When it's is summer in sydney australia what season is it in montreal canada

in Montreal Canada it is winter.

We know that our globe is divided into two hemispheres, one north of the equator, the other south of the equator. Sydney, Australia, is in the Southern Hemisphere, while Montreal, Canada, is in the Northern Hemisphere.

We also know that the difference among seasons is due to the way solar rays reach the surface, which depends on the inclination of the terrestrial axis: during summer the light rays hit the surface almost perpendicularly, resulting in hot temperatures, while during winter the light rays hit the surface with a strong inclination, resulting in low temperatures. 

Due to the tilt of the terrestrial axis, the two hemispheres point towards the Sun in different moments during the year: indeed, seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere (see picture attached).

Therefore, if in Sydney Australia is summer, in Montreal, Canada, it is winter.

Noble gases is far right