Siri has the ability to check the weather for you with results provided by Yahoo.com. It's great for a quick update of the current temperature, or if it's going to rain tomorrow, but it doesn't provide much more than that. Luckily, Siri can also pull search results from WolframAlpha's immense "Computational Knowledge Engine". The ways you can use WolframAlpha are endless, but it's weather information is some of the best out there, all you have to do is know how to ask.
How to do it
The trick to getting results from WolframAlpha is to enable Siri and say "Wolfram" before your question. For the weather say something like "Wolfram, weather forecast for Portland Oregon". Siri will then pull up a long sheet full of useful information.
It has your typical temperature and wind speed, but also adds relative humidity, dew point and even a quick description of the cloud cover. Where it really shines though is in the presentation of the forecast data itself. The weather history and forecast for the last three days and future three days are presented in a neat graph showing the fluctuating temperature and expected cloud cover throughout the day. Also precipitation rate if it's currently raining. Further down you'll find historical temperatures for the location you asked Siri about. A search for Portland, Oregon displayed recorded high and low temperatures as far back as 1975. Additionally it provides the lowest and highest temp for the current day in history, as well as the average high and average low.
There are a few more interesting pieces of information here like sun azimuth, station elevation and if the sun is above the horizon or not. It is an incredible amount of information that you don't even need to download another weather app for.