If it is in the northern hemisphere, then it must be in the southern hemisphere.

If it is summer in the Northern Hemisphere, then it must be winter in the Southern Hemisphere.

Explanation:

If it is summer in Northern Hemisphere, it will be winter in Southern Hemisphere and vice versa. The northern hemisphere and southern hemisphere have opposite seasons. Northern hemisphere is the region lying north of the equator and southern hemisphere is region lying south of equator.

The difference in seasons is because of the tilt of the earth. In December, the northern hemisphere experiences winter whereas southern hemisphere experiences summer because southern hemisphere is tilted towards the sun creating warm temperatures.