Top hats were a fashion must for all respectable gentlemen for a good portion of the 19th century. However the felted beaver fur that was traditionally used in construction proved just slightly too hot for the summer months.
The solution was the introduction of straw top hots for summer wear. In the very early 19th century some were even painted black to resemble their winter counterparts as closely as possible, but eventually leaving the straw its natural color became the style of choice. This one is American in origin and dates to 1832.
To quote Wayne: ‘I did not know that!’
By the way as an aside, top hats were only beaver fur for early part of the 1800s, silk plush became the popular shiny alternative after the 1850/60s which I expect being silk was also less hot than fur felt. Seems to be some confusion about this over at eBay – hence the rather strange epiphets ‘beaver silk" and ‘silk beaver’ – how does a beaver spin silk exactly?!? :-p And silk beaver sounds like you should get it in a plain wrapper in Soho…
