Meteorology

How the weather that reaches a boat gets made: a low, two fronts, and the spin of the Earth. Every picture is live. Drag the sliders.

1. Wind basics

Pressure is like height. Where the air is piled high the pressure is high; where it is thin, low. Air does what water does on a hillside: it starts straight down the slope, from high toward low. On a weather chart the slope is drawn as isobars, lines of equal pressure, and the closer they sit the steeper the hill.

If that were the whole story, wind would blow straight into every low, the low would fill in an hour, and there would be nothing to forecast.

The thing that spoils it is that the Earth turns underneath the moving air. Look down on the North Pole: the planet spins anticlockwise. Slide a puck from the pole toward the equator. While it travels, the ground slides east out from under it, so relative to the ground the puck curves west. West is to the right of "heading south". Now slide a puck the other way, from the equator toward the pole. At the equator the ground is already moving east at 1,670 km/h and the puck carries that speed with it. Further north the ground moves east more slowly, so the puck runs ahead of it, eastward. East is to the right of "heading north". Whichever way you go, right. In the Southern Hemisphere, seen from the South Pole, the planet spins clockwise, and everything bends left instead.

The turntable below is the pole seen from above. The ball is thrown dead straight. Only the rider thinks it curves.

Now put the two facts together on a clock face with the low at the centre. Air at six o'clock heads north toward the centre. Nudged right, it swings east, toward three. Air at twelve heads south, is nudged right, swings west toward nine. Six to three and twelve to nine are both anticlockwise. Every parcel does this, so the whole thing spins anticlockwise.

The pull toward the centre never stops and neither does the nudge. Once the air is circling, "right" points outward, the two balance, and the air just orbits. Near the water, friction slows the air, the nudge weakens, and the inward pull wins a little. So the surface wind spirals in, at about ten to twenty degrees over the sea and more over land, rather than orbiting cleanly. That is why lows fill from below, and why the air in the middle has to rise, which is where the cloud and rain come from.

Highs are the mirror. Air starts outward, is bent right, and that comes out clockwise. And there is a rule you can use on deck with no forecast at all. Stand with your back to the wind and the low is on your left, a little ahead. Southern Hemisphere, your right.

One more thing falls out of the balance, and it matters for sailors. To go round a curve, something has to pull you toward the centre, and the faster you go the more pull you need, growing with the square of the speed. Round a low the inward pull is the pressure difference, which can be as big as the storm makes it, so any speed can find its curve. Round a high the pressure pushes outward and the only inward pull is the nudge, which grows only in proportion to speed. Past a point no speed works. The air cannot make the turn, leaks outward, and flattens the pressure until the turn is gentle enough. So the middle of a high is always calm. Out on a high's edge, where the isobars run nearly straight, the cap is loose, and a low squeezed up against that edge gives a proper gale. Tight curved isobars mean a low. A high's dangerous side is its rim.

2. The front and the low

The sun heats the equator more than the poles, so you might expect the air to fade gently from warm to cold on the way north. It doesn't. It comes in two big blobs with a step between them, and three things make the step.

First, air takes the character of what it sits on. Park it over the tropical ocean for a week and it becomes uniformly warm and moist through its whole depth. Park it over snow and it cools into a uniformly cold, dense, shallow dome. Two uniform blobs can only change where they touch.

Second, cold air will not mix with warm air. It wedges under it. Dense fluid under light fluid is stable, like oil on water, and the turbulence that would blend them is damped. So instead of a blurry mixing zone you get a tilted surface, cold beneath and warm above, and the whole temperature jump lives on that surface. On a cross-section a real front is a slope of about one in a hundred, not a wall. But the change happens across tens of kilometres, not thousands.

Third, the winds squeeze it. Where two air streams converge, the lines of equal temperature get pushed together, the way a pattern on a sponge compresses when you squash it. A gentle gradient goes in, a sharp one comes out, and the second point keeps it sharp.

The step sits around forty to sixty degrees of latitude because the tropical overturning conveyor can carry heat only as far as thirty degrees before it sinks, and the polar dome reaches out to about sixty. Heat piles up at the edge between them. The storms in this chapter exist to ship that pile the rest of the way, which is why they form on the front and nowhere else.

Left alone, the step just sits there. A stationary front. What sets it moving is upstairs.

Zoom out and that ribbon of fast wind wraps the whole planet. The jet stream rides nine to twelve kilometres up, over the boundary between the cold polar air and the warmer air towards the equator. It does not run straight. It snakes in four to six great waves that usually creep east over days, and sometimes stall for a week, while the air inside races along them, often at a hundred knots or more. Each dip carries cold air towards the equator and each crest carries warm air towards the pole, and under the spreading air just east of a dip a low can be born. So in the middle latitudes the weather at any one place arrives from the west, one system after another. Underneath, the turning Earth sorts the moving air into bands: the westerlies beneath the jet, the trade winds blowing west towards the equator, and the polar easterlies round the pole.

