MDT is persistently wetter than CXY in both eras
Annual ratio 1.066 (early) and 1.084 (modern). CIs overlap in 11 of 12 months — the bias direction is fully validated across the 36-year instrumentation gap. July is the sole divergent month (early: MDT drier; modern: MDT wetter), likely driven by convective storm track differences. Mann-Kendall shows no trend in the combined record (p=0.143) — the bias is stationary.
→ Use modern monthly ratios for all homogenization
River valley thermal signature confirmed in both eras
MDT runs 0.7–1.1°F warmer overnight from May through October in both the early and modern periods. The Susquehanna River stores solar energy through summer and releases it nocturnally, keeping MDT's floodplain boundary layer warmer than the upland CXY site. The diurnal cycle shows convergence at solar noon and divergence overnight — exactly as valley-versus-upland physics predicts.
→ Apply warm-season (May–Oct) TMIN offset with confidence
Sign reversal between eras — instrumentation artifact
MDT ran warmer than CXY in summer in the early era (+0.67°F June) but is consistently cooler in the modern era (−0.33°F June). This is an instrumentation era artifact from cooperative observer shelter exposure vs ASOS hygrothermometer siting standards — not a real physical change. Mann-Kendall shows a significant decreasing trend (p=0.000) in TMAX offset across both eras.
→ Do not apply a fixed TMAX offset — use era-specific factors only
Susquehanna moisture signature — perfectly stable modern era
MDT is consistently moister than CXY in every month of both eras. Monthly offsets range +1.8°F to +3.5°F. Peak in March when river-to-air temperature differential is greatest. Modern era Mann-Kendall: p=0.981, Tau=+0.007 — the moisture signal has not shifted at all between 2001 and 2025. The 1962–65 drought collapses the early era offset (MDT goes below CXY) but this is a meteorological anomaly, not structural.
→ Most reliable variable for homogenization — use modern offset
MDT is consistently foggier than CXY — river fog signature
MDT dense fog frequency = 0.8% of hours (early) vs CXY 0.7%. Modern: MDT 0.1% vs CXY ~0.0%. November is the foggiest month at MDT in both eras — when the river retains summer heat while air temperatures drop rapidly, maximizing surface-to-air temperature differential that drives fog formation over the floodplain. Directly relevant to cold air damming event persistence.
→ MDT fog frequency ~14% higher than CXY in early era
CXY receives more snow than MDT — rain-snow line effect
Despite MDT being wetter in liquid equivalent, CXY receives more snowfall — overall MDT/CXY ratio 0.879. MDT's lower elevation and warmer floodplain surface converts marginal precipitation events to rain while CXY at higher elevation records them as snow. This confirms the physical complexity of the two sites and validates the rain-snow line mechanism as an explanation for both the wet precip bias AND the snow deficit at MDT.
→ No modern CXY snow data available — early era only
NW dominates both eras — SE frequency nearly doubled modern
NW sector is the dominant wind direction at MDT in both the early era (30.2%) and modern era (35.9%). The most significant cross-era shift is SE frequency increasing from 10.3% to 18.4% (+8.1%) — the cold air damming signature — while SW decreased by 5.9%. Spring months (March–April) are windiest in both eras. Note: early WSFG (peak gust) vs modern AWND (daily mean) are different measurements.
→ CAD SE flow signal strengthened in modern era
MDT 2–6% higher RH than CXY — confirms moisture boundary layer
Average RH is higher at MDT in every month of both eras. Early era offsets range +0.9% to +5.8%; modern era +1.8% to +3.9%. The seasonal pattern mirrors the dewpoint analysis exactly — both variables are measuring the same Susquehanna River boundary layer moisture enhancement from different angles. Modern era RH shows no trend, corroborating the dewpoint stability finding.
→ Use modern RH offset; corroborates dewpoint independently