Rocket holds up the index card. Three marks step downward, one for each day. "Falling," he says. "Rain is coming."
"Maybe," Raven says. "What else do you notice? The flag changed direction, and the clouds are thicker than yesterday."
Nova compares the card with the official report. "Pressure is down 6 millibars since Wednesday," she says.
"The trend on your card matches the trend in the data. Well done."
"So I was right," Rocket says.
"You were careful," Nova says. "Which is better. Now, how sure are you? Say it as a chance."
Rocket thinks. "Likely rain. Not certain." Raven writes exactly that in the log.
A single reading rarely tells you much. The trend over days tells you more, and several instruments together tell you most.
A falling barometer, thickening clouds and a shifting wind flag point the same way: toward a low and its weather.
A rising barometer, clearing skies and lighter wind point toward a high.
This is why your Station Rooftop log records all five observations, not just one.
| Barometer trend | Clouds | What to expect, with a chance |
|---|---|---|
| Falling | Thickening | A low approaching: clouds and precipitation likely |
| Steady | Little change | Weather likely to continue |
| Rising | Clearing | A high approaching: fairer weather likely |
High on a tall mountain, there are fewer air molecules above you, so the air is thinner and the pressure is lower.
The 1848 guide shows that people have watched the pressure trend for a very long time.
Today the National Weather Service measures pressure with electronic sensors, continuously, at stations across the country.
The "weak low" you met in last week's sea breeze is this idea at work: warmed air rising, pressure dropping.
| Statement | True or false? |
|---|---|
| Air is made of molecules and has weight. | ? |
| Standard sea level pressure is 1013.25 millibars, or 29.92 inches of mercury. | ? |
| Air pressure increases with height. | ? |
| A low is usually accompanied by clouds, precipitation and moderate winds. | ? |
| Rising pressure usually points toward a low. | ? |
Next week you follow the air as it moves. Winds blow toward a low and away from a high, as you read on Thursday.
You will find out where wind comes from, why sea breezes turn around at night, and how the planet's winds are arranged.
Keep marking the straw every day. Next week's wind flag readings will sit beside your pressure trend.
A full week on pressure. Tomorrow is Station Day: the family reads the barometer together.