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Wind and the Ocean Wind • Currents of air • Develop when two adjacent bodies of air have different densities. • Denser air sinks, pushing less dense air upward. • Convection current is produced. • Density of air is affected by temperature and water vapor. • Water vapor in an air mass decreases its density because water vapor is about half as dense as air. Air Pressure • Force of the air mass on earth • High pressure: dry, dense air • Low pressure: moist, less dense air. Ocean Wind • Sea breeze: cooler, denser air above the ocean or lake sweeps over the shore as air warmed by the land rises. -beach areas are usually cooler in the summer than inland • Land breeze: at night, breezes blow off land as the warmer air over the ocean rises. • Ocean winds tend to keep nearby coastal lands relatively warm in the winter and cool in the summer. Global Winds • Heat is not distributed evenly on the earth. • The equator receives about 60 times more sunlight than the poles. • Hot (and usually humid) equatorial air masses are less dense • Cold (and usually dry) polar air is more dense. Global Winds cont. • At the poles cold, dense air sinks then flows toward the equator. • Polar air warms as it moves toward the equator, • At the equator, hot, moist and less dense air rises and moves toward the poles. • Equatorial air cools as it moves toward the poles. • Convection cells are formed. Earth’s Rotation and the Effect on Wind • Rotates from west to east • Spherical so therefore the equator rotates faster than the poles • Cold air from the north pole can’t keep up with the land beneath it • Polar easterlies: blowing from the northeast from poles to 60° N. • Trade winds: blowing from the northeast from 30°N to equator. • Prevailing westerlies: blowing from the southwest from 30°N to 60°N (dominant winds in US) Global Wind Patterns Winds cont. • Doldrums: light shifty winds at the equator (named for lack of wind) • Horse latitudes: light variable winds at 30°N Winds and Surface Currents • Winds can produce currents if they blow from the same direction for long periods. • When wind pushes constantly on the ocean surface, water particles at the surface begin to move but not in the direction of the wind. • The rotation of the earth causes water to form a current at a 90° angle to the wind (Ekman spiral). • Currents will move right in the northern hemisphere and left in the southern hemisphere. Gyres • Circular current • Formed by a combination of prevailing winds, the rotation of the Earth, and land masses that interfere with the movements of water currents. • Due to the Coriolis Effect they rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. • Important because they move the drifting plankton thousands of km across the ocean. Main Ocean Gyres