overheating rather than humidity as I travel between Aust and Singapore as well as working long hours in aircondition room then using it outdoors. Try cleaning the vents of the laptop, and if possible, you could try adding one of those portable-external USB fans as well.
I have an IBM R52 laptop and when I try to turn it on in humid weather it won't turn on. I travel a lot to SE Asia where it is very humid. The only solution is to put it in a warm place for several hours. It will then turn on but after 5-10 minutes shut down. When back in the US in air conditioning it works OK again.

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