Collide a warm humid air mass with a cool dry one, read how the weather changes at a station, and find which setups build a tornado.
Finishing an event adds 25 rows, one every half hour of the twelve hours. Each row has the Trial, the Hour and the three settings you chose. It also has the weather station's Temperature, Dew Point, Pressure, Wind Speed and Rain, the Outcome, and an EF Rating for the trials that produced a tornado.
Every release is its own trial, including a repeat of the same setup, so several trials can be compared. Each release gets small random nudges, so repeats of one setup give a range of results. A line graph of Temperature or Pressure against Hour, grouped by Trial, shows the front passing and the spread between repeats. Plotting Wind Shear against Moisture Setting across trials, coloured by Outcome, shows where tornadoes tend to turn up.
Model details
Three ingredients drive the storm. Moisture is the one setting on the warm humid air. It has four levels: Dry, Damp, Humid and Muggy. Behind each level is a dew point from 45 to 72 degrees Fahrenheit. The weather station's own dew point reading starts at that number and then changes as the air masses pass.
Instability comes from the contrast between the warm moist air below and the cool dry air above it, which you set from 65 down to 35 degrees. Wind shear is the change in wind speed between the ground and the top of the storm, 0 to 45 mph.
Moisture is shown as the shading of the humid layer, and the depth of that layer is held fixed. This is a simplification. A dew point measures how moist the air is, not how much moist air there is. In real storms, how deep the moist layer is matters too, because it decides whether the moisture survives being mixed out in the afternoon.
The cool dry air is an air mass that flows in high up from the northwest, over the top of the warm humid layer. The cut-away and the scene both draw it that way. It is not a wedge pushed along the ground.
Moisture is a gate rather than a contributor. Below about a 50 degree dew point there is not enough water vapour to condense, and the sky stays clear whatever the other two controls say. This is why starting with shear alone gets you nothing at all.
Instability is weighted toward the cool air rather than the moisture, because it is the temperature difference across the layer that decides whether a parcel of air that starts rising keeps rising. Moisture still contributes, since the heat released as water vapour condenses is part of what drives the updraft.
One thing this sim simplifies, and it is worth being straight about because it is the question everybody asks: it is not really that cold air has to sit on top of warm air. Temperature falls with height almost everywhere, almost always, so that arrangement is the ordinary state of the atmosphere and nothing special. What matters is how FAST it falls. A parcel of air that starts rising cools as it goes, and it keeps rising only while it stays warmer than whatever is around it. So the air above has to get colder with height more steeply than the rising parcel itself cools. Forecasters measure exactly that and call it the lapse rate. Dragging the cool air lower in this sim is a way of steepening it.
Two things follow that surprise people. A layer of WARM air sitting on top of the moist air, which sounds like the opposite of what a storm wants, is actually one of the ingredients of the biggest Plains tornadoes. It is called a cap, and by holding everything down it stops a crowd of weak storms from using up the moist air, so when one finally breaks through it rises very fast. The middle layer of the cut-away is drawn as a cap, a layer of warm air. The model does not use it. Its thickness changes only because the cool air's boundary moves, and it has no effect on the outcome. And tornadoes do form in air that is barely unstable at all. Squall-line tornadoes and the ones that come with hurricanes often have very little of the rising-air ingredient and a great deal of wind shear instead. The three ingredients trade off against each other rather than each having a fixed threshold.
Shear only matters once there is a storm for it to act on. It turns a thunderstorm into a rotating supercell, and at high values together with high instability it puts a funnel on the ground. Following NOAA, the mechanism is a horizontal tube of rolling air that the storm's updraft tilts upright into a rotating column called a mesocyclone.
Tornado strength varies with the setup and from one release to the next, and a rating on the Enhanced Fujita scale is exported with each trial. EF is a DAMAGE scale. A real rating is assigned after the event by surveying what the tornado did to buildings and trees, and the wind speeds attached to each rating are the speeds that damage is taken to imply. Nobody puts an instrument inside a tornado.
This sim shows no damage, so it works the scale backwards, from its own wind speed to the rating that speed corresponds to. The tornadoes it makes are rated EF0 to EF4, and EF5 (over 200 mph) is out of reach. The strongest setups give EF3 or EF4. A weak tornado from a setup near the edge of the tornado region gives EF0 or EF1.
