Read

Air pressure is the force of air molecules colliding with a surface. Standard sea-level pressure is 1013.25 hPa, which equals 1013.25 millibars or 29.92 inches of mercury (1 hPa = 100 Pa).

Half of all the air molecules in the atmosphere are within the lowest 5.6 km, so pressure falls quickly with height. Large day-to-day pressure changes come from moving weather systems, marked H and L on weather charts.

Uneven heating by the Sun makes warm air rise (lower pressure) and cool air sink (higher pressure). Air moves from high to low pressure, and a bigger pressure difference gives a faster wind. Highs are often linked with clear skies; lows with wind and unsettled weather, although in the tropics pressure changes little except in tropical cyclones.

Experiment: Pressure and weather log

1. Each day at the same time, record the pressure (hPa), the cloud cover (oktas), the wind (Beaufort force, estimated) and the rain since the last reading (mm).
2. If you can, also note whether a front passed over (check the surface pressure chart).
3. After two weeks, plot pressure against time and mark the rainy days on the same graph.
4. Calculate the mean pressure on rainy days and on dry days.
5. Estimate the uncertainty in each pressure reading (instrument resolution, the time of reading) and in the rain total.
6. Conclusion: is there a link between low or falling pressure and rain in your data? How strong is your evidence from only 10–14 readings?

Day	Pressure (hPa)	Cloud (oktas)	Wind (Beaufort)	Rain (mm)	Front?
1
2
3
4
5
6
7
8
9
10

Questions

1. Sketch a weather chart with a low-pressure centre, a warm front and a cold front. Use the correct symbols and colours.

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2. A storage rain gauge has a rim 127 mm across. How many millilitres of water does it collect when 10 mm of rain falls? (Volume = area × depth)

3. Explain, using pressure differences, why a bigger pressure difference over a short distance gives a stronger wind.

4. Your mean pressure on rainy days was 4 hPa lower than on dry days, from 12 readings with an uncertainty of ±1 hPa each. How confident are you that low pressure brings rain? What would make the evidence stronger?

Sources for every fact on this sheet: echoesof.science/school/weather/sources/

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