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The level describes supporting evidence, not hail size or an accuracy percentage. C1: initial radar suspicion. C2: sufficient radar evidence; a matching warning or lightning jump can also raise C1 to C2. C2 is not yet verified ground evidence. C3: additionally a verified public ground report. In Germany, two independent reports from the German Weather Service's WarnWetter app count for this; nobody checks them individually. C4: on-site confirmations from four separate accounts on a C3 event. Every alert includes its reasoning.

The class describes hail size estimated from radar: class 2 = 20–29 mm, class 3 = 30–44 mm, class 4 = 45 mm and up. Class 1 is below 20 mm, below the hail alert threshold. This estimate does not prove hail on the ground or actual damage. Hail size and confidence level are separate; each alert explains its supporting evidence. In Germany, the DWD hail flag gives no millimetres; alerts there say “hail possible” or “large hail possible”.

A hailstorm is a living thing – and our assessment grows with the evidence. If an event is upgraded or gets stronger, you get an update. If the evidence no longer supports it, you get the all-clear: whoever already acted should also hear about the downgrade. Nothing is changed silently – every version of an event stays traceable.

From the tracked radar cells we compute the storm’s heading and speed and extrapolate the track a short way ahead. If your rule has a base location and the track is heading for it, the alert adds an arrival note with the estimated arrival time (ETA). Important: this is a hint assuming the track holds – not a promise. Storms can turn or fall apart.

The most frequent reasons, in this order: no notification rule is set up (no rule, no alerts). The thresholds are strict – at class 4 and C3, things get quiet. The affected country is not selected, or the track is outside the radius. No delivery channel is checked – then the alert only lands in the inbox. Or the subscription has expired. And: it may simply not have hailed – we would rather send one alert too few than one false alarm.

Open “Getting started” and follow the guide for your device. On iPhone or iPad (iOS/iPadOS 16.4 or later), first add HailSignal to the Home Screen using Safari and open its new icon. Then allow push notifications and select “Web-Push” in your alert rules. SMS is currently not available.

Yes – each account supports up to 20 rules, each with its own area, thresholds and channels, for example for several sites or work areas. Choose countries or US states as the alert area, or a radius of 50 to 300 km around the rule’s base location. The base location is also used for distance, rough travel time and the arrival note.

HailSignal is a web app – no app store needed. “Getting started” explains installation on iPhone, iPad and Android, notification permission and a test message. Afterwards, open HailSignal from its Home Screen icon.

Open the app & push guide →

From the radar and hail products of the weather services for Germany (DWD), Switzerland with border areas (MeteoSwiss) and the contiguous US (NOAA), complemented by official warnings (MeteoAlarm), satellite lightning data (EUMETSAT, NOAA) and ground reports. Every alert carries its sources and licenses – you always see what a statement is based on.

The map shows events for the selected dates and filters, which can include radar indications alone. Notifications also depend on approved sources and coverage, your rule, thresholds, channels and account status. Matching class and confidence filters therefore do not reproduce an exact delivery list. The inbox shows the alerts sent to your account.

Yes. Alerts in English automatically state hail size in inches with the US comparison object (“golf ball”) – you set the alert language in your account. The interface comes in six languages; your browser remembers the choice at the top right.

Both make the alerts better for everyone. “Confirm hail” in the inbox or on the map counts as evidence from the ground – four independent accounts lift an event to the highest level, C4. The feedback after an event (“Was this alert useful for your work?”) is the ground truth we calibrate our thresholds against – an honest no is as valuable as a yes.