# Weather Clouds and Pyrocumulus Reading the Sky

> Weather, Clouds and Pyrocumulus: Reading the Sky is the meteorology page — the cloud taxonomy and what each genus tells you, how an ordinary cumulus becomes a cumulonimbus, the fire-generated storms…

Canonical: https://wiki.soapbox.community/wiki/Weather_Clouds_and_Pyrocumulus_Reading_the_Sky
Section: Plants and preparation
Last updated: 2026-10-04
Publisher: Library of Ashurbanipal (Van Kush Family Research Institute), https://wiki.soapbox.community

**Weather, Clouds and Pyrocumulus: Reading the Sky** is the meteorology page — the cloud taxonomy and what each genus tells you, how an ordinary cumulus becomes a **cumulonimbus**, the **fire-generated storms** that are now a field of their own, the **plant and weather cycles** that growers and farmers actually work to, and the **keyless public APIs** that let any of it be read, charted or built into a scene. Our own free forecast app at **weather.soapbox.community** runs on one of those APIs, client-side, with no key and no account.

## 1. The taxonomy, and why it is a taxonomy

**Luke Howard** named the clouds in **1802** — *cirrus*, *cumulus*, *stratus*, *nimbus* — in Latin, deliberately, so that the system would work in any language. Goethe wrote a poem about it. **It is still the system**, formalised by the WMO into **ten genera** with species, varieties and supplementary features layered underneath, exactly like biological nomenclature.

**The ten genera, by altitude:**
- **High (cirro-, roughly 5–13 km):** **cirrus** (ice, wispy, often the leading edge of an approaching warm front), **cirrocumulus** (high ripples), **cirrostratus** (the thin sheet that makes a **22° halo** around the sun or moon — an ice-crystal optic and a classic "rain within a day" sign).
- **Middle (alto-, roughly 2–7 km):** **altocumulus** (patches, the "mackerel sky"), **altostratus** (featureless grey, sun a vague disc).
- **Low (below ~2 km):** **stratus** (fog that did not touch down), **stratocumulus** (lumpy sheet — **the most common cloud on Earth**, and the reason the planet's albedo is what it is), **nimbostratus** (the steady all-day rain).
- **Vertical:** **cumulus** (the fair-weather puff; its **flat base is the lifting condensation level**, and every cumulus base in the same air mass is at the same height, which is why they look like they are sitting on a shelf — **they are**), and **cumulonimbus**, the thunderstorm.
- **Newest addition:** **asperitas** — the wave-like underside — was formally recognised in **2017**, the first new entry in decades, and it was **proposed by amateur observers** through the Cloud Appreciation Society. **A citizen-science result in the WMO atlas.**

**What a taxonomy buys you** is the same thing the aroma wheels buy: **a shared vocabulary in which disagreement is still informative**. "Looks stormy" is not a datum. "Towering cumulus with a hard cauliflower top and no anvil yet" is.

## 2. From cumulus to cumulonimbus — the mechanics

- **Lapse rate** — how fast temperature falls with height. If the environment cools faster than a rising parcel does, the parcel **keeps** being warmer, so it **keeps** rising. That is **instability**.
- **LCL** (lifting condensation level) — the height at which the rising parcel's water vapour condenses. **That is the cloud base.** Above it, **latent heat release** from condensation **adds energy to the updraft**, which is the engine of the whole system.
- **CAPE** (convective available potential energy, in J/kg) — the total fuel. **CIN** (convective inhibition) — the lid. A **capping inversion** can hold a hot, humid afternoon down for hours and then break, which is why severe storms so often fire late in the day, all at once.
- **Growth:** cumulus humilis → mediocris → congestus ("towering cumulus") → **cumulonimbus**, when the top **glaciates** into ice and spreads out against the **tropopause** into the **anvil**. An **overshooting top** punching above the anvil means the updraft is strong enough to overshoot its own equilibrium — **a sign of a serious storm visible from the ground and from orbit**.
- **Hail** grows by repeated circulation through the updraft, which is why **hail size is a proxy for updraft strength**.
- **The outflow** is the part that hits you: the **gust front** and its **shelf cloud**, the **downburst** and **microburst** (a localised, dangerous, downward blast — the cause of several airliner crashes before wind-shear detection), and in the dry west, the **haboob** when the outflow lifts dust.
- **Wind shear** decides the storm's organisation. Weak shear gives a **single-cell** storm that rains into its own updraft and dies. Strong shear tilts the storm so the downdraft falls **beside** the updraft instead of through it — giving **multicell** lines, **squall lines**, and the rotating **supercell** with its **mesocyclone**, which is what produces most strong tornadoes.

## 3. Pyrocumulus and pyroCb — when the fire makes its own weather

**This is the part of meteorology that has changed most in twenty years, and it deserves the detail.**

### The mechanism

A large fire supplies two things a thunderstorm needs:
1. **Sensible heat** — an enormous, concentrated buoyancy source, far stronger than solar heating of the ground.
1. **Water vapour** — from **the vegetation itself**. Live fuel is largely water; combustion also produces water. **The fire supplies its own moisture.**

The result is **pyrocumulus** (*flammagenitus* in the formal nomenclature) — a dirty, brown-grey convective cloud over the fire. If the atmosphere above is unstable enough, it keeps going and becomes **pyrocumulonimbus** — a **pyroCb**, a **full thunderstorm generated by a fire**.

