Development preview Not validated for operational use.
SpinningWeather

THE FIELD WEATHER WORKSPACE

Good observations.
Clear calculations.

Enter your sling readings, then add the conditions around them. Follow each result back to its method.

IRPG January 2025 Sources checked October 2026

New observation

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01 The observation

Read the wet bulb first, immediately after spinning. Repeat until its lowest reading is stable.

Preview range: whole-degree readings from 33–120°F. Liquid-water wet bulbs only; 109°F dry bulb is excluded pending review.

Enter whole feet from 0–11,000 ft. Region and observation elevation select the pressure band.

02 Time & terrain

These conditions inform the estimated fine fuel moisture.

Percent slope, not degrees.

Relative to the observation site. Choose the full elevation relationship.

Daytime table blocks run 0800–1959. Time conventions and boundary cases remain under review.

Calculated estimates cannot confirm the quality of an instrument reading.

THE OBSERVATION LOG

A record of the conditions.

Saved on this device. Export a copy to keep your records; browser storage can be cleared.

No observations saved yet.

Calculate a reading, then save it to build your local log.

A NOTE FROM THE FIELD

The instrument starts the story.

Use a clean, properly wetted wick and a representative observation site. Record changing conditions and communicate them with your crew. This workspace helps organize a reading; it does not replace field judgment.

SOURCES & LIMITS

Know the method behind the number.

This is a development preview. Its calculation paths and table transcription have not been independently validated for operational use.

01

Humidity & dew point

The provisional method uses the NWS liquid-water wet-bulb equations with a nominal pressure selected from the NWCG elevation and region bands. Wet bulb must be at least 33°F. Relative humidity is rounded to a whole percent for the downstream table lookup.

Equation results are not yet proven equivalent to every published psychrometric table cell. Below-freezing wet bulbs and unsupported conditions are excluded.

02

Fine fuel moisture & ignition

Table-based estimates follow the January 2025 IRPG, the current edition checked in October 2026. Nighttime uses reference fuel moisture plus five percentage points; nighttime ignition probability is withheld. Daytime uses the seasonal terrain and time corrections.

Three Table C correction cells differ from the reviewed Firelab reference code. This preview retains the printed IRPG values; the source disagreement and boundary conventions still require qualified review. A 109°F dry-bulb reading is excluded because the printed reference-moisture bands overlap at that temperature.

03

A traceable observation

Weather inputs, estimated fuel moisture and ignition probability are different types of information. A precise display does not establish measurement accuracy. Keep the paper reference available and verify readings before any operational use.

Read the IRPG (PDF, opens in new tab)Psychrometric tables (PDF, opens in new tab)NWS equations (PDF, opens in new tab; connection required)

Source PDFs are downloaded separately from the calculator.

TAKE IT WITH YOU

Keep the workspace close.

Install Spinning Weather on your home screen. Open it online first and wait for the offline-ready status before testing without a connection.

iPhone & iPad

Open this site in Safari. Tap Share, then Add to Home Screen and confirm Add.

Android & desktop

Use your browser’s Install app or Add to Home screen menu. On desktop, an install icon may appear in the address bar.

Installation does not make this preview operationally validated. Offline availability depends on this device retaining its stored app files.