Hello, I recently setup a PWS - it is a Ambient Weather WS-2902. I monitor the DNS traffic on my network, and noticed in the less then 24 hours DNS requests for rtupdate.wunderground.com has passed 60% of all DNS records on my network - so far at 18,936 hits.
RackSpace has a very logical best practices for DNS TTL here: https://support.rackspace.com/how-to/about-ttl-best-practices/
They recommend using a TTL of 24 hours and then lowering it to 5 minutes the day before any infrastructure changes.
Google.com uses a TLL of 5 minutes. Facebook.com also has a TTL of 5 minutes. Microsoft.com uses a TTL of 1 hour - which I personally think is the best of both worlds between caching and the desire to quickly make changes. Interestingly enough, facebook's API endpoints (api.facebook.com and graph.facebook.com and graph.instagram.com) also use a TTL of 1 hour - which makes a lot of since. The primary domains will be used when a user types it in on a browser, but the API endpoints are almost constantly being hit in the background and so benefit the most when it comes to cached DNS records. I think this same logic would easily apply to rtupdate.wunderground.com - as you can tell its being hit very often.
Thank you for your time and consideration