Why Is It Faster to Fly West to East Rather Than East to West?

Almost every summer, I travel to India. A direct flight from New Jersey, United States to Mumbai, India takes about fifteen hours whereas the return flight takes about sixteen. It is the same route, so why the additional hour for the return flight? 

High up in the atmosphere exists a phenomenon known as the jet stream] . Jet streams are currents of fast moving air that blow  eastward at speeds of about 129 to 255 km/h (80 to 255 mph) and at altitudes above 30,000 feet. Two important factors make up this phenomenon: temperature differences between layers of air masses in the atmosphere and the Earth’s uneven rotation that is faster near the Equator .

The width of a jet stream is a few hundred kilometers and its vertical thickness, or height, is about five kilometers (3 miles). The flight time varies depending on whether the airplane is flying with the flow (eastwards from United States to India) or against it (westwards from India to the United States).