This report describes the typical weather at the Yellowstone Lake (Yellowstone National Park, Wyoming, United States) weather station over the course of an average September. It is based on the historical records from 2001 to 2012. Earlier records are either unavailable or unreliable.
Yellowstone National Park, Wyoming has a humid subarctic continental climate with cool summers and no dry season. The area within 25 mi of this station is covered by forests (90%) and lakes and rivers (7%).
The month of September is characterized by rapidly falling daily high temperatures, with daily highs decreasing from 65°F to 54°F over the course of the month, exceeding 73°F or dropping below 41°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 1:28 hours, implying an average day-over-day decrease of 2.9 minutes.
The shortest day of the month is September 30 with 11:44 hours of daylight; the longest day is September 1 with 13:12 hours of daylight.
The earliest sunrise is at 6:45am on September 1; the latest sunset is at 7:57pm on September 1; the latest sunrise is at 7:19am on September 30; the earliest sunset is at 7:03pm on September 30.
Daylight saving time is observed in 2012 but it neither starts nor ends in September, so the entire month is in daylight time.
This weather station does not reliably report sky cover.
There is either very little precipitation at this location in September, or the weather station does not reliably report the conditions.
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 29% (dry) to 90% (very humid) over the course of a typical September, rarely dropping below 15% (dry) and reaching as high as 98% (very humid).
The air is driest around September 1, at which time the relative humidity drops below 35% (comfortable) three days out of four; it is most humid around September 6, rising above 86% (very 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 21°F (dry) to 41°F (dry) and is rarely below 13°F (dry) or above 47°F (dry).
Over the course of September typical wind speeds vary from 0 mph to 9 mph (calm to gentle breeze), rarely exceeding 12 mph (moderate breeze).
The highest average wind speed of 3 mph (light air) occurs around September 1, at which time the average daily maximum wind speed is 9 mph (gentle breeze).
The lowest average wind speed of 3 mph (light air) occurs around September 27, at which time the average daily maximum wind speed is 8 mph (gentle breeze).