Raven unrolls a printed cloud chart across the lab table: rows of cloud pictures, each with a code.
"Twenty-seven kinds," she says. "Weather observers used to report these every day."
Rocket points at a thin, veil-like cloud. "We saw this yesterday afternoon. Cirrostratus. What does it mean?"
Nova scans the chart. "The chart says a cloud tells a weather story," she says. "What was the sky doing before the veil arrived?"
"It was clear in the morning," Rocket says. "Then the veil came and got thicker."
Raven reads the chart's note aloud: "Cirrostratus increasing in cover and thickness implies a change from fair weather."
Rocket looks at the window. "So the clouds already knew it might rain tomorrow."
Before satellites, a weather observer identified the clouds overhead and estimated their height as part of every observation.
In the 1930s the National Weather Service standardized codes for cloud forms and cloud cover, following the international system.
The result is the NWS Cloud Chart, with 27 categories showing clouds alone and in combinations. Knowing the cloud type told the meteorologist a weather story.
Today, automated instruments in the United States detect cloud height but not type. Human observers in much of the world still report the state of the sky.
The chart and the National Weather Service cloud pages give these readings of the sky. The table below puts them side by side.
| What you see | What it says about the air | What may come next |
|---|---|---|
| Overcast stratus | The atmosphere is stable | Little change in the next few hours |
| Cirrostratus spreading and thickening | A change is on the way | Fair weather may turn rainy or snowy |
| Cirrus arriving on a clear day | High air is moving in ahead of a low pressure system | Often the first sign of a storm system approaching |
| Cumulus on a sunny morning | Air is rising by convection over warmed land | Puffs grow by afternoon and usually fade toward evening |
| Cumulonimbus with an anvil top | Strong rising air | A thunderstorm, with rain and sometimes hail |
| Nimbostratus, dark gray and thickening from altostratus | Deep, steady moisture | Steady rain or snow, base lowering as it falls |
Over land on a clear day, cumulus clouds appear in the morning as the ground warms and air starts to rise.
They grow through the day and then more or less dissolve toward evening as the heating stops.
This daily rhythm is convection you can watch, the same engine that drives the wind.
| Statement | True or false? |
|---|---|
| The NWS Cloud Chart has 27 categories. | ? |
| Overcast stratus means a storm is minutes away. | ? |
| Automated instruments in the United States report cloud type. | ? |
| Cumulus over land usually fades toward evening. | ? |
| Cirrostratus getting thicker can mean fair weather is ending. | ? |
Excellent reading. Tomorrow you will explain cloud colors, contrails and fog, and review the week.