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Pieris rapae
I can't really recall the size, but it may be a Small White like a previous spotting of mine...but this one was bigger than my first one.
Meyer Ranch Opens Space Park, meadow area
I agree with bayucca about the ID of this butterfly.
Here is the bugguide.net page for the butterfly in question:
http://bugguide.net/node/view/3259
Pieris rapae are called Cabbage Whites or Cabbage Butterflies here in the USA. I think this one is female since it has two spots on each forewing.
Pieris brassicae, or the Large White, are not found in the US so that one can be ruled out.
Actually you can't separate the Small White and the Large White (Pieris brassicae, if this is found in US?) only by size. A large Small White might be larger than a small Large White ;-)...