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A look at how water vapor turns into clouds and the various forms of precipitation.
Have you ever wondered why a cold soda 'sweats' on a hot summer day? It isn't leaking—you are actually witnessing the air around the can turn from an invisible gas into liquid water right before your eyes!
Quick Check
If the temperature drops but the amount of water vapor stays the same, does the relative humidity go up or down?
Answer
It goes up, because cold air has a smaller capacity, making the existing water fill a larger percentage of the 'sponge.'
As air cools, its relative humidity rises. Eventually, it reaches a temperature where it is 100% saturated. This specific temperature is called the dew point. If the air cools even further, the extra water vapor must turn back into liquid. This process is called condensation. But water can't just turn into a cloud on its own; it needs a 'landing pad.' These tiny particles of dust, salt, or smoke are called condensation nuclei. When billions of these tiny droplets cluster together, a cloud is born!
Quick Check
What are the two 'ingredients' needed for water vapor to condense into a cloud?
Answer
The air must cool to its dew point, and there must be particles (condensation nuclei) for the water to stick to.
When cloud droplets get too heavy for the air to hold up, they fall as precipitation. The form they take depends on the temperature of the air layers they fall through. Rain falls when the air is warm all the way down. Snow forms when the air stays below freezing () from the cloud to the ground. Sleet happens when snow melts into rain in a warm layer but refreezes into ice pellets before hitting the ground. Hail is different; it forms inside violent thunderstorms where strong winds toss ice chunks up and down until they grow heavy enough to fall.
Predict the precipitation type based on these layers: 1. Cloud Layer: (Snow forms) 2. Middle Layer: (Snow melts into rain) 3. Ground Layer: (Rain freezes into ice pellets) Result: This 'sandwich' of cold-warm-cold creates sleet.
A hailstone starts as a small ice crystal. 1. An updraft (rising wind) pushes it up into the freezing top of a storm cloud. 2. It collects a layer of liquid water which freezes. 3. Gravity pulls it down, but another updraft kicks it back up. 4. If it cycles 10 times, it grows 10 layers of ice. 5. It only falls when its weight exceeds the force of the updraft : .
What happens to the dew point when the air becomes more humid?
Which form of precipitation requires a strong updraft in a thunderstorm to form?
Condensation nuclei are the tiny particles like dust that water vapor sticks to when forming clouds.
Review Tomorrow
In 24 hours, try to explain to a friend why a '50% humidity' day feels different in the winter than it does in the summer.
Practice Activity
Watch the weather report tonight. Look for the 'Dew Point' and 'Humidity' stats. If the dew point is very close to the current temperature, look outside for fog or dew!