Why does it rain instead of just staying cloudy forever?
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Why does it rain instead of just staying cloudy forever?
The short answer
It rains because warm air can hold more invisible water than cold air. When a puff of moist air rises and cools, the amount of water it can hold shrinks below the water it is already carrying, so the extra water is forced to clump into drops and fall.
How it works
Air everywhere carries invisible water vapor it picked up from oceans, lakes, and puddles. Warm air can hold a lot of that water; cold air can hold much less. When air rises into the sky it cools down, and as it cools its limit drops. Once the air is holding more water than it now can, the leftover water condenses onto tiny floating specks, builds into bigger and bigger droplets, and finally falls as rain.
What people get wrong
Many kids think a cloud fills up like a sponge and rain is it spilling over, or that rain happens when clouds bump into each other. Really, clouds are not solid containers. Rain comes from a change in temperature: cooling the air shrinks how much water it can hold, and the water it can no longer carry turns into drops.
The catch
Warm air can carry lots of water, but it stays invisible as vapor, which is why a warm day can feel muggy and sticky even under a clear blue sky. Cold air makes drops easily because its limit is so low, but it cannot pick up much water in the first place, which is why very cold places are often dry deserts of ice.
Questions kids ask
Where does the water in the air come from?
It evaporates from oceans, lakes, rivers, puddles, and even plants. The sun warms that water and turns it into invisible water vapor that floats up and mixes into the air all around us.
Why does air get colder as it goes up?
Higher up there is less air pressing down, so a rising puff of air spreads out and expands. Spreading out makes it cool down, the same way air rushing out of a bike pump or a spray can feels cold.
If cold air holds less water, why does it rain more in warm places?
Warm air can pick up far more water to begin with. So when warm, moist air rises and cools, there is a lot of water to squeeze out, which is why hot, humid places often get heavy downpours.
What is the difference between a cloud and rain?
A cloud is made of tiny droplets so small and light they float. Rain happens when those droplets bump together and grow big and heavy enough that the air can no longer hold them up, so they fall.
For grown-ups
Air's saturation vapor pressure rises steeply with temperature (the Clausius–Clapeyron relation). As a moist parcel rises it expands and cools adiabatically; once it reaches its dew point it is saturated, and further cooling forces the excess vapor to condense onto cloud condensation nuclei, forming cloud droplets. Those droplets grow and coalesce until they are heavy enough to fall as rain.