# How accurate is ERA5 weather data?

We are using weather data to feed building energy models.

Historically, we would do that with weather from the closest weather station, which can be far away (up to 100 km, I guess).

Recently, we've been investigating the use of ERA5 weather data. This is a fantastic database for us because it provides access to historical data for any given place in the world, for free or close to free.

However, I noticed on a few examples an offset with the weather from the weather station. I won't detail this here.

The point of my question is that I can find ERA5 values but I have no idea of the expected accuracy. In other words, if it says 20°C, does this mean 20°C+-0.2°C, or 20°C+-2°C?

I'd like to tell our engineers how reliable the data I'm providing is.

I guess the accuracy is better for recent data since there are more actual data to fit the model. It may depend on many other factors and vary from one physical parameter (temperature, humidity,...) to another.

Is there any place where I could find such information in a relatively synthetic way?

I've been searching a bit with keywords such as "ERA5" "precision" "accuracy" but found no short answer, if any.

• I've never used ERA5. Do they also include irradiance (e.g. Global Horizontal Irradiance), or just wind/temperature/humidity? Jan 26 at 8:21
• @EricDuminil there's much more than this. See CDS. Or Oikolab for easier access to commonly used data. Jan 26 at 10:11
• Thanks. I tried to download a few locations over a few time ranges, and didn't manage to get any solar radiation data. Jan 26 at 14:24

Time series of monthly and globally averaged ERA5 ensemble spread from 1979 to 2018 at indicated pressure levels for (a) temperature ($$K$$), (b) zonal wind ($$m s^{−1}$$), (c) ozone (partial pressure in $$mPa$$) and (d) specific humidity (in percent of the 1981 to 2010 mean value at the pressure level in question).