On the iPhone, you will find the upgrades page by tapping on the icon in the top right of the location bar then tapping on Remove Ads. If you are on a version prior to 1.8, this page is found by swiping the bottom panels in the app to the left and tapping on the upgrades panel. On the iPad, simply tap on the icon in the top right of the location bar and tap on the menu item called upgrades. From that page, tap on the remove ads option.
After purchasing a subscription, we recommend that you create a Weather Underground ID if you don't have one, so that we have a backup of your subscription in our records. This way, you can carry the subscription to other Weather Underground apps or have the ability to restore the purchase if you ever change your device or need to reset your device.