### Why it matters

- **pyroCb make lightning**, and that lightning starts **new fires**, often **ahead of** the existing one and outside any containment line. **The fire reproduces itself through the sky.**
- **They inject smoke into the stratosphere.** This is the finding that reframed the field. A pyroCb's updraft can deliver smoke aerosol **above the weather**, where it does not rain out and instead circulates for **months**. The **2019–20 Australian "Black Summer"** injected smoke into the stratosphere at a mass comparable to **a moderate volcanic eruption**, produced **self-sustaining smoke-charged vortices** that persisted for months, and measurably affected **stratospheric chemistry and ozone**. The **2017 British Columbia** event was the case that first made the scale undeniable.
- **They are lethal to firefighters.** The convective column **collapses** when the updraft fails, dumping the whole downdraft onto the fireground and producing **sudden, violent, direction-reversing winds**. Fatality investigations repeatedly identify this.
- **Fire whirls and fire tornadoes** are a related phenomenon: vorticity stretched by the fire's updraft. The **Carr Fire (California, 2018)** produced a fire-generated vortex with estimated winds in the **EF3** range, which killed a firefighter. The **1923 Great Kantō earthquake** firestorm and the **1871 Peshtigo** fire are the historical reference cases for fire generating its own circulation at scale.

### Fire weather, as it is actually forecast

- **The downslope winds** that drive the worst fires are **foehn** winds — air forced over a range, drying and warming as it descends: **Santa Ana** and **Sawtooth** (southern California), **Diablo** (northern California), **Chinook** (Rockies), **Zonda** (Argentina), **Bora** and **Mistral** (Mediterranean, cold variants). **A foehn is a compression-heating phenomenon**, and the **föhn wall** on the upwind side is where the moisture was left behind.
- **Indices:** the **Haines index** (lower-atmosphere stability and dryness), and the newer **Hot-Dry-Windy (HDW) index**, which multiplies the vapour-pressure deficit by wind speed — simpler and better-validated for **extreme** fire behaviour.
- **Fire danger rating systems** — the US **NFDRS**, Canada's **FWI**, Australia's **Fire Behaviour Index** — all combine fuel moisture, weather and drought indices.

## 4. Plant cycles and weather cycles

**Phenology** is the study of biological timing, and growers use it as arithmetic.
- **Growing degree days (GDD)** — accumulated heat above a crop-specific base temperature. **Plants develop on heat accumulated, not on dates.** GDD is why planting guides are converting from calendars to degree-day targets, and why climate shift shows up as phenology shift first.
- **Chill hours / vernalisation** — the opposite requirement. Many fruit trees and biennials need a **minimum accumulated cold** before they will break dormancy or flower. **Insufficient winter is a real crop failure mode**, and it is one of the clearest local signals of a warming climate.
- **Photoperiodism** — the night-length switch. **Short-day** plants flower when nights lengthen; **long-day** plants when nights shorten. **The plant measures the dark period, not the light one**, which is why a brief light interruption at night blocks flowering in a short-day plant. Cannabis is **short-day**, except for **day-neutral** autoflowering types from *ruderalis* genetics — see Plant Training, Topping, FIMing, Cloning and Grow Lighting.
- **VPD — vapour-pressure deficit** — is the variable that actually governs transpiration, and it is **the single most useful number in controlled-environment growing**. It combines temperature and humidity into "how hard the air is pulling water out of the leaf." **Relative humidity alone is nearly useless** because its meaning changes with temperature. Too low VPD and stomata stay open with no transpiration pull; too high and they close and growth stops.
- **Frost** — **radiative** frost (clear, calm, still nights, cold air pooling in hollows — which is why orchards sit on slopes, not in valley bottoms) versus **advective** frost (a cold air mass arriving, wind and all, which no amount of row cover fixes).
- **The large cycles:** **ENSO** (El Niño / La Niña), the **monsoon**, the **NAO** and **PDO**. These are the **seasonal-forecast** timescale — months, not days — and they are what actually shift a growing season or a fire season. **Köppen** classification is the standing map of what climate a place has at all.
- **And the smell of it.** You really can smell rain coming: **ozone** carried down by a storm's downdraft from aloft, **geosmin** released from wet soil by *Streptomyces*, and plant oils flushed off dry surfaces — collectively **petrichor**. Humans detect geosmin at around **5 parts per trillion**. See Aroma Wheels Euphoric Odours and the Science of Aromatherapy and A Field Guide to Smell Molecules.

