Final answer:
A rainshadow desert forms in places where mountain ranges act as barriers to the movement of water vapor, leading to arid conditions on their leeward side due to the rain shadow effect.
Step-by-step explanation:
A rainshadow desert forms on the leeward side (downwind side) of a mountain range. This phenomenon occurs when moist air from the ocean rises over a mountain range, cools, and precipitates its moisture on the windward side. The air that descends on the leeward side is dry and causes little to no precipitation, creating arid conditions. This is known as the rain shadow effect, responsible for many of the world's deserts, including the North American deserts like Mohave and Sonoran. Regions such as eastern Washington and eastern Oregon in the United States, western China, and the Atacama region of Chile also experience this effect, resulting in semi-arid or desert conditions due to the presence of high mountain ranges that block moisture.
The rain shadow effect is particularly evident where high mountains prevent precipitation-bearing clouds from reaching regions, as seen with the Himalayas affecting western China and the mountain ranges in the western United States. The Great Plains, for example, are semi-arid largely due to this phenomenon. Hence, the correct answer to the question 'A rainshadow desert forms ________.' would be 'in places where mountain ranges act as barriers to the movement of water vapor, creating arid conditions on their leeward side due to the rain shadow effect.'