How to make the iOS 12 lock screen weather widget appear

The lockscreen weather widget.

First, you'll want to ensure the Weather app has constant access to your location. Open Settings, select Privacy, then Location Services. Scroll down to Weather, and make sure "Always" is enabled.

With that done, you'll have to set up a Do Not Disturb schedule. This quiets incoming calls and notifications, with the option to enforce the setting strictly or only while a device is locked. The latter can be useful if you go out to late-night concerts, say, or have your phone on as a baby monitor. You can make exceptions for favorite contacts to avoid missing emergency calls.

If you don't already have a DND schedule, there's a menu under the top level of the Settings app. Pick a stretch when you know you'll be fast asleep — you don't want to accidentally miss notifications when you're awake, or worse, get woken up by that 3 a.m. turn in Words With Friends.

Before leaving the DND menu, toggle on Bedtime mode. This is the option that makes the widget appear.

On waking up the next day, a weather summary should appear front and center when you pick up your iPhone, including a brief text forecast and current conditions. That's the only time the widget will show up, sadly — maybe things will be different in iOS 13.