As such meteorologists in India and especially in TN never refer the word 'AGNI NATCHATHIRAM'. Perhaps ancient Tamil people in their climatology, might have classified this period [in the month of May]as hot summer period and named it as 'AGNI Natchathiram. There has been no reference in meteorological glossary, since hottest days may extend in June too.
But the wisdom of the ancient Tamils has to be considered.
Posted by Mr. Ponnachi G Son