It does.
It’s natural but short lived. In addition to thunderstorms here are two more examples
1) Beta decay for certain radioactive isotopes naturally occurring on Earth produce positrons.
2) In cosmic ray showers hitting Earth’s upper atmosphere, both muons and antimuons are produced about equally. Each antimuon decays into a positron, a muon-type antineutrino and an electron-type neutrino. There’s even muonium, an exotic atom made up of an antimuon and an electron, which was discovered in 1960 and is given the chemical symbol Mu.
http://en.wikipedia.org/wiki/Muonium