Final answer:
The most likely factor for the difference in climate between the two sides of the mountains in Washington is the Westerlies, which result in varying weather patterns due to orographic lift.
Step-by-step explanation:
The factor most likely responsible for the difference in climate between the two sides of the mountains in Washington is Westerlies, global winds that blow from west to east. Westerlies are significant in affecting weather patterns on the western coast of continents, especially in the temperate zones. When these winds encounter mountain ranges, they are forced upward, causing the air to cool and moisture to condense, leading to precipitation on the windward side of the mountains. This phenomenon is known as orographic lift, and it results in a wet climate on the windward side and a dry, rain-shadow effect on the leeward side.
While the Gulf Stream does impact climate, it influences the Atlantic Ocean and, consequently, Western Europe. Therefore, it is not responsible for the climatic differences on either side of the mountains in Washington, which is located on the Pacific Coast of the United States. Similarly, latitude-based pressure belts, like those at 60° N and 30° N, do influence global climate patterns widely, but they are not the primary reason for the local differences in climate observed near mountain ranges.