Yes when summer time is happening in the USA then Australia is experiencing winter time and cold temperatures right now.
Like right now here in the USA where I'm at it's summertime and hot out but if I were in Australia then it would be winter and cold.
When summer is over with and winter comes to the USA then Summer Starts in Australia.
The reason that summer and winter are different times between USA and Australia is due to the earths axis being tilted.
And during the summer the suns rays are hitting the earth at a steep angle.