Reading the radar
CSENHow to read a weather radar
Colours, rain intensity and what the timeline is for — a practical guide to the precipitation radar.
FAQ
What do the radar colours mean?
Colours map to reflectivity in dBZ: blue/green = light rain, yellow/orange = steady rain, red = downpour, purple = hail or a strong storm core. The exact scale is in the map legend.
How often does the radar update?
ČHMÚ publishes a new frame every 5 minutes, about a minute after measurement. Slejvak picks it up automatically — no need to refresh.
How do I tell a storm is heading my way?
Play the animation and watch the cores move — or use the rain countdown: pick your location and Slejvak computes when precipitation arrives, from which direction and how fast it is moving.
Why does the radar show rain when it is dry outside?
Most often it is virga — precipitation that evaporates before reaching the ground. Radar measures reflectivity aloft, not rain at the surface. It can also be a very weak echo at the sensitivity limit.
How far ahead can radar predict rain?
Extrapolating echo motion (nowcasting) is reliable for roughly 30–60 minutes. Beyond that a numerical model is more accurate — on Slejvak the ICON-EU and ECMWF forecast.