Factoring in the wind chill, the temperature must have been below minus 30 degrees.
Literal
Wind-chill-index [object-を] consideration [target-に] include-[if-と], temperature [topic-は] below-zero thirty-degrees below was [certainty-に違いない].
風速冷却指数 is 'wind chill index.' 考慮に入れると means 'if you take into account.' ~に違いない expresses strong certainty: 'must have been.' A technical/weather sentence.