The predominant wind is from the east, and the air cools aid forms rainclouds as it tries to rise over the mountains in the center of the island. Then warms again as it descends down the eastern slopes.

So the eastern (Hilo) side is pretty lush jungle, and the west(Kona) is desert. With snowy mountains in between.

[0]https://www.climate.gov/news-features/featured-images/rain-s...