This report describes the typical weather at the Phoenix Sky Harbor International Airport (Phoenix, Arizona, United States) weather station over the course of an average September. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
Phoenix, Arizona has a hot desert climate. The area within 25 mi of this station is covered by shrublands (85%), croplands (5%), forests (4%), built-up areas (3%), and grasslands (2%).
The month of September is characterized by falling daily high temperatures, with daily highs decreasing from 102°F to 95°F over the course of the month, exceeding 108°F or dropping below 87°F only one day in ten.
Over the course of September, the length of the day is gradually decreasing. From the start to the end of the month, the length of the day decreases by 58 minutes, implying an average day-over-day decrease of 1.9 minutes.
The shortest day of the month is September 30 with 11:51 hours of daylight; the longest day is September 1 with 12:50 hours of daylight.
The earliest sunrise is at 6:03am on September 1; the latest sunset is at 6:52pm on September 1; the latest sunrise is at 6:22am on September 30; the earliest sunset is at 6:14pm on September 30.
Daylight savings time (DST) is not observed in 2012.
The median cloud cover ranges from 19% (mostly clear) to 32% (mostly clear). The trend is for the sky to be clearing with the median cloud cover decreasing by 14% from the start to the end of the month.
On September 30, the clearest day of the month, the sky is clear, mostly clear, or partly cloudy 78% of the time, and overcast or mostly cloudy 19% of the time.
On September 1, the cloudiest day of the month, the sky is overcast, mostly cloudy, or partly cloudy 45% of the time, and clear or mostly clear 51% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around September 1, occurring in 24% of days. Precipitation is least likely around September 30, occurring in 15% of days.
Throughout September, the most common forms of precipitation are thunderstorms, light rain, and moderate rain.
Thunderstorms are the most severe precipitation observed during 65% of those days with precipitation. They are most likely around September 1, when it is observed during 16% of all days.
Light rain is the most severe precipitation observed during 24% of those days with precipitation. It is most likely around September 1, when it is observed during 5% of all days.
Moderate rain is the most severe precipitation observed during 11% of those days with precipitation. It is most likely around September 1, when it is observed during 2% of all days.
Either snow is exceptionally unlikely to fall at this location in September or this station does not reliably report precipitation types.
The relative humidity typically ranges from 18% (dry) to 52% (mildly humid) over the course of a typical September, rarely dropping below 9% (very dry), or exceeding 76% (humid).
The air is driest around September 29, at which time the relative humidity drops below 22% (dry) three days out of four; it is most humid around September 1, rising above 40% (comfortable) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a typical September, the dew point typically varies from 40°F (dry) to 62°F (mildy humid) and is rarely below 26°F (dry) or above 71°F (muggy).
The time of the month between September 1 and September 30 is the most comfortable, with dew points that are neither too dry nor too muggy.
Over the course of September typical wind speeds vary from 0 mph to 13 mph (calm to moderate breeze), rarely exceeding 20 mph (fresh breeze).
The highest average wind speed of 7 mph (light breeze) occurs around September 1, at which time the average daily maximum wind speed is 13 mph (moderate breeze).
The lowest average wind speed of 6 mph (light breeze) occurs around September 30, at which time the average daily maximum wind speed is 11 mph (gentle breeze).