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The Big Island: Earth Image of the Week July 23, 2010
Satellite Image Hawaii's Big Island
Prevailing wind patterns determine the types of vegetation that cover various parts of Hawaii.
Encountering a sunny day across the entire tropical island of Hawaii is nearly impossible.

But the image to the right, captured on July 6, 2010, by NASA’s Aqua satellite, shows what is about as cloud-free a day as can be found.

Usually, the windward coast along the east of the island is enshrouded in cloud cover due to moist air being lifted upslope along the flanks of Mauna Kea and Mauna Loa volcanoes.

That lift is also responsible for the lush vegetation that is clearly visible from Waimea in the north to Hilo and the active volcanic region to the south.

Rainfall across the Big Island can range from more than 100 inches per year in Hilo to about 10 inches per year along the Kona coast to the west.

Desert or arid conditions prevail in many areas downwind from where the terrain has wrung out most of the moisture in the air available for rainfall.

The resulting stark terrain that prevails in most western parts of the island is clearly visible in the image.

Also seen is a small plume of volcanic vapor and debris spewing from the Kilauea volcanic caldera. The prevailing easterly winds had blown that vog, or volcanic smog, off Hawaii’s southwestern coast when the image was captured.

That created a pool of the hazardous gas hovering over the waters of the Pacific to the south and west of the island’s famed Kona coast.

Vog is produced from a chemical reaction between sulfur dioxide gas emissions of the Kilauea volcano and sunlight, oxygen, dust particles and water in the air. It creates a hazy atmosphere. Daytime onshore breezes and nighttime off shore breezes can carry the vog back and forth across Kona.

Image: NASA Modis Rapid Response System