"Is it winter now?"
Зараз - at this moment, immediately
Тепер - nowadays, lately.
Тепер should be used for the longer periods of time and зараз for the things that are true only at some short moment (second, minute, hour)
But lately native speakers started to use зараз where you would usually need тепер and "forgot" about тепер.
As winter is quite a long period of time, тепер would be more appropriate here