CREATE TABLE table_148578_1 (altitude__km_ VARCHAR, yield__approximate_ VARCHAR)

What was the altitude of the yield of 1.4 megatons?
SELECT altitude__km_ FROM table_148578_1 WHERE yield__approximate_ = "1.4 megatons"
SELECT "altitude__km_" FROM "table_148578_1" WHERE "1.4 megatons" = "yield__approximate_"