Skyfire ratings are all based on statistics. The method it uses calculates where the light has the best chance of being great based on the inputs it's given and the rules we have defined in the code. It is designed to show higher ratings when the clouds are forecast to be in good configurations for great light and to show lower ratings when skies are fully overcast and are not expecting any color during sunset.
The results are overlaid onto maps to provide guidance on the best locations. It is important to know that the rating at a location is for the clouds directly overhead. So use this to plan your shoot; if you know an area in the distance will be lighting up and you can see it from your location, you should be ok.
Rating of 1
A rating of 1 means that it's likely going to be a grey and overcast sunset. Perhaps time to break out that neutral density filter and try some long exposures or focus on B&W pictures, or use city lights as a nice contrast at blue hour.
Rating of 2
A rating of 2 means that there are no clouds forecast, so it's going to be a blue sky evening. On these days you can focus on the golden hour light right after sunrise and before sunset. It will be full of rich warm tones and provide long dark shadows. Then after the sun goes down the Earth Shadow and Band of Venus can provide for some excellent color near the horizon. This is the band of scattered sunlight in the atmosphere with the darkness underneath the shadow of the Earth blocking the light.
Rating of 50%
A rating of 50% is a "could go either way" sort of situation. We will continue to add clarity and refine the model so that this won't be necessary long term.
Rating of 60%
This is where the fun starts to happen, photography wise. At ratings of 60% and up the chance of great light starts getting progressively better.
Rating of 80%
The sky is starting to look even better, with better conditions for more spectacular light. I would definitely be out shooting.
Rating of 90%
This is when the sky is showing great potential based on the models. A lot of these events are along the edges of storms where it can be clear along the horizon and cloudy overhead. This situation can lead to some of the best and most colorful sunsets typically known as "gap light".