Question: when will the weather start cooling down in arizona?
Answer: National Weather Forecast - Phoenix Info The heat tends to "break" - cool evenings, days in the 80s - about mid-October. The coldest part of the year is usually late December or early January, with highs in the 60s, lows in the 40s. A rare cold streak would be a high in the 50s, low in the upper 20s.