Weather differs from climate. Weather is what happens over a short period of time (e.g., a few days), while climate is a prevailing pattern of weather in a specific region; scientists usually measure climate in 30-year periods. Landforms, and large bodies of fresh and salt water, can affect both short-term weather and long-term climate.
The Spinning Earth
Because Earth's rotation is counterclockwise -- as viewed from a point above the North Pole -- major weather systems in the Northern Hemisphere generally move from west to east. As these systems travel over landforms or bodies of water, they may gain or lose heat and moisture content.
Mountains and Rainfall
Tall mountain ranges, like the Andes of South America and the Rockies of North America, act as an obstacle to traveling air masses, forcing them to rise over their lofty peaks. When this happens, air temperatures drop; as the water vapor cools, fog forms, and rain or snow may fall on the windward side of the mountain. When the same air mass descends on the other side of the mountain, it contains a minimal amount of water vapor. As a result, a "rain shadow" or dry climate develops on the far side of the mountain.
Air masses that travel across large bodies of water often pick up a substantial amount of water vapor. In the case of an ocean, the air mass may contain considerably more moisture when it reaches the far shore. Therefore, the climate of such coastal regions tends to be wetter; the Pacific Northwest is a well-known example of this effect.
Lakes, Bays and Gulfs
Like the oceans, a large lake, bay or gulf can act as a moderating influence on climate, resulting in cooler summers and warmer winters. For example, North America's Great Lakes modify the temperature of air masses that travel across them, which produces a comparatively mild climate. At the same time, these air masses pick up a large amount of moisture from the lakes, which precipitates annually on the downwind shores in the form of heavy rain and snow.