September in Phoenix represents a pivotal shift in the desert climate, marking the end of the oppressive summer heat and the beginning of more comfortable conditions. While the city remains warm, the extreme temperatures that define July and August start to subside, offering residents and visitors a more inviting environment for outdoor activities. Understanding the average Phoenix temperature in September involves looking at historical data, daily variations, and the transition towards the milder fall season.
Typical Temperature Ranges
The average high temperature in Phoenix during September usually sits between 100°F (37.8°C) and 105°F (40.6°C) early in the month, gradually cooling down to the mid-90s°F (35°C) by the end of the month. Overnight lows are significantly more comfortable, averaging between 75°F (23.9°C) and 80°F (26.7°C) throughout the month. This diurnal temperature swing, where the difference between day and night can be 20°F (11°C) or more, is a defining characteristic of the desert climate and provides some relief even during the warmest days.
Daily Temperature Variations
Phoenix experiences significant temperature fluctuations within a single 24-hour period, especially in September. Mornings often begin pleasantly, with temperatures in the low 80s°F (27-28°C) before the day heats up rapidly. The afternoon peak typically occurs between 2:00 PM and 5:00 PM, reaching the upper 90s to low 100s°F. By sunset, temperatures drop quickly, making evening activities far more enjoyable than the midday conditions.
Historical Climate Data
Long-term records show that September averages in Phoenix have been gradually warming, consistent with broader climate trends. The overall monthly average temperature typically lands around 90°F (32.2°C). While days can still reach 110°F (43.3°C) or higher, these extreme occurrences are less frequent than in the peak summer months. The cooling trend as the month progresses is reliable, with the first half being noticeably hotter than the second.
Comparison to Other Months
Looking at the progression of the year, September offers a distinct improvement over the summer months. June, July, and August often see average highs of 104°F to 108°F (40-42.2°C), with frequent days exceeding 110°F (43.3°C). In contrast, September begins with highs near 105°F (40.6°C) and concludes with temperatures more akin to a comfortable spring day. This places it as the most transitional and increasingly pleasant month of the late summer season.
