Drag Racing Weather & Density Altitude Tracking

Why weather data matters for drag racing

Two passes at the same track on the same day can run two different ETs because the air changed. Cooler air, lower humidity, or higher barometric pressure all mean more oxygen molecules per cubic foot — the engine makes more power, the car runs quicker. Hot, humid, low-pressure days do the opposite. Density altitude is the single number that summarizes those three inputs.

Capturing weather on every run is what lets you compare runs apples-to-apples across a season. Without it, your trend chart is noise — the car looks faster in October just because the air got cool. With it, you can answer "did the setup change actually do anything" because the model can correct for the conditions.

How weather is captured

There are four ways to attach weather to a run, in priority order:

• Live auto-fetch from NOAA/NWS or Open-Meteo when you open the Log Run dialog
• Photo of a Kestrel, Davis WeatherHawk, AcuRite, or weather-app screen — AI extracts temp/RH/pressure/DA
• Historical fetch for runs older than 2 hours, pulled from the ERA5 reanalysis archive
• Manual entry — every field is editable, and density altitude auto-recomputes when you change inputs

Auto-fetch from the nearest weather station

When you open Log Run, the app looks up the track's coordinates and pulls live conditions from the closest NOAA station (or Open-Meteo, for tracks outside the US or when NOAA is down). The fetched values populate air temp, RH, barometric pressure, wind speed, wind direction, and density altitude — usually within a second or two of opening the dialog.

The fetch surfaces the timestamp of the observation so you can see how fresh it is. If the closest station is 20 miles away and reads differently from what's happening at the strip, you can override with a Kestrel photo or type the numbers in directly. The DA recomputes live as you edit.

Photo capture from Kestrels and weather apps

A photo of a Kestrel 5500 (or 5700, or Davis, WeatherHawk, AcuRite, smartphone weather app, even the track's weather board) goes through the same AI vision pipeline as the time slip. The model reads the display and extracts temp, RH, pressure, DA, elevation, and wind speed. Units convert automatically: °C to °F, hPa to inHg, m to ft, knots to mph.

If the Kestrel shows DA directly, that value is used as-is. If it doesn't but it shows temp + RH + pressure + elevation, density altitude is computed client-side using the same psychrometric formulas a Kestrel uses internally. The number that lands in your run is the DA your engine actually felt.

Wind direction and headwind math

Wind speed alone doesn't tell the predictor much — a 12 mph crosswind matters far less than a 12 mph headwind. The logbook captures wind direction in degrees (0 = north, 90 = east, 180 = south, 270 = west) and stores each track's compass orientation. When the predictor needs a headwind component, it projects the wind vector onto the track orientation:

Headwind mph = wind speed × cos(angle between wind direction and reversed track heading).

A direct headwind = full component. A direct tailwind = full negative component. A crosswind has zero headwind effect. The empirical ET sensitivity is about 0.010 seconds per mph of headwind, applied as a positive (slower) adjustment for headwind or negative (faster) for tailwind.

Historical weather for past runs

If you're back-filling runs from a stack of paper slips, the app pulls weather from the ERA5 reanalysis archive for any run older than 2 hours. That historical fetch is high-resolution (hourly, ~9 km grid) and goes back to the 1940s — so a logbook page from 1998 can still attach realistic conditions to every row.

You can also leave the weather block empty on imported runs and the predictor will simply weight them lower. Real captured weather is always better than no weather, but the system doesn't break without it.

Public live weather for every drag strip

Every track in the DRL directory has its own public page at /tracks/<track-name> with live atmospheric conditions and computed density altitude. No account required. Bookmark your home track and you can pull up current DA from the trailer five minutes before time trials, even before you sign in.

The track page refreshes its weather block on every page load — no manual refresh, no caching staler than your last navigation. Six values are surfaced: air temperature, humidity, station pressure, density altitude, wind speed plus a compass direction arrow, and the source attribution (which NOAA station and how many miles away, or "Open-Meteo gridded" for international tracks).

Below the live block is a "Why density altitude matters at <track>" section with a one-tap CTA: "Open DA calculator for <track>" that jumps to the public calculator with the strip's elevation pre-filled. The page also shows today's sun chart (sunrise, solar noon, sunset, day length) and the three nearest other tracks for quick navigation.

• Six weather cells: air temp, humidity, station pressure, DA, wind, source.
• NOAA METAR observations for US tracks; Open-Meteo gridded forecast as fallback.
• Sun chart computed locally from track coordinates (works without JavaScript via the prerendered page).
• Track facts: sanction body, elevation, strip length, surface, compass heading.
• Three nearest tracks with road-distance estimates, for quick comparison shopping if conditions look bad at your home strip.
• Deep-links into the DA calculator pre-filled with the strip's elevation and name.