Field manual · Domestic observation

Three instruments, one habit, and weather you can actually read.

A barometer tells you almost nothing in isolation and almost everything in sequence. Enter your reading now and your reading three hours ago; the dial and the verdict below are computed from those two numbers alone.

Set hand — the reading 3 hours ago

−3.5 hPa / 3 h

Falling

A definite fall. Expect thickening cloud and freshening wind, and rain within a few hours if a frontal sequence is already overhead.

With a south-westerly wind, Buys Ballot's law puts the centre of low pressure roughly to the north-west. Surface friction draws air inward, so the true centre normally sits 15–30° further round than that.

1013.25: hPa — standard mean sea-level pressure
≈ 8 m: rise in altitude per 1 hPa lost, near sea level
3 h: the tendency window used in synoptic reports
10: cloud genera in the WMO classification
8: oktas — eighths — in a completely covered sky
22°: radius of the halo thrown by cirrostratus

02 · Reference

What the interpreter is doing

Weather services record pressure at three-hourly synoptic hours and report both the amount of change and its shape. Three hours is long enough for a real trend to separate itself from the small diurnal wobble, and short enough to catch a system while there is still time to act on it.

The descriptors below are the conventional marine bands. They describe rate, not weather: the same fall means something different in a tropical airmass than in a winter westerly, which is exactly why the log in guide 03 is worth keeping.

The wind line applies Buys Ballot's law. Stand with your back to the wind in the Northern Hemisphere and low pressure lies to your left; in the Southern Hemisphere, to your right. Surface friction pulls air inward, so the true centre usually sits 15–30° further ahead of you than the rule suggests.

Pressure tendency descriptors by 3-hour change
Change / 3 h	Descriptor
under 0.5 hPa	Steady
0.5 – 1.5 hPa	Slowly rising or falling
1.6 – 3.5 hPa	Rising or falling
3.6 – 6.0 hPa	Rapidly rising or falling
6.1 hPa and over	Very rapidly rising or falling

03 · Guides

Three things worth learning properly

Setting and reading an aneroid barometer

Set it before you trust it. An aneroid measures pressure by the flex of an evacuated metal capsule, and nothing in that mechanism knows your altitude. Pressure falls by roughly 1 hPa for every 8 metres of height near sea level, so an instrument 80 m up reads about 10 hPa low until it is corrected. Turn the adjusting screw on the back until the needle matches the mean-sea-level pressure published for a nearby official station, then leave it alone for a season.

The brass pointer on the face — the set hand, moved by the knob on the glass — is not a forecast. It is a bookmark. Move it onto the needle every time you read, and the gap you find at the next reading is your tendency, which is the number that carries the information.

The engraved words Rain, Change and Fair were laid out for a mid-latitude climate at sea level, and they describe the wrong variable anyway. A steady 1005 hPa can be a bright dry day; a falling 1020 hPa can bring rain by evening. Read the movement, not the label.

Older aneroids carry friction in their linkage. A single light tap on the glass before reading releases a needle that is lagging behind. If that tap moves the needle by more than about a hectopascal, the movement wants servicing rather than tapping.

Identifying clouds without a key

The WMO International Cloud Atlas sorts every cloud into ten genera, and you can separate most of them with two questions: how high, and what shape.

Height first. Cirrus, cirrocumulus and cirrostratus live above about 6 km, are built of ice crystals, and look fibrous or finely granular. Altocumulus and altostratus occupy roughly 2–6 km. Stratus, stratocumulus and cumulus sit below 2 km, while cumulonimbus and nimbostratus span several levels at once.

Shape next, using your hand at arm's length as a protractor. If the individual cloudlets in a patterned sheet are narrower than your little finger, you are looking at cirrocumulus. Between one finger and three, altocumulus. Wider than three fingers, stratocumulus.

The sequence matters more than any single cloud. A warm front announces itself in order: cirrus thickening to cirrostratus — watch for the 22° halo around sun or moon — then altostratus, through which the sun looks like a lamp behind ground glass, then nimbostratus and hours of steady rain. Read that progression against a falling barometer and you have a forecast assembled entirely from your own garden. Turreted altocumulus on a summer morning, small towers rising from a shared base, points instead at instability aloft and often precedes afternoon thunderstorms.

Keeping a log that is worth re-reading

A log earns its keep when the entries are comparable, which means fixed times and fixed methods. Pick two or three times you can hit every day, write down the time zone, and never skip a reading because the weather is dull — the dull days are the baseline everything else is measured against.

Record at minimum: pressure and the three-hour change, shade temperature, wind direction (named for where it comes from) and speed, cloud cover in oktas, the genera present, and precipitation since the last entry.

Siting decides whether any of it means anything. The thermometer belongs in shade with free airflow, 1.25–2 m above short grass and well clear of walls and paving, which keep radiating heat long after sunset. A rain gauge wants level ground, a rim about 30 cm up, and a clear distance from any obstacle of at least twice that obstacle's height.

Two habits sharpen the record. Write your own observation before you look at any forecast, so the forecast cannot colour what you think you saw. And leave a column for what happened next, so that after a month you can check which of your local rules actually paid.

04 · Notes

Sources

World Meteorological Organization, International Cloud Atlas — the reference for genera, species and the ten-genus scheme used above.
WMO-No. 8, Guide to Instruments and Methods of Observation — the source for thermometer and rain-gauge exposure.
C. H. D. Buys Ballot (1857), on the relation between wind direction and the pressure gradient.

Written as general guidance for amateur observers. For warnings, flooding and any decision where safety depends on the forecast, use your national meteorological service.