircraft. There are no true "all weather" aircraft at all, those called all weather fighters just operate in worse conditions than most. In WWII fog/ poor visiblity was a genuine problem in that getting into the air was relatively easy - just open the throttle and go upwards :lol: But landing when you can't find the airfield or see the runway is not a good idea. The RAF invented FIDO - Fog Intensive Dispersal something - large drums of flammable material (oil?) along both edges of the runway which provided lights showing the runway position and at the same time caused thermal updraughts that lifted the fog locally. But IIRC that was for bomber squadrons and I can't remember how much use it saw. I have, somehere in my notes, some figures on typical conditions showing how many days per year there are when it's worthwhile putting aircraft up - Europe only. If I find them I'll post.
Weather itself is rarely a problem to planes once they are flying and provided they don't need to do much else. However landing and taking off in bad weather is very dangerous. Especially when loaded with fuel and high explosives! Furthermore being up in the sky is pointless when you can't see the ground. Especially given that there was no GPS to navigate by. Therefore it usually wasn't worth the risk to fly in bad weather, the opponents air power wasn't usually around to attack and you couldn;t enage ground targets either strategically (unless they were big, like bombing a city in general) or certainly tactically. You also need to remember that the vast majority of airstrips were grass and therefore vulnrable to weather themselves. FNG
