The heat waves in the 1930's are very interesting. I read about them a year or so ago more in depth. Basically our poor farming methods out in the midwest were completely destroying the soil leaving it unable to retain moisture combined with a very dry and hot decade that was only made hotter because we weren't doing the methods we started using after the dust bowl era. It's also interesting too that there were some downstream effects from there constantly being so much heat over the US from this that effected Asia which is neat. But yeah the soil was basically unable to retain moisture which allowed it to get superheated compared to how it normally is. Then after that we started practicing more efficient farming which allowed it to not get as hot. Remember too the 1930's dust bowl era is mostly a US phenomenon besides the downstream effect in Asia I mentioned.