Why is Vladivostock so cold, while Dalian is so warm? Over at SE History we're pondering the relation of temperature to Russian motives in establishing its Port Arthur colony in the late 1800's.

Wikipedia says that Vladivostok is cold due to the Siberian High. Meanwhile, in my answer, I repeated what I learned decades ago in my high school earth science class about ocean currents and why Port Arthur was warmer. (Port Arthur was located near the modern large city of Dalian)

Another astute SE member in the comments pointed out that trade winds are more responsible than ocean currents for weather effects.

What is the truth for these two places? (Note that we are asking more about Winter than summer, as both places would make fine summertime 19th century ports) 

Also, was Vladivostok significantly colder 120 years ago than it is now?
Vladivostok (this is the spelling I've always seen) is located at 43º08'  N whereas Dalian lies at 38º05' N.   Five degrees make a difference.  Also, Vladivostok gets more influence from the cold Oyashio current than does Dalian.  For these reasons summers are warmer in Dalian and winters are colder in Vladivostok.