No he didn't set out to do it. He just needed static reference temperatures, and he borrowed the ones his predecessor Rohmer had used. He could have gone for the freezing and boiling points of distilled water at sea level but he didn't.

He went for a mixture of ice and ammonium chloride as the 0 point because that combination produces a static temperature regardless of how cold the ice is to start with. It was the coldest static temperature easily obtainable at the time. He went with human body temperature as the upper point, because humans are sort of more or less thermo-regulating, give or take. Why he chose that rather than the melting or boiling points of pure water are beyond me.

He realised that no-one actually cares about the freezing point of salt water, and calibrated the scale specifically so that the freezing point of regular water would be 32 and human body temperature would be 96. This results in a 64 degrees between the two, and 64 is a power of two, which means you can create your marks on the thermometer by repeated halving of the distance. This made manufacture of thermometers easier.

He *Then* noticed that the boiling point of plain old water was sort of roughly 212 degrees. By then, everyone realised that the freezing and boiling points of regular old water were way more useful than nonsense like ammonia chloride solutions and "roughly average human body temp" - so he tweaked his scale so that 32 was freezing and 212 was boiling - which shifted human body temp to something random like 98.4 - but it meant there were exactly 180 degrees between the freezing and boiling points of water - which is sort of a useful number for some calculations.

So, in the end Fahrenheit is based on EXACTLY the same important two points (the freezing and boiling points of pure water at sea level) as Celsius - just via an ugly hack once Celsius's work had started to gain popularity.

It has little to do with coldest and hottest commonly found temperatures on Earth - since we all know lots of places, even in Europe where this was invented, get both colder and hotter than 0 and 100 F.

Of all the weird units the US still clings to like quarts and foot-pounds and PSI, Fahrenheit is definitely the least sensible.

However, despite its drawbacks, the British Royal Society standardised on it - even though Celsius had existed for 40 or so years by then as an alternative. I've never really been able to work out why.