The Station Plot

Eight or nine numbers, one small diagram: how every dot on a synoptic chart carries the whole atmosphere on its back.

What the symbol contains

Before a forecaster can draw anything, someone has to get the observations onto a page. The solution, standardised by the World Meteorological Organization and in use since the early twentieth century, is a dense little glyph called the station plot — a circle and a set of surrounding symbols that pack eight or nine meteorological measurements into a space roughly the size of a thumbnail.

The circle sits at the centre, and its fill tells you cloud cover at a glance: empty means clear sky, fully filled means overcast, and a range of intermediate fills covers the quarters between. Extending from the circle is a wind shaft, an arrow-like line whose direction shows where the wind is coming from, and whose barbs encode speed: each short barb is five knots, each long barb ten, and a solid triangle fifty. A forecaster scanning a chart does not read numbers; the whole Atlantic wind field becomes a field of sticks, its gradients immediately visible.

Around the circle, temperature and dew point — in degrees Celsius in most of the world, Fahrenheit in the United States — sit upper-left and lower-left respectively. The gap between them is the dew-point depression, a quick index of humidity: when the two converge, cloud or fog is likely. Upper-right is the current sea-level pressure in millibars, written as a three-digit code (the leading one or nine is dropped, and the decimal omitted, so 1013.4 becomes 134). Below it, a pair of symbols report the pressure tendency — how the barometer has moved in the past three hours — which a forecaster often reads before anything else, because a falling barometer is a warning in its own right.

The present weather symbol, a number from a standardised table of roughly one hundred codes, sits to the left of the circle. It distinguishes continuous moderate rain from intermittent drizzle, thunderstorm with hail from thunderstorm without: the granularity that a single icon on a phone screen deliberately discards. Below it, a past-weather symbol records what was happening in the previous hour. The circle itself may carry a supplementary symbol for low cloud type, with mid and high cloud types noted above.

Designed to be read, not decoded

The elegance of the station plot is that it was built for the synoptic chart — the map assembled every three or six hours from thousands of simultaneous observations worldwide. Laid across a chart, station plots are dense enough to reveal structure: a cluster of falling pressures defines a low before the isobar is drawn; a sharp temperature gradient marks where a front is about to be analysed. The symbols are positioned consistently around the circle so that a trained eye does not read each variable consciously but takes in the whole configuration as a single visual event.

The WMO's FM 12 SYNOP code — the international format in which surface observations are transmitted — maps onto the station plot almost one-to-one, and this is not coincidental. The code and the diagram evolved together, both designed for humans receiving observations ↗ by telegraph and needing to plot them by hand without confusion or ambiguity. Standardisation was the point: a plot made in Valentia, Ireland, had to be legible to someone analysing a chart in Bergen or Washington without recourse to a legend.

Today, most analysis software plots the data automatically, and many forecasters work at screen rather than paper. The underlying station model, though, is unchanged. What changed is the volume: where a mid-century forecaster might have worked with a few hundred surface reports, ECMWF's data assimilation system ↗ ingests millions of observations per analysis cycle, many of them not surface stations at all. The station plot remains the legible face of the oldest and densest part of that dataset — the global network of human-staffed and automated surface stations still reporting every six hours, as agreed, as they have been doing for the better part of a century.