CHMI climate normals (1991-2020)

A climate normal is a long-term average of a weather variable, such as air temperature or precipitation, over a thirty-year reference period. The World Meteorological Organization (WMO) recommends recalculating normals every ten years; the current period is 1991-2020. The Czech Hydrometeorological Institute (ČHMÚ) publishes maps of annual and monthly normals of air temperature and precipitation for the whole of Czechia.

How the maps are made

ČHMÚ uses so-called technical series: measurements from selected stations that have been homogenised and gap-filled. It interpolates the station values onto a regular grid with a resolution of 500 m, taking altitude into account. As a result, a normal can be read even for places with no station.

How to read the figure

A normal shows how warm a place is on average and how much rain it gets, and allows comparison with other places. Counts of characteristic days are often added:

• summer day: maximum daily air temperature of at least 25 °C
• frost day: minimum daily temperature below 0 °C
• tropical day: maximum daily temperature of at least 30 °C

Altitude tells a lot: higher places tend to be cooler and wetter. On a property website the normal is one of the inputs from which the location analysis builds its description of the climate.

What to watch out for

A normal is a thirty-year average, not a forecast. Individual years differ from it; ČHMÚ therefore also publishes maps of how each year and month deviates from the normal. Studies based on ČHMÚ measurements show that frost days have become less frequent in recent decades. The 500 m grid also misses local differences such as a cold valley, a windy ridge or the warmth of a built-up town.

Frequently asked questions

Why 1991-2020 and not the last thirty years?

Under WMO rules, the reference period moves forward every ten years and ends in a year ending in zero. The next period will be 2001-2030. The older 1961-1990 normal is still used as a baseline for assessing climate change.

Is a climate normal related to the climatic region in BPEJ?

Not directly. The climatic region is the first digit of the BPEJ soil code and is used to assess farmland. ČHMÚ climate normals describe actual temperature and precipitation values.