Question

Why cyclones rotate clockwise in northern hemisphere?

Answer 1

 Actually the tropical cyclone winds rotate in anti or counter-clockwise/cyclonic in the northern hemisphere and clockwise/anti-cyclonic in southern hemisphere around the low pressure, because of the Coriolis Effect (which deflects the wind direction to the right side of the actual movement of the wind in the northern 

Answer 2

Hurricanes and cyclones are low pressure systems. When they form you'd expect the surrounding higher pressure air to flow in radially. But minute forces, caused by the earth's rotation, make the wind flowing towards the low pressure rotate - anticlockwise northern hemisphere, clockwise southern. They're called Coriolis forces. Now imagine a low pressure forming in the southern hemisphere. The first tiny breezes trying to fill the low pressure are made to rotate in a clockwise direction. The low pressure deepens and what was a breeze is now a cyclone. If the cyclone now moves north across the equator the Coriolis forces - now anticlockwise - will try to reverse the rotation, but they're far too small to have any effect. The cyclone will continue to rotate in a clockwise direction.