paragraph: Weather occurs because of unequal heating of the atmosphere. The source of heat is the sun. The general principles behind weather can be stated simply: The Sun heats Earths surface more in some places than others. Where it is warm, heat from the sun warms the air close to the surface. If there is water at the surface, it may cause some of the water to evaporate. Warm air is less dense, so it rises. When this happens, more dense air flows in to take its place. The flowing surface air is wind. The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall. 
question: How are clouds formed?
answer: The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall