How is weather formed?
Even in Antarctica, the driest place on Earth, there is moisture in the air. If the air were completely dry, more heat radiated from the surface would be lost to space. Fortunately for life on Earth, air contains water vapor that absorbs energy very well. What's even more fortunate is that the water vapor in the air can be continuously replenished. In a continuous cycle, water evaporates from land and sea and collects into clouds. Then rain, snow or other forms of precipitation are produced, and the entire process is self-cyclical.
Air has a weight that exerts pressure. The more air, the greater the weight and the stronger the pressure. The depth of air—the thickness of the atmosphere—varies depending on the topography of the Earth. There is less air at the top of the mountain, so the atmospheric pressure is lower than in the valley.
Air pressure is also affected by temperature, and the temperature indicates the degree of molecular movement. Air molecules are constantly moving back and forth against each other, and any surrounding molecules may happen to collide with them. This impact in turn generates heat. So the stronger the air pressure - that is, the more molecules there are hitting each other - the hotter the air. In addition, the more molecules that are moving, the more space they occupy. So, for a given same volume of warm air and cold air, the former contains fewer molecules than the latter. The smaller density of warm air means that it is lighter and rises more easily than the denser, heavier cold air that tends to sink.
Water molecules in the atmosphere are constantly changing back and forth between three states: gas, liquid and solid. Rain falling from a cloud means that more water molecules leave the gaseous state and form droplets (condensation) than water molecules enter the gaseous state (evaporation).
These two processes, condensation and evaporation, are going on all the time in the air around us, but the speed will vary depending on the temperature. For example, on a cloudless and sunny day, heat accelerates the rate of evaporation, preventing small water droplets in the air from surviving for too long. So, there are more water molecules returning as water vapor than there are in the form of small water droplets. As the air cools, the rate of evaporation decreases until fewer water molecules evaporate than condense: at this point, the air is said to be saturated, and the water vapor usually condenses into small droplets, forming clouds, mist, and fog.