Usually, it is measured in terms of K-value. Storms are often K=4, K=5, or K>=6 (Major or Severe storm). 'The K index measures the deviation of the most disturbed horizontal component of the magnetic field.'
Here is a nice description of what K-value means -- in practical terms, you need a certain K-value to have a chance to see the Aurora at...
You have to admire the Aristotelian tightness of the logic: There is no such thing as a vegetable. Pizza, clearly, exists. Therefore, Pizza is not a Vegetable.
