Average Weather at Knoxville Iowa, United States
At Knoxville, the summers are warm, humid, and wet; the winters are freezing, dry, and windy; and it is partly cloudy year round. Over the course of the year, the temperature typically varies from 18°F to 87°F and is rarely below -2°F or above 96°F.
Based on the tourism score, the best time of year to visit Knoxville for warm-weather activities is from early June to mid September.
The hot season lasts for 3.8 months, from May 25 to September 18, with an average daily high temperature above 76°F. The hottest day of the year is July 20, with an average high of 87°F and low of 69°F.
The cold season lasts for 3.0 months, from November 28 to February 28, with an average daily high temperature below 44°F. The coldest day of the year is January 22, with an average low of 18°F and high of 34°F.
Average High and Low Temperature
The figure below shows you a compact characterization of the entire year of hourly average temperatures. The horizontal axis is the day of the year, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature
At Knoxville, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.
The clearer part of the year at Knoxville begins around June 9 and lasts for 4.5 months, ending around October 26. On August 26, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 70% of the time, and overcast or mostly cloudy 30% of the time.
The cloudier part of the year begins around October 26 and lasts for 7.4 months, ending around June 9. On February 12, the cloudiest day of the year, the sky is overcast or mostly cloudy 55% of the time, and clear, mostly clear, or partly cloudy 45% of the time.
Cloud Cover Categories
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days at Knoxville varies significantly throughout the year.
The wetter season lasts 5.6 months, from April 6 to September 26, with a greater than 28% chance of a given day being a wet day. The chance of a wet day peaks at 46% on June 7.
The drier season lasts 6.4 months, from September 26 to April 6. The smallest chance of a wet day is 10% on January 12.
Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation at Knoxville changes throughout the year.
Rain alone is the most common for 11 months, from February 2 to December 28. The highest chance of a day with rain alone is 46% on June 7.
Snow alone is the most common for 1.2 months, from December 28 to February 2. The highest chance of a day with snow alone is 5% on January 11.
Daily Chance of Precipitation
To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Knoxville experiences extreme seasonal variation in monthly rainfall.
The rainy period of the year lasts for 11 months, from February 6 to December 28, with a sliding 31-day rainfall of at least 0.5 inches. The most rain falls during the 31 days centered around June 16, with an average total accumulation of 4.6 inches.
The rainless period of the year lasts for 1.2 months, from December 28 to February 6. The least rain falls around January 20, with an average total accumulation of 0.3 inches.
Average Monthly Rainfall
We report snowfall in liquid-equivalent terms. The actual depth of new snowfall is typically between 5 and 10 times the liquid-equivalent amount, assuming the ground is frozen. Colder, drier snow tends to be on the higher end of that range and warmer, wetter snow on the lower end.
As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Knoxville experiences some seasonal variation in monthly liquid-equivalent snowfall.
The snowy period of the year lasts for 4.1 months, from November 14 to March 18, with a sliding 31-day liquid-equivalent snowfall of at least 0.1 inches. The most snow falls during the 31 days centered around December 25, with an average total liquid-equivalent accumulation of 0.4 inches.
The snowless period of the year lasts for 7.9 months, from March 18 to November 14. The least snow falls around July 19, with an average total liquid-equivalent accumulation of 0.0 inches.
Average Liquid-Equivalent Monthly Snowfall
The length of the day at Knoxville varies significantly over the course of the year. In 2017, the shortest day is December 21, with 9 hours, 12 minutes of daylight; the longest day is June 20, with 15 hours, 10 minutes of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:38 AM on June 14, and the latest sunrise is 2 hours, 9 minutes later at 7:47 AM on November 4. The earliest sunset is at 4:43 PM on December 7, and the latest sunset is 4 hours, 7 minutes later at 8:49 PM on June 27.
Daylight saving time (DST) is observed at Knoxville during 2017, starting in the spring on March 12, lasting 7.8 months, and ending in the fall on November 5.
Sunrise & Sunset with Twilight and Daylight Saving Time
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.
Knoxville experiences extreme seasonal variation in the perceived humidity.
The muggier period of the year lasts for 3.7 months, from May 28 to September 20, during which time the comfort level is muggy, oppressive, or miserable at least 14% of the time. The muggiest day of the year is July 21, with muggy conditions 57% of the time.
The least muggy day of the year is February 9, when muggy conditions are essentially unheard of.
Humidity Comfort Levels
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 Knoxville experiences significant seasonal variation over the course of the year.
The windier part of the year lasts for 7.7 months, from October 2 to May 25, with average wind speeds of more than 10.2 miles per hour. The windiest day of the year is April 1, with an average hourly wind speed of 12.6 miles per hour.
The calmer time of year lasts for 4.3 months, from May 25 to October 2. The calmest day of the year is August 3, with an average hourly wind speed of 7.9 miles per hour.
Average Wind Speed
The predominant average hourly wind direction at Knoxville varies throughout the year.
The wind is most often from the north for 3.4 months, from January 6 to April 18, with a peak percentage of 35% on March 8. The wind is most often from the south for 7.5 months, from April 18 to December 1, with a peak percentage of 44% on July 2. The wind is most often from the west for 1.2 months, from December 1 to January 6, with a peak percentage of 32% on January 1.
Best Time of Year to Visit
To characterize how pleasant the weather is at Knoxville throughout the year, we compute two travel scores.
The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Knoxville for general outdoor tourist activities is from early June to mid September, with a peak score in the last week of August.
The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Knoxville for hot-weather activities is from late June to late August, with a peak score in the third week of July.
For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.
Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.
Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.
Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.
Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.
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 Knoxville typically lasts for 6.3 months (192 days), from around April 12 to around October 20, rarely starting before March 25 or after April 30, and rarely ending before October 1 or after November 11.
Time Spent in Various Temperature Bands and the Growing Season
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.
Based on growing degree days alone, the first spring blooms at Knoxville should appear around April 3, only rarely appearing before March 21 or after April 21.
Growing Degree Days
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 experiences extreme seasonal variation over the course of the year.
The brighter period of the year lasts for 3.7 months, from May 2 to August 24, with an average daily incident shortwave energy per square meter above 5.9 kWh. The brightest day of the year is July 3, with an average of 7.0 kWh.
The darker period of the year lasts for 3.3 months, from November 2 to February 10, with an average daily incident shortwave energy per square meter below 2.9 kWh. The darkest day of the year is December 21, with an average of 1.8 kWh.
Average Daily Incident Shortwave Solar Energy
For the purposes of this report, the geographical coordinates of Knoxville are 41.300 deg latitude, -93.117 deg longitude, and 850 ft elevation.
The topography within 2 miles of Knoxville contains only modest variations in elevation, with a maximum elevation change of 187 feet and an average elevation above sea level of 865 feet. Within 10 miles also contains only modest variations in elevation (302 feet). Within 50 miles contains only modest variations in elevation (623 feet).
The area within 2 miles of Knoxville is covered by cropland (88%) and artificial surfaces (10%), within 10 miles by cropland (81%) and trees (11%), and within 50 miles by cropland (86%).
This report illustrates the typical weather at Knoxville, 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
Knoxville 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 Pella Municipal Airport, Chariton Municipal Airport, Newton Municipal Airport, Des Moines International Airport, Grinnell Regional Airport, Ankeny Regional Airport, Osceola Municipal Airport, and Oskaloosa Municipal Airport.
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.