Cloud Cover is one of 24 user-selectable climate variables in the Historical Climate KPI. It shows the daily time series of mean daily cloud cover fraction (0–100%), recorded from 1940 to the present. Each data point is color-coded against 1961–1990 reference percentiles, highlighting anomalous values relative to the historical climatology.
This is an exploratory visualization tool — it produces no A–E quality scores but provides the complete time series for detailed climate analysis.
Daily values are extracted from the ERA5-Land reanalysis for the site's coordinates:
where the percentile intervals cover levels [10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 99] of the 1961–1990 reference distribution. Colors range from cool tones (near-normal) to warm tones (extreme anomalies).
Data are sourced from the Open-Meteo Historical Weather API (ERA5-Land reanalysis, ECMWF) at approximately 10 km spatial resolution with daily coverage from 1940 to the present.
Line Chart. A daily time-series chart showing mean daily cloud cover fraction (0–100%) over the selected period, with each data point color-coded by severity relative to the 1961–1990 climatology.
Purpose: Answers "How has cloud cover at this site varied over time, and are there anomalous periods?"
Description: The chart displays daily values (%) as colored dots connected by a line. The user selects "Cloud Cover" from the variable dropdown. The time range is adjustable. Each point is colored from blue (cool/normal) through yellow to red (extreme anomaly) based on the 1961–1990 percentile distribution. A tooltip shows the exact value, date, and percentile rank on hover.
How it's calculated: For each day, the daily value is compared to the 1961–1990 reference percentiles for the same variable and location. The color is assigned based on which percentile interval the value falls into.
Interpretation example:
If the chart shows a cluster of red dots in July 2023 for cloud cover, it means those days had values in the extreme upper percentiles of the 1961–1990 distribution — significantly warmer/higher than the historical norm for that period.
| Source | Provider | Coverage | Resolution | Period |
|---|---|---|---|---|
| ERA5-Land Reanalysis | ECMWF via Open-Meteo | Global | ~10 km | 1940–present, daily |
Daily values are queried from the Open-Meteo Historical Weather API using the site's centroid coordinates. For variables with min/max support (including temperature), three values per day are stored: daily minimum, daily mean, and daily maximum. For single-value variables, only the daily mean (or daily sum for precipitation-type variables) is stored.
The severity color is computed by mapping the value into the 1961–1990 percentile distribution, which is pre-computed per variable and location. The 11 percentile levels (10th through 99th) define 12 color bands from deep blue through white to deep red.