Chance of rain and accumulated millimeters: they are not the same thing

“70% chance of rain” and “30 mm expected” may sound like two different ways of saying that it is going to rain. They are not. One describes the likelihood of precipitation occurring. The other describes how much water could fall. For anyone who needs to make decisions in the field, that difference matters far more than simply knowing whether the forecast icon shows a rain cloud.

What chance of rain means

The probability of precipitation indicates how likely it is that rain will occur at a given location and during a given period. An 80% chance does not mean it will rain for 80% of the day. Nor does it necessarily mean that a large amount of rain will fall. There can be a high chance of rain while the expected accumulation remains low. The opposite can also occur: there may be less certainty about the exact timing, but a potentially significant amount of precipitation if the event does occur. This is why looking only at the percentage can lead to an incomplete conclusion.

Millimeters answer a different question

Precipitation measured in millimeters estimates the accumulated amount of water. As a reference, 1 mm of rain is approximately equivalent to 1 liter of water per square meter. So, if the forecast estimates 20 mm, the relevant information is not only that “it may rain,” but also the volume of water expected during that period. In the field, that difference can completely change how a forecast is interpreted. A high chance of light rain is not the same as an event expected to produce significant accumulation over several hours.

The accumulation period matters too

Whenever accumulated precipitation is discussed, there is always a time window behind it. Ten millimeters in one hour and 10 mm spread throughout an entire day represent the same total accumulation, but not necessarily the same situation. That is why it is important to understand what an alert is measuring. In Vienor, rain rules use accumulated precipitation within the configured monitoring window. If you set a threshold of 20 mm, for example, the system adds up the forecast precipitation within that window and generates an alert when the total reaches or exceeds that value. It does not wait for a single hour to show 20 mm.

So, what is the probability useful for?

It is still useful information. Vienor also retrieves the probability of precipitation from the forecast and can include it as context in an alert. But the probability alone does not determine whether a rain rule is triggered. The main condition is the accumulated amount. This makes it possible to configure an alert around a more concrete question:

Is enough rain expected over the next few hours that I would want to know about it?

The threshold depends on the decision

There is no single number of millimeters that matters to everyone. The same accumulation may be irrelevant to one task and decisive for another. Its importance can also vary depending on the soil, crop, stage of the season, and the work planned for that field. This is why Vienor does not set one rain value for every user. Each rule can have its own threshold. You may want an alert when forecast accumulation exceeds 10 mm in one field and use a different criterion for another field. The tool monitors the condition you define.

Rain and spraying should not be considered separately either

Precipitation matters for more than just the water that reaches the soil. It can also affect operational decisions. One of Vienor's specialized features is spraying windows, which analyze weather conditions hour by hour to indicate whether conditions are favorable, marginal, or unfavorable for an application. The variables considered include wind, humidity, and forecast rain. This shows why an isolated weather value often says very little on its own. A forecast of 5 mm may seem insignificant. But if those 5 mm are expected only a few hours after a planned application, they take on a different importance.

An alert does not try to replace the forecast

Vienor does not create its own weather forecast. For alerts, it uses Open-Meteo data and periodically evaluates that data against the rules configured for each field. The difference lies in the automation. Instead of opening an app several times throughout the day to check how the expected accumulation has changed, you can define in advance how much rain requires your attention. Vienor reevaluates those conditions every 30 minutes. If the forecast changes and an accumulation exceeding your threshold appears within the configured window, Vienor can generate an alert and send it by push notification or email.

Forecasts and alerts serve different purposes

A forecast shows you information. An alert looks for a condition within that information. For rain, separating probability from accumulated amount helps you configure much more useful rules. Because “it may rain” does not necessarily answer the question that matters. That question may be:

How much rain is expected, and when does that accumulation become relevant to what I need to do?

That is the kind of condition you can monitor automatically with Vienor.

Configure your own rain alerts

In Vienor, you can define a precipitation threshold for each field and choose the time window you want to monitor. The system automatically evaluates the forecast every 30 minutes and can analyze weather conditions within an hourly forecast horizon of up to 3 days. When the forecast accumulation reaches the value you configured, Vienor can notify you by push notification or email.

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