Why is the jet there at all? A sharp temperature contrast at the surface forces a strong wind directly above it, and the sharpest contrast on Earth is the step between polar and tropical air. Zoom back in to a single wave. On the downstream side of a dip, as the air rounds the bend, it spreads apart, and spreading air aloft means less air stacked over that spot. Surface pressure falls right there, on the front. A low is born.

Air rushes toward the low and, from chapter one, curls anticlockwise. East of the low the flow is from the south, shoving warm air north; that leading edge is the warm front, now moving. West of the low the flow is from the north, shoving cold air south; that edge is the cold front. The low itself climbs into the cold air as it travels east. The fronts never move on their own. They are the edges of two air masses that a low has set spinning.

And it feeds itself. Lifting warm air and sinking cold air releases the energy stored in the contrast, which deepens the low, which spins harder, which lifts more warm air. A small kink grows into a full storm in a day or two.

3. Cold catches warm

Both fronts are edges of the same cold air, and a front moves at the speed of the cold air. The warm air pushes nothing; it is light, it rides on top. So the cold front moves as fast as the cold air behind it advances, and the warm front moves as fast as the cold air ahead of it retreats.

Advancing is easy. Cold air is dense and hugs the ground, and it spills forward under the warm air by itself, like cold air out of an open freezer running along the floor. Give it a wind behind it and the nose steepens into a blunt wedge that undercuts the warm air efficiently. Retreating is slow. Nothing drives the cold air out; it clings, and the ground friction that steepened the advancing nose drags the retreating tail into a long thin sheet that the warm air has to climb. That is why a cold front's slope is one in fifty to one in a hundred and a warm front's is one in a hundred and fifty to one in three hundred, and why the warm front's cloud arrives a day early as high cirrus.

The wind helps the cold side too. On the west of the low the isobars are tighter, so the wind behind the cold front is stronger and blows more squarely across it. Ahead of the warm front the wind runs more along the front than through it. Typical result: a cold front at fifteen to thirty knots, a warm front at ten to fifteen.

So picture the two fronts as hands on a clock, both hinged at the low. The warm hand points east and barely moves, drifting a little clockwise as the low climbs north. The cold hand starts pointing west and swings anticlockwise through south. It catches the warm hand south-east of the hub first, and the join spreads outward along both fronts over a day. The pinched-off wedge of warm air is lifted off the ground and the low begins to fill. The join is the occluded front, drawn with both symbols.

Some lows do it differently: the cold front detaches and slides past at right angles while the warm front wraps round the back of the low. The same wind rules apply.

4. From the deck

Cut the storm in half and look at it side on, and the clouds sort themselves by height. Ahead of the warm front the warm air slides up over the cold on a long ramp, about a kilometre of height for every hundred and fifty kilometres, and cloud forms along it: the highest and thinnest first, far ahead, then lower and thicker as the ramp comes down to the sea. Behind the warm front, near the sea, there is only a low grey lid. The cold front is steep, so its cloud is too: one tower to the top of the weather. Tap a cloud, or its name, to see what it is and what it tells you.

Those are the clouds a front brings. The whole family has ten members, and their names are a code. Three Latin roots give the shape: cirrus, a curl of hair; cumulus, a heap; stratus, spread out in a layer. A prefix gives the height: cirro- for high, and alto- for middle, which is odd, since altus is Latin for high, but that is the rule. Nimbus marks the two great rain-makers, though plenty of others drizzle or shower. So altocumulus is middle-level heaps, and cumulonimbus is a heap that rains.

Now stand on a boat and let the whole thing pass over you, the low to your north. Here is what the model looks like from below, strip by strip: the sky, the barometer, the wind.

A day out, the first sign is high cirrus, often with a halo round the sun or moon: the ice crystals at the top of the warm front's long ramp. The barometer has started to ease down. The wind is backing, coming round from west through south-west toward south as the low approaches. While the low is far and weak, the back-to-the-wind rule still points north, at the general low pressure toward the pole; by the time the steady rain sets in it points straight at this low.

The cirrus thickens to a milky sheet, then a grey one where the sun is a smudge and light rain begins, then a dark one, and it rains steadily for hours. The pressure is falling fast now. Then the warm front passes. The rain eases to drizzle, the air turns mild and damp, the cloud drops to a low lid, visibility goes, and the wind veers and freshens. The barometer still falls, but slowly. You are in the warm sector, and you will stay there for a few hours to a day.

The cold front arrives from the west as a dark bank, sometimes a shelf. The wind veers round to the west, and in a sharp front further, to the north-west; the temperature drops, a squall of heavy rain or hail passes, and the barometer turns upward. Behind it the sky clears to blue with fat cumulus and showers, the visibility is wonderful, and the wind stays fresh from the west and slowly eases as the high builds in. Pressure rising, wind veered, cold and clear: it is over.

In the Southern Hemisphere, mirror everything: the low passes to your south, and the wind backs and veers the other way round. Flip the switch in the header and watch the strips flip.

You are in the Southern Hemisphere now, so everything is mirrored: the low passes to your south, and the wind veers as the low comes in, then backs as the fronts go through. Flip the switch back to compare.

And the game. Chart hidden, same boat. Read the three strips at the cursor, decide where the low must be, click, then reveal.