How is the weather formed?

Even in Antarctica, the driest place on earth, the air contains moisture. If the air is completely dry, more heat radiated from the surface will be lost in space. Fortunately for life on earth, the air contains water vapor, which can absorb energy well. Fortunately, the water vapor in the air can be continuously replenished. In the continuous circulation, water evaporates from land and sea and gathers into clouds. Then rain, snow or other forms of precipitation are produced, and the whole process is self-circulating.

Air has the weight to exert pressure. The more air, the greater the weight and the greater the pressure. The depth of the air-the thickness of the atmosphere, varies according to the topography of the earth. There is less air at the top of the mountain, so the air pressure is lower than that in the valley.

Air pressure is also affected by temperature, which indicates the degree of molecular movement. Air molecules are constantly moving back and forth with each other, and any molecules around them may collide. This impact in turn generates heat. Therefore, the stronger the air pressure-that is, the more molecules collide with each other, the higher the air temperature. In addition, the more molecules move, the more space they occupy. Therefore, for a given volume of warm air and cold air, the former contains fewer molecules than the latter. The smaller the density of warm air, it means that it is lighter and easier to rise than the denser, heavier and sinking cold air.

Water molecules in the atmosphere constantly change back and forth between gas, liquid and solid. Rain falling from the clouds means that more water molecules leave the gas state and form small water droplets (condensation), while water molecules enter the gas state (evaporation) from the state of small water droplets.

These two processes, condensation and evaporation, are always going on in the air around us, but the speed will be different because of different temperatures. For example, on a clear, cloudless and sunny day, heat will accelerate evaporation and prevent water droplets in the air from surviving for too long. Therefore, there are more water molecules returning to water vapor than those existing in the form of small water droplets. When the air cools, the evaporation rate will decrease until the evaporated water molecules are less than the condensed water molecules: at this time, we say that the air is saturated and the water vapor usually condenses into small water droplets, forming clouds, fog and dense fog.