Rocket lies on the rooftop watching a dark gray cloud slide over a bright white one. "Why is that one gray?" he asks. "Is it dirty?"
Raven shades her eyes, keeping her back to the Sun. "What do you notice about how thick it is?"
"Thicker," Rocket says. "Much thicker."
Nova hovers low. "Think about where the light has to go to reach your eyes," she says.
High above, a jet draws a thin white line across the blue. "And that one is new," Rocket says. "A cloud that was not there a second ago."
"A cloud needs vapor, cold air and specks," Raven says. "A jet engine gives all three."
Rocket grins. "Then I can read that one too."
Sunlight is white light, a mix of every color. Cloud droplets scatter all those colors equally, so we see the cloud as white.
As a cloud thickens, less sunlight passes through it. The National Weather Service explains that this is what gives a cloud its gray color.
A cloud with no direct sunlight on it may even reflect the sky and look bluish. Haze and dust can turn clouds yellow, orange or red.
Cirrus clouds light up long before other clouds at sunrise and fade later at sunset, often glowing yellow or red.
A contrail is the white trail behind a high-flying airplane. The engine exhaust contains water vapor, and the air up there is very cold.
The U.S. Geological Survey notes that the vapor condenses into what is essentially a cirrus cloud.
Fog is a cloud at ground level. Unlike clouds above, it does not need rising air. Humid air cools on the cold ground to its dew point.
When a stratus layer breaks up and dissipates, blue sky shows through. Evaporation has won over condensation.
| Sight | Why it happens |
|---|---|
| White cloud | Droplets scatter every color of sunlight equally |
| Gray cloud | The cloud is thick, so less sunlight gets through |
| Bluish cloud | No direct sunlight; the cloud reflects the sky |
| Contrail | Water vapor from a jet engine condenses in very cold air |
| Fog | Humid air cools to its dew point on the cold ground, with no rising air needed |
| Week review | True or false? |
|---|---|
| A cloud is made of tiny water droplets or ice crystals, not vapor. | ? |
| Every droplet forms on a tiny speck called a condensation nucleus. | ? |
| Rising air warms up as it expands. | ? |
| Cirro means curl of hair, cumulo means heap, strato means layer, nimbo means rain. | ? |
| Cumulonimbus is a thin, high ice cloud. | ? |
Strong week, cloud reader. Tomorrow is Station Day: a family cloud watch with the chart.