How Does the Wind Start to Blow?

Wind is the circulation of air. We are surrounded by air. We all need air to breathe. There is cold air and hot air. Hot air is lighter than cold air. When hot air goes up, cold air comes down so that we have enough air. The air on ground and sea goes up when it becomes hot. Then cold air from somewhere else comes to replace it. The flowing of air is wind. Because the earth is very big, the flow of air is also great. Therefore, the wind is sometimes very strong.