Nice observation! As a matter of fact, most masculine nouns ending in vowels other than आ and consonants show no changes for their plural form (which is understood from the context), while most female forms have a distinct plural form.
That is to count to figure out how far away the lightening is. The correct plural of Mississippi would be Mississippis but for this counting you only use the singular. Some say to count, one potato, two potato, etc.