## 5. The APIs — all of these are free, and most need no key

**This is the practical section. Every one of these is a public endpoint.**
- **Open-Meteo** (open-meteo.com) — **no API key** for non-commercial use. Forecast, **historical reanalysis (ERA5)**, air quality, marine, and **geocoding**. **This is what weather.soapbox.community uses**, fetched **entirely in the browser** so our server never touches it. The best starting point by a distance.
- **US National Weather Service** — *api.weather.gov*. **No key.** Official US forecasts, alerts, observations, and gridded forecast data. Requires a User-Agent header identifying you.
- **NOAA NCEI** — the climate archive: **GHCN** daily station records, normals, and the long series. This is where "is this year unusual here" is actually answered.
- **GOES and Himawari** satellite imagery — open, and the **channel to watch for pyroCb** is the combination of visible, infrared brightness temperature (a **cold** cloud top over a fire means it has gone deep) and fire-detection bands.
- **NASA FIRMS** — **active fire detections** from MODIS and VIIRS, near-real-time, free. Overlay this with GOES cloud-top temperature and **you can watch a pyroCb form from your desk.**
- **Copernicus / CAMS** — European reanalysis and atmospheric composition, including **smoke aerosol** transport.
- **USGS** — streamflow and flood gauges, for the other end of the same storm.
- **Lightning** — Vaisala and the **GOES GLM** (Geostationary Lightning Mapper) products.
- **House rule for using any of them:** fetch **client-side or on a timer, never on the request path**, soft-fail to a friendly message, and cache. That is how SoapBox Data Hub and the rest of the surfaces are built — see SoapBox Tools.

## 6. Building a scene from this

**Why a world-builder should care** — and this is the thread back to Dudael.

A believable sky is **not a texture**. It is a **state**, and it is fully specified by a small list of numbers that the APIs above will hand you for any place and any date:
- **Place and date** → **sun angle** and day length (pure geometry, no API needed), **Köppen** zone, and the **seasonal** expectation.
- **Temperature, dew point and pressure** → the **LCL**, which **tells you how high the cloud bases sit**. Humid lowland: low, heavy bases. High desert: bases so high the cumulus look small and far.
- **Instability (CAPE) and shear** → **which cloud genus** belongs in the shot, and whether it is building or decaying.
- **Wind** → the **direction everything leans**, including smoke and dust.
- **Aerosol and smoke load** → the **colour** of the light. **This is the single biggest lever on the mood of an outdoor scene** and the one most often got wrong: smoke reddens and flattens, sea air cools and softens, dust browns.
- **And the smell layer** — the 4D time-mapped model in the aroma page: petrichor before the rain, smoke downwind, hot dust at noon, cut grass in late afternoon.

**The discipline is the same one the whole library runs on:** look the numbers up for the real place and the real date rather than inventing them, because **the real ones are free and they are more interesting than the invented ones.**

## 7. Safety, briefly and specifically

- **Lightning: "when thunder roars, go indoors."** The **30-30 rule** — if the flash-to-bang gap is under 30 seconds the storm is within about 10 km; wait **30 minutes** after the last thunder. **Most lightning deaths occur before the rain or after it has passed.** A car is safe; a shelter with no walls is not.
- **Flash flood:** moving water, not depth, is what kills. **Never drive into it.** Desert washes flood from storms you cannot see.
- **Heat:** **wet-bulb globe temperature** is the real metric, because humidity determines whether sweating works at all. A **wet-bulb temperature around 35 °C is survivable for only a few hours** regardless of fitness or acclimatisation.
- **Fire:** **a pyrocumulus column above a fire is a warning sign, not a spectacle.** Column collapse produces sudden, violent, unpredictable winds, and pyroCb lightning starts new fires outside the line.
- See Routes for Backpacking Pilgrimages for the field version of all of this.

## Sources

- Howard L., *On the Modifications of Clouds* (1803).
- World Meteorological Organization, *International Cloud Atlas* (current edition) — the genera, species and varieties, and the 2017 addition of *asperitas* and the *flammagenitus* special cloud.
- Fromm M. et al., "The untold story of pyrocumulonimbus", *Bulletin of the American Meteorological Society* 91 (2010).
- Peterson D. A. et al., "Wildfire-driven thunderstorms cause a volcano-like stratospheric injection of smoke", *npj Climate and Atmospheric Science* 1 (2018) — the British Columbia case.
- Khaykin S. et al., "The 2019/20 Australian wildfires generated a persistent smoke-charged vortex rising up to 35 km altitude", *Communications Earth & Environment* 1 (2020).
- Solomon S. et al., on Australian wildfire smoke and stratospheric ozone chemistry, *PNAS* (2022).
- Lareau N. P., Nauslar N. J. and Abatzoglou J. T., "The Carr Fire vortex: a case of pyrotornadogenesis?", *Geophysical Research Letters* 45 (2018).
- Srock A. F. et al., "The Hot-Dry-Windy index: a new fire weather index", *Atmosphere* 9 (2018).
- Haines D. A., "A lower atmosphere severity index for wildland fire", *National Weather Digest* 13 (1988).
- Bear I. J. and Thomas R. G., "Nature of argillaceous odour", *Nature* 201 (1964) — petrichor.
- Open-Meteo, NWS *api.weather.gov*, NOAA NCEI, NASA FIRMS and Copernicus CAMS public API documentation.