Weather: pressure, wind and forecasting
Air pressure in hectopascals and why it falls with height, pressure gradients and wind, fronts and cloud formation, the Beaufort scale, and numerical weather prediction. With a two-week pressure and weather log.
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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).[9]
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.[9]
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.[7][8][9]
Clouds form when moist air cools until it is saturated; water vapour condenses on tiny dust, salt or smoke particles, and millions of cloud droplets are needed for one raindrop. At a cold front, cold dense air pushes under warmer air; at a warm front, warm lighter air rises over colder air. An occluded front forms when a cold front catches up with a warm front.[6][5][10]
Wind speed is described with the Beaufort scale: force 2 light breeze is 4–6 knots (2–3 m/s), force 6 strong breeze 22–27 knots (11–14 m/s), force 8 gale 34–40 knots (17–21 m/s) and force 12 hurricane 64 knots (33 m/s) or more. To convert knots to mph, multiply by 1.15.[12]
Numerical weather prediction: millions of observations a day from satellites, land and marine instruments, upper-air soundings and radar are blended by data assimilation into starting conditions. Supercomputers then solve the equations of the atmosphere forward in time, and meteorologists interpret and adjust the output.[11]
Some weather measurements are simple calculations: the distance to a lightning strike in kilometres is roughly the number of seconds between the flash and the thunder divided by 3. A tipping-bucket rain gauge records rain in steps of 0.2 mm.[3][4]
Try it: Pressure and weather log
You need
- A barometer, a digital weather station, or a phone with a pressure sensor (and a free barometer app)
- A rain gauge (home-made or school)
- A record sheet or spreadsheet
- Optional: the Met Office or national weather service surface pressure charts
Safety
Do not climb onto roofs or walls to place instruments. Stay indoors during thunderstorms; do not take readings while there is lightning.
What to do
- 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).
- If you can, also note whether a front passed over (check the surface pressure chart).
- After two weeks, plot pressure against time and mark the rainy days on the same graph.
- Calculate the mean pressure on rainy days and on dry days.
- Estimate the uncertainty in each pressure reading (instrument resolution, the time of reading) and in the rain total.
- 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?
What happens, and why?
Low-pressure systems generally bring wind and unsettled weather, while highs are often linked with clear skies, so you may find lower mean pressure on rainy days. A phone sensor reads the pressure at your own height, which is lower than the sea-level value on weather charts, because pressure falls with altitude; this offset is the same every day, so changes can still be compared. Two weeks is a small sample, and the 'rules' are only rules of thumb.[8][9][4]
Play: Which Beaufort force?
Convert if needed (1 knot ≈ 1.15 mph) and choose the Beaufort force: 2 = 4–6 knots (2–3 m/s), 6 = 22–27 knots (11–14 m/s), 8 = 34–40 knots (17–21 m/s).
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11