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I'm in California, so I tend to think in American units (i.e., Fahrenheit), and I have those units selected in Meterologist. But sometimes I need to talk about the weather with friends in Europe, so it would be handy to have international conversions (i.e., Celsius) readily available.
Now, what I've done is to set up two versions of my local City, one in American units, one international. This works, but it occurs to me that it would be cool to make use of the menubar option key functionality to switch between two user-defined sets of units. In my case, I could have American units by default, but if I pressed the option key, the menus would switch to international units.
I think this suggestion is an excellent one. Alternatively you could allow users to select all of the units to display so that it could show C or F or both if desired.