Zambretti Weather Forecaster

Station readings are reduced with hydrostatic balance and the ideal-gas law, assuming a dry-air temperature lapse of $L=0.0065\,\mathrm{K/m}$:

Temperatures in this equation are kelvins. PDAS, Hydrostatic equations, pp. 1–3, supplies the geopotential conversion and linear-gradient relation (equation 15). Constants are $g_0=9.80665\,\mathrm{m/s^2}$, $R_d=287.05287\,\mathrm{J/(kg\,K)}$ and $R_e=6356750\,\mathrm{m}$. Already-reduced pressure bypasses this multiplier.

The forecast follows the documented pywws variant (attached source s4, p. 1; same core as s1). Its 950–1050 hPa normalization is the identity here. The 3-hour change is divided by 3 h: rising at or above +0.1 hPa/h, falling at or below −0.1 hPa/h, otherwise steady. The 16-sector wind offset and warm-half-year offset are added to $p_0$. Southern wind sectors rotate by 180°. In the warm half-year, rising pressure receives +3.2 hPa and falling receives −3.2 hPa; steady receives zero.

The unrounded index is $0.1740(1031.40-p)$ for rising, $0.1553(1029.95-p)$ for falling, or $0.2314(1030.81-p)$ for steady pressure, with $p$ in hPa. Round to the nearest integer and clamp to 0–13, 0–9, or 0–16 respectively, then use the branch-specific letter mapping. Outlook sentences are our own paraphrases. The letter number A=1…Z=26 supports checks; it is not a probability or severity scale.

• Fixed UK-oriented 950–1050 hPa scale; no fitted local climate calibration.
• The barometric reduction extrapolates a constant dry-air lapse below the sensor; it is an estimate of meteorological sea-level pressure, not a complete operational QFF reduction or an aviation altimeter setting.
• Prefer the preceding 12-hour mean outside air temperature, following the NOAA NWS explanation of pressure reduction. Humidity, inversions, temperature history beyond that mean, and local lapse changes are omitted.
• The trend input is an actual 3-hour sea-level pressure difference. Only its category is used: larger changes within a category do not strengthen a forecast.
• Warm half-year means April–September north, October–March south. Wind direction means FROM that sector. Calm omits the wind offset.
• Illustrative dial and terrain are schematic, not a geographic cross-section. Diagram and chart pressures are labelled in hPa.

Accuracy is unknown. No published validation was found in the sources reviewed (25 October 2026). We do not endorse frequently repeated percentage-accuracy claims. This empirical outlook covers approximately the next 12 hours; it gives neither precipitation probability nor a quantitative storm warning.

The two PDAS cases validate only the pressure reduction against published standard-atmosphere numbers, within table rounding. Four separate executions of the official pywws function validate software agreement across all trend branches and the southern transformation. None measures forecast skill. A location-specific study would need timestamped forecasts, observed outcomes, defined scoring rules, and comparisons with persistence or climatology.

Input elevation is limited to −500…3000 m and outside temperature to −60…55 °C as practical tool limits, not validated meteorological bounds. An elevation above 1000 m prompts a caution; use a reliable externally reduced sea-level reading when available. A sea-level value outside 950–1050 hPa or an index beyond its branch limits triggers a saturation note. At very rapid pressure changes, inspect the time series: two endpoints can conceal reversals. Do not use this outlook as the sole basis for hazardous-weather decisions.