Reading the Weather

An instrument is only useful if you know what its reading means. This is the
short version of how to read the two numbers that actually forecast.

The number that forecasts is not the pressure

A barometer reading of 1008 hPa tells you almost nothing on its own. What tells you
something is the tendency: how much it has changed, and in which direction, over the
last three hours. That is why every barometer worth owning has a set hand — you lay it over the
pointer when you take a reading, and the gap between the two hands three hours later is the
figure you want.

A fall of three hectopascals in three hours is a change worth noticing. Six is a front arriving
soon. A barometer that is rising slowly and steadily is the most reliable predictor of a quiet day
that exists, and it costs nothing to read.

Why a trace beats a dial

A dial gives you the pressure now and, with a set hand, the tendency over one interval. A
recording barograph gives you the shape of the change across a week as one continuous
line — and the shape is where the information is. A slow sag and a sharp drop can arrive at the
same reading by very different routes and mean very different things. You can see a front coming
off the slope of the line hours before the sky shows you anything.

Humidity: the wet-bulb depression

Relative humidity is derived, not measured directly. The reference method is a psychrometer:
two thermometers, one with a wetted wick, swung through the air. Evaporation cools the wet bulb,
and the difference between the two readings — the depression — gives you relative humidity off a
table. Because that difference is a physical consequence rather than the output of a sensor, a
psychrometer does not drift. It is why laboratories keep one to audit their electronic
hygrometers.

Wind: read the land, check the instrument

The Beaufort scale is not a historical curiosity — it is a way of reading wind speed from what
is happening around you, which is faster than walking to an anemometer and works when the
anemometer is not there. Force 4 is dust raising and small branches moving. Force 6 is large
branches in motion and the wind whistling in wires. Learn six of the thirteen and you will
estimate wind within a force almost every time.

The instrument is there to check your judgement, and to record what your judgement
cannot.

Siting matters more than the instrument

A good rain gauge sited badly reads worse than a cheap one sited well. Keep the rim level and
clear of its own post, away from anything that can shelter it or funnel water into it. Put
thermometers in shade with free air movement, never on a sunlit wall. Put an anemometer as high
and as clear as you reasonably can — the number is meaningless if a building is accelerating the
air past it.