What is this falling from the sky, is it rain?
Clarity comes in degrees. At Weather Underground we had received some user feedback that when it was snowing, there was some confusion in exactly what our forecast was saying would happen. Internally we estimate how much precipitation may come down, but those estimates (QPF, Quantitative Precipitation Forecasts) are all based on liquid precipitation and our UI expressed everything based on those forecasts. The issue was that we could say in words that there may be 2-4 inches of snow, but the numerical representation would still show the amount of liquid which might be .25" which gave the appearance of contradictory information. Snow is a challenge because there is not a good automated way to measure depth in the way rain can be. The National Weather Service handles it by manually measuring snowfall at it's locations, but those locations are too far apart for the granularity we need to work at.
After some discussions with our meteorological colleagues we came up with a reasonable system for estimating snow fall and accumulation throughout the day which is tremendously helpful compared to the daily accumulation totals normally used. The next challenge was integrating the new information into the UI in a way that clarified what was happening instead of having the new information add confusion.
The approach we took was a conditional hierarchy. If there was a chance of snow, that would override the area normally used to display rain in the UI. This allowed us to weave the information in when it was needed without requiring a separate area dedicated to a condition that for many places was useful for only small parts of the year, if at all. From our testing we found that in the case of snow, any amount of snow was more important to most people than any amount of rain, so the hierarchy matched up to people's expectations. The end result was the interface change was very small, and the additional information was very useful.