Google intern Joey Scarr had a busy first month at the search giant's Sydney office, home of much of the Google Maps API engineering team. Scarr added the weather layer, available on the Google Maps website, as an optional layer in the API. The resulting feature even gets you at some of the data, making it close to an official Google Weather API.
The weather layer adds current temperatures and conditions via map markers. There are a few settings, such as label color and units of measurement, but otherwise the layer does most of the work. Info windows with detailed forecasts can be disabled.
In addition to the weather layer, the announcement post also references a new cloud layer. Developers can activate cloud cover imagery for the furthest-out seven zoom levels.
We would love to see an official Google API for weather, but the company is likely not allowed to redistribute the data. It gets weather data via the Weather Channel API.