Because of America's size the climate is very diverse but is generally temperate. In the northern parts of the United States the climate is at its coolest with conditions going below breezing during the winter months. Conditions are particulary cool and some what bitter in the plains, as well as the Midwest and Northeast. The climate can go from one extreme to another with areas such as Florida and Hawaii experiencing tropical conditions where in Alaska the climate is more arctic like. Along the great plains west of the Mississippi River conditions are more semiarid turining into more arid like in the Great Basin in the southwest. In sunny California the climate is more Mediterranean with temperatures rising as high as 57 degrees in California's Death Valley during the summer months. Hurricanes are common in the United States and is home to some of the world's worst tornadoes, particulary in the Midwest. Tornadoes usually occur between April and June. Spring and Autumn which is also known as Fall are usually mild and sunny with wet weather occuring in some areas, although this is more so in the Pacific Northwest.