page thirty-five

The Glass

Thirty-four rooms, and not one of them has had any weather. Today's number sends a depression or two across an invented sea, draws the bureau's chart for midday tomorrow, and leaves the shipping forecast unwritten. Sailors call the barometer the glass. Its lines are all you get.

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The chart

Isobars join places of equal pressure, every four hectopascals. The wind follows them: stand with your back to it and the low is on your left. The closer the lines, the harder it blows. Drag across the chart to measure. The forecast is checked at each area's white point.

Chart
Drag across the chart to measure.

The geostrophic wind scale

Measure square across the isobars from one line to the next. Find that spacing along the top and the latitude down the side. The figure is the wind over the sea: the speed that balances the pressure gradient against the turning of the earth, cut to seven tenths for friction. Your ruler lights up its column.

The forecast

A direction and a Beaufort force for every area, read off the forecast chart for midday tomorrow. Direction is where the wind comes from. Within one compass point and one force counts. Where the gradient is too slack for force 3, any direction will pass.

Forecast
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The arrow keys walk to other days. Every day is a different sea, different areas and different weather.

What is real here and what is not

Forecasters read winds off charts like this one for a century before the computers took over, and the rules still hold. The sea, the islands and the bureau are the day's number.

  1. 01

    Buys Ballot's law

    Christophorus Buys Ballot set it down in 1857: in the northern hemisphere, with your back to the wind, low pressure is on your left. Air pushed toward the low is turned to the right by the earth's rotation until it runs along the isobars instead of across them.

  2. 02

    The geostrophic wind

    When that turning exactly balances the push, the wind speed is the pressure gradient divided by the air's density and the Coriolis parameter, 2Ω sin(latitude). Tight isobars mean strong wind, and the same spacing blows harder further south, where the turning is weaker. The scale on this page is that formula.

  3. 03

    Friction at the surface

    Near the sea the wind drags. Over open water it is usually taken as about seven tenths of the geostrophic speed and backed some fifteen degrees, turned in a little toward the low. The page's own answer uses exactly those figures, at each area's point.

  4. 04

    The rest is simplified

    No fronts, no curvature correction for winds around a tight low, no gusts, no squalls. The systems are smooth bells of pressure that drift and deepen in a straight line, and the weather and visibility in the bulletin are flavour, not graded. Real forecasters also had the barograph, the ship reports and a lot of tea.

Thirty-five rooms, and one number doing all the work. Today it moves the air over a sea nobody sails and leaves you to tell the boats.