Every autumn the atmosphere over North America gets interesting. The Gulf is still warm, the first cold fronts are coming down from Canada, and the hurricane season has not finished. It is the second severe-weather season of the year, and the one most people forget about.
"Severe" has a precise meaning to a forecaster. A thunderstorm earns the word when it produces hail at least an inch across, wind gusts of 93 km/h (58 mph) or more, or a tornado. Everything below that bar is just weather. The six plates in the Severe series are five kinds of weather that clear it, one of them only on a bad day, and one event that is not weather at all but belongs in the same set of warnings. Here is what each one is actually doing.
The anvil: a storm hits the ceiling
A thunderstorm is an updraft. Warm, humid air rises, cools, and condenses into a tower of cloud that keeps climbing as long as the air inside it is warmer than the air around it. The climb ends at the tropopause, the boundary between the troposphere and the stratosphere, which sits anywhere from about 8 kilometres up near the poles to 18 at the equator; a big summer storm over the central United States tops out around 15 or 16. Above it the temperature stops falling, the updraft loses its buoyancy, and the cloud top spreads sideways into the flat sheet that gives the cumulonimbus incus its name. Incus is Latin for anvil.
The strongest storms have updrafts of 50 metres per second, fast enough to hold a baseball-sized hailstone in the air while it grows. The lightning underneath carries a typical peak current of around 30,000 amperes, which is why a bolt can heat the air in its channel to several times the surface temperature of the Sun.
The Anvil plate draws the whole storm in section, with one square red flag with a black centre beside it. That flag is the storm warning in the coastal warning display system, which is worth knowing on its own: one red pennant is a small-craft advisory, two pennants a gale warning, one square flag a storm warning, two square flags a hurricane warning. The flags turn up across the series.
The funnel: the storm starts to spin
Most thunderstorms do not rotate. A supercell does. When the wind changes speed and direction with height, the air near the ground rolls like a log; a strong updraft can tilt that roll upright and begin to spin as a whole. The rotating updraft is called a mesocyclone, a column of rotation several kilometres across that runs up through much of the storm's height. A tornado is what happens when that rotation tightens and reaches the surface.
The visible funnel is condensation: the pressure inside the vortex is low enough that water vapour condenses into cloud. The dark skirt at the base is everything the wind has picked up. The Enhanced Fujita scale rates a tornado after the fact by the damage it leaves; an EF5, the top of the scale, means winds above 320 km/h (200 mph) and a track that can run for tens of kilometres.
The Funnel plate shows the tornado on the ground under its parent supercell, with the map symbol a forecaster uses for it. If you teach earth science, that pairing of the thing and its notation is the point: the symbol is the storm compressed to one mark on a chart.
The eyewall: the strongest wind on the planet
A hurricane is a different machine. Where a supercell is driven by one updraft, a hurricane is a ring of thunderstorms wrapped around a column of sinking air, powered by the heat released when warm ocean water evaporates and then condenses again in the storm. The calm centre, the eye, can be 30 to 60 kilometres wide. The ring immediately around it, the eyewall, is where the air rises fastest and the wind is strongest.
A category-five storm on the Saffir-Simpson scale has sustained winds of at least 252 km/h (157 mph). The lowest pressure ever measured in an Atlantic hurricane is 882 hectopascals, against a normal sea-level pressure of 1013. From orbit the structure is unmistakable: the spiral bands feeding in, the clean eye, the eyewall as a bright ring.
The Eyewall plate is that view from above, with two square red flags beside it. Two square flags with black centres is the hurricane warning, the top of the display.
The haboob: the storm pushes a wall of dust ahead of itself
Thunderstorms end in a downdraft. Rain falling through dry air evaporates and cools the air around it, which sinks and hits the ground and spreads out as a gust front, the leading edge of the storm's outflow. Over desert, that front scoops up dust and carries it as a wall that can stand a kilometre and a half high and stretch a hundred kilometres across. The word haboob is Arabic, from a root meaning to blow, and it reached English by way of the dust storms of Sudan; Arizona sees them every summer monsoon.
The wall arrives before the rain does. Visibility drops to a few metres in seconds, which is why the real hazard is on the highway, not in the sky. The Haboob plate carries two red pennants: the gale warning.
The spout: the gentle cousin
Most waterspouts are not tornadoes over water. A fair-weather waterspout forms from the surface up, under a line of developing cumulus clouds, along a boundary where two air masses meet. It is thinner, slower and shorter-lived than a tornado: a typical one lasts fifteen minutes from the first dark spot on the water to the rope-like funnel that marks its end, with winds that rarely pass 120 km/h. Boats still give them room. The Spout plate stands its vortex on a ring of spray, with the single red pennant of a small-craft advisory.
The fault: not weather, but a warning all the same
The sixth plate breaks the pattern on purpose. An earthquake is what the ground does, not the sky, but it shares the series' logic: a violent event, a scale to measure it, a notation to record it. A strike-slip fault moves sideways, the two blocks of crust grinding past each other, and when it ruptures the break can unzip along the surface at kilometres per second. The first waves to arrive at a seismograph are the P-waves, compressional, moving through the crust at around 6 km/s; the slower, more damaging S-waves follow.
The Fault plate shows a magnitude 7.8 rupture crossing the ground, with the seismograph trace of the moment it arrived and a line of alternating triangles and half-discs. On a surface weather chart that line is a stationary front, the boundary between two air masses that is not moving. Here it marks a different boundary that has just moved four metres.
Why it is a series
Severe weather is one of the few parts of physics everyone has watched out of a window. Each of these plates is a diagram of a process, the way a textbook would draw it, with the numbers that define it and the symbol a forecaster would use for it. Every one comes as a tee and a hoodie, printed on the front, made when it is ordered.
If you teach, the flags make a good five-minute lesson on their own. If you are buying for someone who checks the radar before the forecast, start with the Severe series and pick the storm they would stand outside to watch.