A person is not only dehydrated during summer time but also in cold dry weather.
During cold dry weather we loss more fluids due to loss of water in respiratory parts.At that time our blood vessels constrict which makes it difficult for blood to flow freely to extremities.
During winter time one feels less thirsty.It is so because sweat evaporates very quickly in cold weather which decreases our thirst response. At that time more urine is also produced.
Hence the statement given in the question is true(T).