Rain can form in two ways. In tropical areas, where temperatures are warm, any water droplets in the clouds join together to form raindrops that are heavy enough to fall from the clouds. Elsewhere, rain starts life as snow in the freezing temperatures of the high clouds. As the snow falls nearer the ground, it will turn to rain if the temperature is above freezing.
While it may be tempting to say that rain comes from clouds, you can also say that rain is clouds, giving up on their dreams of being water vapor and falling back down to Earth, where they start their journey through the precipitation cycle again. If you want a better understanding of why rain comes down from clouds, start with that precipitation cycle, the mechanism through which water moves from the Earth to the atmosphere and back again.
The amount of water available on Earth never changes. But its state (liquid or gas/vapor) does, and that’s all thanks to thermal energy from the sun. As liquid water is heated by the sun, it receives enough energy to break its molecules apart and transform into water vapor.
The warmer the air, the more water vapor it can hold. That warm, moisture-saturated air rises, along with the water vapor it contains, and as it rises it cools. Once the air has cooled past the “dew point,” it condenses around “condensation nuclei,” which are usually teeny-tiny particles of dust, smoke or even salt that are suspended in the air. (If you’ve ever looked through a shaft of sunlight and seen dust particles dancing in the air, that’s a great visual.)
The tiny water droplets that initially form are what you see as clouds – and if you pay close attention to clouds in the sky, you’ll see that they’re constantly shrinking and growing in response to the warring forces of evaporation and condensation.
Water vapor that has condensed into tiny droplets and formed clouds is well on its way to becoming rain – but it’s not there yet. For now, the water droplets are so tiny that the air currents keep them aloft, just as swirling particles of dust can stay in the air. But as those droplets continue to rise, buoyed by rising bodies of warm air, they have two routes for making it back to Earth.
The first is when water droplets collide and coalesce with other droplets, eventually becoming heavier than the uplift of the air around them, at which point they fall down through the cloud. Or, through something called the Bergeron-Findeisen-Wegener process, the ice process of precipitation or simply the Bergeron process, the droplets rise high enough to freeze into ice crystals, attracting more water vapor to themselves and growing quickly until they’re heavy enough to fall as snow or melt and fall as rain.
Picture Credit : Google