Raven spreads a weather map across the roof table. A blue H sits over one region, a red L over another.
"What do you notice about the sky under each one?" she asks.
Rocket studies the satellite picture beside it. "Clear under the H. A swirl of clouds under the L."
Nova projects an old page beside the map. "People noticed the same thing nearly two centuries ago," she says.
"They watched the barometer fall before wind and rain, and rise before fine weather."
"So our straw is a forecaster," Rocket says. "A slow one."
"A careful one," Nova says. "Read what the data say before you decide what they mean."
The National Weather Service glossary defines a low as a region of low pressure, marked L on a weather map.
A low center is usually accompanied by precipitation, extensive cloudiness and moderate winds.
A high is a region of high pressure, also called an anticyclone, marked H on a weather map.
Scientists at the University Corporation for Atmospheric Research explain why. Pressure varies partly because the Earth is not equally heated by the Sun.
Where air is warmed, it often rises, giving lower pressure. Winds blow toward a low, and the air rises where they meet.
As the air rises, its water vapor condenses, forming clouds and often precipitation.
Winds blow away from a high, and air from higher in the atmosphere sinks down to fill the space.
| Low (red L) | High (blue H) | |
|---|---|---|
| Pressure at the center | Lower than around it | Higher than around it |
| Winds near the surface | Blow toward the center | Blow away from the center |
| Air at the center | Rises | Sinks |
| Usual sky | Clouds, often precipitation | Clearer |
The National Weather Service air pressure page reproduces barometer rules from a weather guide written by Rev. Dr. Brewer in 1848.
The table below reproduces several of them. They are old observations, not modern forecasts, but they show the pattern.
| Barometer | What the 1848 guide expected |
|---|---|
| Falls suddenly | High wind |
| Falls in very hot weather | Thunder |
| Falls much and stays low in fair weather | Much wet in a few days, and probably wind |
| Rises high and stays high in wet weather | Continued fine weather in a day or two |
| Rises suddenly very high in wet weather | Fine weather that will not last long |
| Rises in winter | Frost |
| Statement | True or false? |
|---|---|
| A low is usually accompanied by precipitation, extensive cloudiness and moderate winds. | ? |
| Air sinks at the center of a low. | ? |
| A high is also called an anticyclone. | ? |
| The 1848 guide expected a sudden fall to bring fine, calm weather. | ? |
Your jar barometer cannot give an exact number. It can show the trend: rising, falling or steady.
The old rules and the modern glossary agree on the big picture. Falling pressure often leads toward a low and its clouds.
Rising pressure often leads toward a high and clearer skies. "Often" matters. Forecasts give chances, not promises.
Sharp map reading. Tomorrow you use rising and falling pressure in everyday life.