Why is my campaign grayed out when I try selecting it to create a weather-based rule?

A campaign will show up grayed out when you already have it set up in another rule since only one setting per campaign is allowed as of now.

What will happen to the locations that are being targeted by my campaign but are not selected in the tool?

The tool will only work with the locations that are specified in the settings. If any of the locations already targeted/excluded in the campaign align with the new rules, the tool will take action accordingly.

For example, if you already have London targeted for the campaign, and a new setting is created with the location as “any city in the United Kingdom”; since London is in the scope of the rules, the tool will target/exclude or make changes based on the conditions.

However, if you also have New York targeted in the campaign, the tool will not make any changes to that location as it is not in the scope of the rules.

How does the tool behave when the condition is based on "any city in ...."?

If the condition is built on “any city in ....”, even if one of the cities of that location is matching the rules - the specified action will be carried out.

Which service is being used to fetch the weather data?

We use the weather data from OpenWeather API.

How often is the weather information fetched from the API?

We get forecast data twice a day, i.e., 7 AM UTC and 7 PM UTC, from the API. The data that was fetched the latest, is stored and used by us.

To clarify:

If you run the tool between 7 AM UTC and 7 PM UTC, it will analyze rules based on the forecast data received at 7 AM UTC.
If you run the tool between 7 PM UTC and the next day at 7 AM UTC, it will work with the forecast data received at 7 PM UTC from the previous day.

How accurate is the weather data?

OpenWeather claims that the error in the accuracy of temperate prediction is less than 2 degrees and the reliability is above 90%. You can read more about this here.

What does the "After N days" forecast period mean in terms of weather conditions?

The "After N days" forecast period refers to N days from today, excluding the current day. For Example: Suppose today is Monday. Selecting "After 2 days" means you're looking at Wednesday, which is two days from today.