In the US, It is called summer because it is in the Northern Hemisphere. Summer is considered the hottest season of the month in the US. But since Australia is in the Southern Hemisphere, that season is called winter. We swap seasons around the farther away the place is. The axis of the Earth is tilted, which means that in summer we (in the US) are leaning towards the Sun. As we travel around the Sun, we begin to tilt away and it becomes winter. This is why when we have winter, Australia has summer.
