Average Weather in September at RAF Northolt United Kingdom
Daily high temperatures decrease by 6°F, from 69°F to 63°F, rarely falling below 57°F or exceeding 78°F.
Daily low temperatures decrease by 5°F, from 53°F to 49°F, rarely falling below 41°F or exceeding 60°F.
For reference, on August 1, the hottest day of the year, temperatures at RAF Northolt typically range from 56°F to 73°F, while on February 7, the coldest day of the year, they range from 35°F to 46°F.
Average High and Low Temperature in September
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on September. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature in September
The month of September at RAF Northolt experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 47% to 54%.
The clearest day of the month is September 2, with clear, mostly clear, or partly cloudy conditions 53% of the time.
For reference, on December 27, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 74%, while on July 15, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 58%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. At RAF Northolt, the chance of a wet day over the course of September is increasing, starting the month at 21% and ending it at 26%.
For reference, the year's highest daily chance of a wet day is 32% on December 30, and its lowest chance is 20% on September 11.
Probability of Precipitation in September
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during September at RAF Northolt is gradually increasing, starting the month at 1.5 inches, when it rarely exceeds 2.8 inches or falls below 0.4 inches, and ending the month at 1.9 inches, when it rarely exceeds 3.5 inches or falls below 0.5 inches.
Average Monthly Rainfall in September
Over the course of September at RAF Northolt, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1 hour, 53 minutes, implying an average daily decrease of 3 minutes, 54 seconds, and weekly decrease of 27 minutes, 16 seconds.
The shortest day of the month is September 30, with 11 hours, 41 minutes of daylight and the longest day is September 1, with 13 hours, 34 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month at RAF Northolt is 6:13 AM on September 1 and the latest sunrise is 47 minutes later at 7:00 AM on September 30.
The latest sunset is 7:48 PM on September 1 and the earliest sunset is 1 hour, 6 minutes earlier at 6:41 PM on September 30.
Daylight saving time is observed at RAF Northolt during 2018, but it neither starts nor ends during September, so the entire month is in daylight saving time.
For reference, on June 21, the longest day of the year, the Sun rises at 4:43 AM and sets 16 hours, 39 minutes later, at 9:22 PM, while on December 21, the shortest day of the year, it rises at 8:05 AM and sets 7 hours, 49 minutes later, at 3:54 PM.
Sunrise & Sunset with Twilight and Daylight Saving Time in September
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The chance that a given day will be muggy at RAF Northolt is essentially constant during September, remaining around 0% throughout.
For reference, on July 31, the muggiest day of the year, there are muggy conditions 1% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in September
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed at RAF Northolt is gradually increasing during September, increasing from 10.3 miles per hour to 10.8 miles per hour over the course of the month.
For reference, on January 3, the windiest day of the year, the daily average wind speed is 13.2 miles per hour, while on August 3, the calmest day of the year, the daily average wind speed is 9.6 miles per hour.
Average Wind Speed in September
Wind Direction in September
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
The growing season at RAF Northolt typically lasts for 6.7 months (205 days), from around April 12 to around November 3, rarely starting before March 15 or after May 4, and rarely ending before October 13 or after December 2.
The month of September at RAF Northolt is reliably fully within the growing season.
Time Spent in Various Temperature Bands and the Growing Season in September
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days at RAF Northolt are increasing during September, increasing by 266°F, from 1,483°F to 1,749°F, over the course of the month.
Growing Degree Days in September
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy at RAF Northolt is rapidly decreasing during September, falling by 1.6 kWh, from 4.6 kWh to 3.0 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in September
For the purposes of this report, the geographical coordinates of RAF Northolt are 51.553 deg latitude, -0.418 deg longitude, and 138 ft elevation.
The topography within 2 miles of RAF Northolt contains only modest variations in elevation, with a maximum elevation change of 128 feet and an average elevation above sea level of 132 feet. Within 10 miles contains only modest variations in elevation (531 feet). Within 50 miles contains significant variations in elevation (1,017 feet).
The area within 2 miles of RAF Northolt is covered by artificial surfaces (79%) and grassland (21%), within 10 miles by artificial surfaces (67%) and grassland (19%), and within 50 miles by cropland (44%) and grassland (25%).
This report illustrates the typical weather at RAF Northolt year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
RAF Northolt has a weather station that reported reliably enough during the analysis period that we have included it in our network. When available, historical temperature and dew point measurements are taken directly from this weather station. These records are obtained from NOAA's Integrated Surface Hourly data set, falling back on ICAO METAR records as required.
In the case of missing or erroneous measurements from this station, we fall back on records from nearby stations, adjusted according to typical seasonal and diurnal intra-station differences. For a given day of the year and hour of the day, the fallback station is selected to minimize the prediction error over the years for which there are measurements for both stations.
The stations on which we may fall back include but are not limited to London Heathrow Airport, London Weather Centre, Luton Airport, Bracknell / Beaufort Park, Farnborough Airport, London Gatwick Airport, London City Airport, and RAF Odiham.
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.