In weather forecast, we keep hearing about wind speed and wind direction. But how does wind form?

  Wind is a natural phenomenon mainly caused by the rotation of the earth. Have you heard about Coriolis Effect in physics class? If not, no big deal. Coriolis Effect occurs because of the rotation of Earth and the inertia of the mass experiencing the effect. To just make it simple to understand, the rotation of the Earth creates winds in the atmosphere because the atmosphere is not sticking to the surface of the Earth, while the earth rotates under the atmosphere.

Wind can also caused by temperature difference, and that is what we hear about on weather forecast. The North Pole and South Pole are colder than areas near the Equator. Hot air rises from the equator, moves to the both poles, where it cools down and sinks back to the surface. This creates a circulation of air, which is what we call wind. Temperature difference also causes air to change their densities and pressures. Denser air (which is cold) usually goes down, and air with less dense (which is hot) usually goes up. Also, air with higher pressure will move towards the ones with lower pressure. Although this is true, wind doesn't blow in a straight line. Again, that is because of the rotation of the Earth. This is why we don't have straight winds except when wind is blowing along the Equator.