Understanding climate
The ideas this site puts on every city page, explained: what a climate normal is, where the ERA5 data comes from, how to read a warming trend, and what the Köppen codes mean.
What is a climate normal?
A climate normal is the average of a meteorological quantity — temperature, precipitation, sunshine — computed over a long reference period, which the World Meteorological Organization sets at thirty years.
ERA5: what is a climate reanalysis?
ERA5 is the climate reanalysis produced by ECMWF for the Copernicus Climate Change Service: a physically consistent reconstruction of the state of the Earth's atmosphere, hour by hour, from 1940 onwards, served on a grid of 0.25° — roughly 28 kilometres.
How to read a warming trend
A warming trend is the slope of a line fitted by linear regression to a series of annual means; it should only be asserted when the accompanying statistical test shows it cannot reasonably be explained by chance.
The Köppen-Geiger classification, letter by letter
The Köppen-Geiger classification sums up the climate of a place in a two- or three-letter code built from monthly temperature and precipitation alone: the first letter gives the broad zone, the second the rainfall regime, the third the thermal nuance.
Climate and weather: what is the difference?
Weather describes the state of the atmosphere at a given place and moment, while climate describes the statistics of those states over several decades: the same physical quantities, observed at two timescales that do not obey the same rules.
Frost days, hot days, tropical nights
Climate indicators count how many days per year cross a given threshold — frost, strong heat, rain, a warm night — and so describe the distribution of a climate where an annual average merely summarises it.