(Updates with weakening)

MIAMI, Sept 15 (Reuters) - Tropical Storm Ingrid, the ninth named storm of the 2007 hurricane season, fell apart over the open Atlantic on Saturday and was downgraded to a tropical depression as it fought unfavorable atmospheric conditions.

The weather system was likely to continue to weaken due to wind shear -- the difference in wind direction and speed at different altitudes -- but some computer models still indicated it could strengthen again if it clings on as a depression for a few days, the U.S. National Hurricane Center said.

The depression had top sustained winds of 35 miles per hour (55 km per hour) as it swirled around 510 miles (820 km) east of the Lesser Antilles islands of the Caribbean at 11 a.m. EDT (1500 GMT), the hurricane center said.

It was moving west-northwest at 10 mph (16 kph), on a track that was likely to keep it over water well north of the islands.

The 2007 Atlantic hurricane season has been active, though far less ferocious than record-breaking 2005 when 28 storms spawned 15 hurricanes, including Katrina, which flooded New Orleans and killed 1,500 people on the U.S. Gulf Coast.

On Thursday, Hurricane Humberto slammed into the Texas-Louisiana border area with an unexpectedly powerful punch that killed at least one person.

Before that, two maximum-strength Category 5 hurricanes, Dean and Felix, struck Mexico's Yucatan Peninsula and Central America respectively -- the first time on record that two top-ranked storms on the five-step Saffir-Simpson scale made landfall in the same year.

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