Weather targeting granularity for AdWords
For a USA-wide campaign in AdWords, your weather targeting will be at county-level granularity, rather than city level. This is to ensure that you remain within the 10,000 location limit that Google imposes for your campaign.
If you wanted to target down to city level granularity, then you would need to create 2 campaigns, and we have a guide on how to do this. Scroll right down to the bottom section titled 'TARGETING THE USA'
Weather targeting granularity for Facebook
For a Facebook campaign your targeting will be broken down to custom locations by WeatherAds with an average radius of around 40km. For areas with high urban density such as large cities, the radius points will be much more granular, and then for unpopulated rural areas, they may be slightly broader.
If you wish add weather targeting to a Facebook campaign targeting the USA, Canada or Australia, please read our guide on Weather Targeting Country-Wide Facebook Campaigns For USA, Canada and Australia before importing your campaign into WeatherAds.