Radar is the backbone of short-term severe weather detection and it measures something indirect.
Reflectivity
Energy returned from particles in the atmosphere.
Which is converted to estimated rainfall by formula.
Doppler velocity
Movement towards or away from the radar.
Which is how rotation is detected.
Dual polarisation
Pulses in two orientations distinguishing particle shape.
Which separates rain, hail and debris.
Limitations
Beam height increasing with distance and terrain blocking.
Which produces coverage gaps.
What the colours actually represent
Reflectivity, which is how much energy came back, converted into estimated precipitation intensity by an assumed relationship.
Which works well for rain and less well for snow, hail and mixed precipitation.
The bright colours indicating heavy rain on an app are the output of that conversion rather than a direct measurement of what is falling.
Debris signatures
Dual polarisation detecting lofted debris.
Which confirms a tornado is on the ground.
Ground clutter and anomalies
Buildings, birds and atmospheric conditions producing false returns.
Which processing removes imperfectly.
Coverage gaps
Mountains blocking and beams overshooting at distance.
Reading radar apps
Useful for nowcasting and not a substitute for warnings.
Why apps and official warnings sometimes disagree
Consumer apps display raw or lightly processed radar; warnings are issued by meteorologists interpreting radar alongside observations, models and reports.
Which is why a warning can be issued when the app looks clear, and why a red blob on an app is not a warning.
Radar shows what is happening in the atmosphere at beam height, which over distance can be well above the ground.
Nowcasting
Short-range prediction from radar movement.
Which is what rain-in-the-next-hour features use.
Hail detection
Dual polarisation distinguishing hail from heavy rain.
Which improved warnings substantially.
Network gaps
Terrain and spacing leaving areas poorly covered.
Using radar well
For timing and movement rather than for decisions about severity.
Why understanding the system matters
Weather warnings are the output of a large, expensive, internationally coordinated system that most people interact with only through an app icon. It is one of the clearest examples anywhere of publicly funded science producing something everyone uses daily without noticing.
The practical value of understanding it is in reading warnings correctly. A cone is not a map of the storm, a watch is not a warning, a hundred-year flood is not once a century, and a radar app is not a warning service. Each of those misunderstandings has cost lives, and each is corrected in a sentence.
Where the authoritative information is
National meteorological services issue the warnings, publish the forecasts, explain the products and archive the data, all freely. Local emergency management agencies publish evacuation zones, shelter locations and preparedness guidance specific to the hazards where you live.
Those two sources answer almost every question anyone has about weather risk, and both are considerably more reliable than aggregated coverage, which frequently reproduces the misunderstandings described above.
A general note
Warning terminology, alert thresholds and emergency procedures differ between countries and between agencies. Anything with safety consequences should be checked against the service responsible for the area you are actually in.
How the warning system is actually organised
National meteorological services observe, model and forecast; they issue watches, warnings and advisories against defined criteria; and local emergency management agencies decide what action to take in response, including evacuations, shelter openings and road closures.
Those are two different organisations with two different jobs, which is why a national forecast office issues a hurricane warning and a county decides who evacuates. Knowing which one to follow for which question saves a great deal of confusion during an event.
What is worth doing before a season starts
Find out which hazards actually affect where you live, check whether your address sits in a flood or surge zone, identify where you would shelter, confirm that emergency alerts are enabled on your phone, and know how you would get information if the power and the network both failed.
None of that takes long, all of it is free, and every element of it becomes considerably harder to arrange once a warning has been issued. Emergency managers say the same thing after every major event: the households that coped were the ones who had decided in advance.
Where the authoritative information is
National meteorological services issue the warnings, publish the forecasts, explain what each product means and archive the underlying data, all freely. Local emergency management agencies publish evacuation zones, shelter locations and preparedness guidance specific to the hazards where you actually live.
Between them those two sources answer almost every question anyone has about weather risk, and both are considerably more reliable than aggregated coverage, which routinely reproduces the misunderstandings that cause harm.
A general note
Warning terminology, alert thresholds and emergency procedures differ between countries and between agencies, and they are revised as understanding improves. Anything with safety consequences should be checked against the service responsible for the area you are in rather than against a general description.