This report describes the typical weather at the Porterville Municipal Airport (Porterville, California, United States) weather station over the course of an average September. It is based on the historical records from 1992 to 2012. Earlier records are either unavailable or unreliable.
Porterville, California has a cold semi-arid steppe climate. The area within 25 mi of this station is covered by grasslands (57%), forests (20%), croplands (16%), and shrublands (5%).
The month of September is characterized by falling daily high temperatures, with daily highs decreasing from 93°F to 84°F over the course of the month, exceeding 99°F or dropping below 75°F only one day in ten.
Over the course of September, the length of the day is decreasing. From the start to the end of the month, the length of the day decreases by 1:05 hours, implying an average day-over-day decrease of 2.2 minutes.
The shortest day of the month is September 30 with 11:49 hours of daylight; the longest day is September 1 with 12:55 hours of daylight.
The earliest sunrise is at 6:28am on September 1; the latest sunset is at 7:23pm on September 1; the latest sunrise is at 6:51am on September 30; the earliest sunset is at 6:40pm on September 30.
Daylight savings time is observed in 2012 but it neither starts nor ends in September, so the entire month is in daylight time.
The median cloud cover is 0% (clear) and does not vary substantially over the course of the month.
On September 15, a representative day, the sky is clear or mostly clear 79% of the time, partly cloudy 1% of the time, and mostly cloudy or overcast 4% of the time.
The average probability that some form of precipitation will be observed in a given day is 1%, with little variation over the course of the month.
Throughout September, the most common forms of precipitation are thunderstorms.
Thunderstorms are the most severe precipitation observed during 100% of those days with precipitation. They are most likely around September 25, 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 24% (dry) to 64% (mildly humid) over the course of a typical September, rarely dropping below 17% (dry), or exceeding 81% (humid).
The air is driest around September 2, at which time the relative humidity drops below 28% (dry) three days out of four; it is most humid around September 9, rising above 57% (mildly humid) 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 41°F (dry) to 56°F (comfortable) and is rarely below 34°F (dry) or above 61°F (comfortable).
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 10 mph (calm to gentle breeze), rarely exceeding 12 mph (moderate breeze).
The highest average wind speed of 5 mph (light breeze) occurs around September 1, at which time the average daily maximum wind speed is 10 mph (gentle breeze).
The lowest average wind speed of 5 mph (light breeze) occurs around September 28, at which time the average daily maximum wind speed is 9 mph (gentle breeze).