Rocket squeezes a soft ball of air out of a plastic bottle onto his wrist. "Why is that cool?" he asks.
Raven is lying on her back on the rooftop, naming clouds. "Puffy. Flat. Wispy. There has to be a system."
Nova hovers between them. "There is," she says. "Four Latin words. And Rocket just found the other half of the puzzle."
"I did?" Rocket squeezes the bottle again.
"Air that spreads out gets cooler," Nova says. "What happens to air as it rises higher, where there is less pressure?"
"It spreads out," Rocket says slowly. "So it cools. And cool air hits its dew point."
Raven points up. "Which means every cloud up there is sitting on a column of rising air."
The most common way air reaches its dew point is by rising. As a parcel of air rises, the pressure around it drops, so the parcel expands.
Expanding air cools. The National Weather Service gives the rate: about 5.5 degrees Fahrenheit for every 1,000 feet the parcel rises, until it is saturated.
Once the parcel reaches saturation, which is 100 percent relative humidity, vapor condenses onto the nuclei and a cloud droplet is born.
The flat base of a cumulus cloud marks the exact height where the rising air reached its dew point.
Clouds keep changing because condensation and evaporation happen at the same time. When condensation wins, a cloud grows. When evaporation wins, it fades.
Cloud names are built from four Latin roots. Learn these four and you can read almost any cloud name.
| Latin root | Meaning | What the cloud looks like | Made of |
|---|---|---|---|
| Cirro | Curl of hair | High, white, wispy, hair-like | Ice crystals |
| Cumulo | Heap | Detached puffs like cotton balls, flat base, sharp edges | Rising air, water droplets |
| Strato | Layer | Broad blanket with soft, blurry edges | Air lifted gently, not by convection |
| Nimbo | Rain | Thick clouds that drop rain or snow | The thickest clouds |
Cirro-form clouds are the high, wispy ones that often appear first ahead of a low pressure storm system.
Cumulo-form clouds show that air is rising by convection, the same rising you studied in week 4.
Strato-form clouds come from air rising gently over a wide area. They tend to occur along and north of warm fronts.
Nimbo-form means a rain cloud. Most precipitation falls from nimbo-form clouds.
| Statement | True or false? |
|---|---|
| Rising air expands and cools. | ? |
| The flat base of a cumulus cloud marks the dew point height. | ? |
| Strato means heap. | ? |
| Most precipitation falls from nimbo-form clouds. | ? |
| A cloud stops changing once it has formed. | ? |
Nice work. Tomorrow is Station Lab: you start a cloud log and learn the ten basic cloud types.