Google Maps has had the real-time weather layer available since this past August, which gives Maps users a new way to check the current weather as well as see the forecast for the next few days. The layer also has a feature that toggles current cloud formations above earth, too, so you'll know if you can leave the shades at home.
Starting today, the weather layer will now be available to the Google Maps API so developers can play with the weather (too bad "play with the weather" there isn't in any kind of X-Men sense) when they're building up their own maps. Developers can find the Maps API in the "weather" library as two different classes: "WeatherLayer" and "CloudLayer." I'm sure you can imagine the function of each.
The weather layer has different functions to accomodate for what part of the world you reside in, so you can see the temperature in fahrenheit or celsius, see the wind speeds in miles or kilometers, and so on. Fortunately, we all measure cloudiness in the same way so at least that feature is standard.
Google's being greedy about being able to embed the map with the weather layer so you can navigate around and see how it works, so below is just a screenshot. If you want to see the weather layer in action, you'll have to go to Google Maps or go tinker with the example that was provided on the Geo Developers Blog.