Hi greg.staunton, long time no speak!
It sounds like a great idea, I'd suggest using an api or data import into a CDO such as https://openweathermap.org/current, then linking the CDO to contacts with a city field.
In the campaign there could be a Compare Custom Object Fields or segment with Linked Contact in Custom Object to look for the contacts country in the CDO and filter on temperature being greater than a set value.
Nice idea John. How would you go about bringing in the data via Eloqua? Curious what that process would look like. Thank you!