The funnel changes shape as well as size. A weak one is a thin rope. A strong one stands as a wedge nearly as wide at the ground as it is at the cloud. Width and strength are only correlated in real life, not the same thing. Some narrow tornadoes have done violent damage.
The storm always arrives from the right and leaves to the left, and that is not arbitrary. The camera faces west with south on the right, which is where the warm humid air comes from, and a Plains supercell travels with the wind a few kilometres up, which in a setup like this blows from the southwest toward the northeast. From where you are standing that is right to left, closing on you and then pulling away to the north. The direction is the same every run on purpose, for the same reason the view does not move: every student's screen matches, so a teacher can say “watch the left of the picture” and be right for all of them.
How FAR the storm travels is up to you, though. Storms move with the mean wind, and the wind is what you set on the shear rail, so more shear means a faster storm: it starts further out and ends up further away in the same twelve hours. Its closest approach stays at the same hour either way, which keeps the readings and the picture in step. One real consequence follows: a fast-moving supercell leaves a longer damage track, which is where long-track tornadoes come from.
The funnel is drawn, not solved. There is no fluid dynamics here: the storm's shape is art driven by a small model of the three ingredients.
The setup decides what is likely. Each release then adds four small random nudges: how unstable the air is, how much shear there is, how early the front arrives, and how strong the storm is.
Most repeats of one setup give the same kind of storm, a little stronger or weaker and a little earlier or later. A setup near the edge of a boundary can give a different kind of storm. A setup well inside one almost never does.
That spread is what a forecaster means by a probability, and it is why one trial is not enough evidence. Every release is a separate trial in the table, so a class can read the spread there.
The weather station stands near the path the storm takes. You watch from a field a few kilometres away. That is why the station can record the wind and pressure of a storm passing over it while the picture shows the funnel in the distance.
Two things are deliberately not to scale. Twelve hours of an afternoon and evening run in about thirty seconds, and the storm's whole life from first cloud to funnel is compressed to fit inside that. And the two air masses are drawn with roughly three times the contrast they really have. In life a warm humid air mass announces itself as a milky horizon and a dew point reading, not as a coloured bank you can point at, but a control you are told to drag has to be visible where the instruction says it is.
Everything else is close to real scale: a cloud base at about 2 kilometres, a storm about 5 kilometres away, and a funnel a few hundred metres across. That is what the fence posts, the power poles and the grain elevator are there for. You are standing close enough that the storm's anvil and its huge cauliflower tower are above the top of the view, which is what anyone under a supercell actually sees.
The cut-away on the left is a diagram, not a window: its vertical scale is honest to 3 kilometres, but the rolling tube of air drawn in it is far larger than life. A real one is a few hundred metres across and completely invisible, since air is transparent. It is drawn at all because it is the thing the storm is about to stand upright, and you cannot watch that happen to something you cannot see.
The sound is optional and off until you switch it on with the speaker button in the toolbar. Nothing is a recording: the wind, the rain and the tornado's roar are built out of filtered noise whose loudness and tone are driven by the same numbers the weather station is reporting, so what you hear is what the table records. The thunder is the interesting one. Each bang is scheduled for the time sound actually takes to cover the distance to the bolt, about three seconds every kilometre, so early in the event you see the flash a long time before you hear anything and by the time the storm is overhead they arrive together. Counting the gap is how you work out how far away a storm is, and here it falls out of the model rather than being asserted.
The only mark the storm leaves behind is the crop in the foreground, laid flat by the outflow and staying flat. A flattened gap where the funnel crossed the tree line was built and then cut, and the reason is worth knowing: twenty metres of tree five kilometres away is about four pixels on your screen, so flattening it moves three of them and nobody can see it. Damage on the ground is worse still. A track 500 metres wide at that distance takes up roughly a ten-thousandth of the height of the view, because from an eye a metre and a half up everything that far away is edge-on. You cannot see a tornado's track from the ground at all, and that is exactly why the people whose job is to rate tornadoes survey them from aircraft.
No buildings, animals or people are ever in the tornado's path, and nothing is shown being wrecked or thrown.