After an afternoon thunderstorm, it occurred to me how little I know about the actual mechanism of rain.
OK, so evaporated moisture gathers in the the air and then further gathers into clouds. Is that cohesion, or what is the mechanism that gets the gathering under way?
Then a time comes when some mechanism causes some or all of the moisture falls back to earth as rain. What mechanism? And what makes it happen not in isloation but rather propagating through the clouds? And even the biggest, nastiest clouds give up rain over time – why doesn’t it happen that at least sometimes that this mechanism spreads with such speed that the rain falls in one tremendous, earth-squashing sploosh?
Clouds form when moisture in the air condenses into tiny droplets. When the temperature drops below the point where the atmosphere becomes 100% saturated with water vapor (the dew point), the rate of cloud formation increases, and then indiviual droplets begin to grow as more and more water condenses out. Eventually, the droplets become too large for the air currents to keep them up, and they fall as rain. Part of the reason that rain happens over time is that in order for a droplet to form in the first place, there has to be a “seed”–usually a speck of dust or even an ionized air molecule. Until a seed is available, the moisture in the air stays there as vapor. As raindrops fall our of the cloud, air currents bring in more seed particles to keep the process going.
Water absorbs a fair amount of heat when it changes from a liquid to a gas. This heat is called the heat of vaporization and amounts to ~640 calories/gram of water. In cooking terms, that’s the same amount of energy as it takes to heat 1/4 cup of ice to boiling; just to evaporate 1 gram of water ! When water condenses, the process is reversed, and the vapor gives up its excess heat to the atmosphere. The process of heat transfer tends to be slow because air isn’t a very good conductor. Condensing water warms the atmosphere, and that speeds evaporation. Obviously some sort of steady state is reached in which net water is either vaporizing or condensing, but the process takes time. Which explains both why clouds don’t dump all their water in an instant, and why water heated in a pan doesn’t suddenly flash into steam the moment it hits 100°C.