This style of hat is referred to as a Sou'wester. This name was originally given to hats worn by sailors at sea to protect them from rain and the stormy sea. It is characterized by a waterproof material and a brim that widens at the back to protect the neck. The shape was eventually adopted and used in fashionable women's wear, such as this example. The bright color, flowers, and straw indicate that this hat was likely worn seasonally in the spring and summer.