Kalvard Weather Bulletin

Kalvard Weather Bulletin

Kalvard is a coastal region of six districts — a fishing port on an exposed northern arm, four islands offshore, a fjord head under a fell ridge, an observatory on the ridge itself, a sheltered inland valley and a mild southern peninsula — and it does not exist. Everything printed on this sheet is produced by one small atmospheric model running in your browser, sampled at whatever hour the clock has reached.

The point of the piece is that the panels are not four decorations of the same mood; they are four readings of one field. When the rain band on the radar crosses the coast, the isobars on the chart kink at the same line, the comma cloud on the infrared picture is wrapped around the same low, the meteogram shows the wind veering and the temperature dropping in that hour, and the seven-day table has already used that depression to decide tomorrow. Watch the clock run and you can follow a single depression from its arrival on the western edge to its occlusion and decay four days later.

What the model does

Depressions and anticyclones cross the domain from the west on analytic tracks — each has a genesis time, a course, a deepening phase and a decay. Because the whole system is a function of time rather than an integration, the state at any hour, behind or ahead of the clock, is simply evaluated. That is what makes the forecast panels honest: they call exactly the same function the analysis panel calls, only later.

Everything else follows from the pressure field. Wind is the geostrophic balance of the pressure gradient, then turned across the isobars and slowed by surface friction — more over land than sea, and accelerated over the tops. Fronts hang off each depression in the classic arrangement and occlude as it matures, with the air mass, and so the temperature, changing sharply across them. Rain and cloud come from the fronts, the air mass, advected noise and the terrain: the fells wring out the windward slope and leave the valley behind them dry. The satellite channels are that cloud field rendered as brightness temperature (infrared) and as reflected sunlight (visible), with real solar geometry — which is why the visible channel dims towards evening and goes black at night, and why the infrared channel keeps working.

Confidence in the seven-day table is not decoration either. Each day is run five times, four of them with the depressions nudged off track by an amount that grows with lead time, and the grade is the spread of the answers.

All the controls sit in the masthead of the sheet itself.

Control	Effect
Clock	How fast simulated time advances. At one hour per second a frontal passage takes about a minute; at six hours per second you can watch a system live and die. Hold freezes the analysis, which is also the default if your system asks for reduced motion.
Season	Jumps the date. Changes the temperature baseline, the length of the day, the sea temperature lag, and whether the precipitation falls as rain or snow.
New synoptic situation	Reseeds the weather. The geography stays put — Kalvard is a place, not a random field.
District	Click a name in the index, or a station on the chart, to follow it. The observation block, the hourly meteogram, the written forecast and the seven-day table all move with the selection.
Chart tabs	Surface analysis, infrared, visible, radar — the same hour, four instruments.

Reading the chart

The station plots use the ordinary conventions: the circle is filled in eighths of sky cover, the barb gives the wind in knots (a half barb is 5, a full barb 10, a pennant 50), temperature and dew point sit to the left, and the figure to the right is the pressure in tenths with the leading digits dropped — 024 is 1002.4 hPa. Fronts are blue for cold, red for warm and purple for occluded, with the pips always on the cold side.