The barometer and the go/no-go decision
Read the barometric tendency, keep an hourly log, check the forecast against the barometer and the sky, then decide whether to go: the window and the tide.
How the lesson starts
What matters is not the pressure now, but how much it has changed in the last three hours: the barometric tendency.
The forecast words, with their figures from the Met Office glossary: rising (falling) slowly, 0.1 to 1.5 hPa in three hours; rising (falling), with no other word, 1.6 to 3.5 hPa; quickly, 3.6 to 6.0 hPa; very rapidly, more than 6.0 hPa.
Steady means the same pressure as three hours ago. Now rising (falling): it had been falling (rising) or steady in the three hours, but at the time of observation it was definitely rising (falling). More slowly: rising or falling at a progressively slower rate through the three hours.
The hourly log
Write the barometer reading in the deck log every hour, with the time. One reading on its own tells you nothing: the tendency comes from taking away the reading of three hours before.
With the hours written down you also see the shape: whether it is falling faster and faster, slowing down, or has turned round.
A word of warning: the pressure also rises and falls a little through the day, the daily oscillation, and in settled weather that can look like a tendency.
What you practise inside
- The barometer
- Putting it together
- Go or no-go
This is a preview. The lesson itself is interactive: you work through it step by step, and every answer is checked as you go.