THE BURN WINDOW

Every afternoon around five o'clock, the day's fire weather shows its hand. Temperature has crested, humidity has bottomed out, and the burn window — that stretch of the afternoon when fuels are at their most willing — is wide open.

These charts follow that daily moment at three RAWS stations spanning very different Northern California country: Redding Airport on the hot valley floor, Reader Ranch in the foothills, and Big Bar down in the Trinity River canyon.

Each day keeps only two numbers — the highest temperature and the lowest relative humidity — and from those two, the peak vapor pressure deficit: how hard the air was wringing the fuels that afternoon.

fire seasons, June 1 through October 31, , and the current season running through today. The 10-hr fuel stick rides along too — as a daily average, the only form WRCC's daily summaries carry.

The completed seasons don't get lines. They get pooled into one climatology, ranked, and drawn as shading. For each date the record is cut at the 60th, 80th, 90th and 97th percentiles, and the darkest band runs open above the 97th. A line up there is beating 97 percent of everything on record; a line under the shading entirely is somewhere in the ordinary lower sixty. The bands stop at the 97th deliberately — pooling is what lets a 90th or a 97th mean something, but a record is a single afternoon, and widening a window around one afternoon only smears it sideways. Season maxima live in the scorecard at the foot of the page.

Each date borrows the ten days either side of it, which is the difference between a percentile and a coincidence. seasons give only readings for the sixteenth of August, and a 97th percentile drawn from numbers is just the largest of them wearing a label — at Redding Airport on that date it came out at 8.37 kPa against a record of 8.38. Widening to the surrounding three weeks raises the sample past three hundred, and the upper cuts start describing the distribution instead of its single worst day.

On the humidity and fuel-stick charts the ranking runs the other way, so the shaded end is always the dangerous end: the 90th there means drier than ninety percent of the record, not wetter. Only the current season is drawn to begin with — the chips bring back any earlier year to sit beside it, and Percentiles switches the shading off.

The percentile idea is lifted from the NFDRS index charts that fire managers read, but only the idea. Those charts hang consequences on their thresholds — what fire does above one line, what it doesn't below another — and those consequences are calibrated for ERC and BI, indices built from fuel models, wind and slope, which is why the two rarely sit at the same percentile on the same day. These are percentiles of temperature, humidity and vapor pressure deficit. The ranking carries over; the verdict doesn't, so none is offered here.