You have probably seen them on book covers, university banners, or app widgets: a barcode-like strip of blue and red vertical lines known as warming stripes (or climate stripes). Originally created by climate scientist Ed Hawkins in 2018, they reduce complex multi-decade meteorological reanalysis into a single visual statement.
But what do the colors actually represent on a localized, city-by-city basis?
1. Anomaly vs. Absolute Temperature
The most common misunderstanding about climate stripes is assuming blue means "cold winter" and red means "hot summer."
In reality, each vertical stripe represents a full calendar year relative to a long-term baseline average (typically the 30-year climatological normal from 1971–2000 or 1991–2020):
- Deep Blue: A year where the annual mean temperature was significantly below the baseline (e.g., -1.5°F cooler).
- White / Neutral: A year that matched the 30-year normal within a fraction of a degree.
- Deep Red: A year where the annual mean was substantially above the baseline (+1.5°F to +2.5°F warmer).
2. The Local Shift: Why City Baselines Diverge
Global climate stripes show a steady, unmistakable transition from left-hand blues to right-hand reds over the past 150 years. But when you look at individual cities in Eranorms, localized patterns emerge:
- Urban Heat Island Amplification: Cities like Phoenix, Austin, and New York warm faster at night because asphalt and concrete trap daytime heat, elevating the annual baseline.
- Marine Dampening: Coastal cities like Seattle and San Francisco show high annual temperature stability, but their winter overnight lows show steady upward drift.
- Diurnal Range Variations: High-altitude metros like Denver experience 30°F daily swings, making single-year anomalies feel like rapid swings between extreme freezes and record summer heatwaves.
3. Why 70-Year Reanalysis Matters
Single-day forecasts tell you whether to bring an umbrella. Decadal reanalysis (combining NOAA station records with ERA5 atmospheric data) tells you whether an unseasonably warm December is an outlier or part of a 40-year baseline shift.
Eranorms puts this history directly into your hands — giving you custom stripes, box plots, and station normals for hundreds of global locations.