Average Weather in December in Riversdale South Africa
Daily high temperatures increase by 3°F, from 77°F to 80°F, rarely falling below 69°F or exceeding 87°F.
Daily low temperatures increase by 3°F, from 59°F to 62°F, rarely falling below 53°F or exceeding 66°F.
For reference, on January 26, the hottest day of the year, temperatures in Riversdale typically range from 63°F to 81°F, while on July 12, the coldest day of the year, they range from 46°F to 66°F.
Average High and Low Temperature in December
The daily average high (red line) and low (blue line) temperature, with 25th to 75th and 10th to 90th percentile bands. The thin dotted lines are the corresponding average perceived temperatures.
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on December. 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.
The month of December in Riversdale experiences decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 30% to 21%.
The clearest day of the month is December 31, with clear, mostly clear, or partly cloudy conditions 79% of the time.
For reference, on May 11, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 42%, while on February 11, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 87%.
The percentage of time spent in each cloud cover band, categorized by the percentage of the sky covered by clouds.
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Riversdale, the chance of a wet day over the course of December is essentially constant, remaining around 11% throughout.
For reference, the year's highest daily chance of a wet day is 18% on April 8, and its lowest chance is 10% on December 21.
Probability of Precipitation in December
The percentage of days in which various types of precipitation are observed, excluding trace quantities: rain alone, snow alone, and mixed (both rain and snow fell in the same day).
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 December in Riversdale is essentially constant, remaining about 1.0 inches throughout, and rarely exceeding 3.0 inches.
Average Monthly Rainfall in December
The average rainfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average liquid-equivalent snowfall.
Over the course of December in Riversdale, the length of the day is essentially constant. The shortest day of the month is December 1, with 14 hours, 15 minutes of daylight and the longest day is December 21, with 14 hours, 26 minutes of daylight.
Hours of Daylight and Twilight in December
The number of hours during which the Sun is visible (black line). From bottom (most yellow) to top (most gray), the color bands indicate: full daylight, twilight (civil, nautical, and astronomical), and full night.
The earliest sunrise of the month in Riversdale is 5:16 AM on December 5 and the latest sunrise is 10 minutes later at 5:26 AM on December 31.
The earliest sunset is 7:31 PM on December 1 and the latest sunset is 18 minutes later at 7:49 PM on December 31.
Daylight saving time is not observed in Riversdale during 2018.
For reference, on December 21, the longest day of the year, the Sun rises at 5:19 AM and sets 14 hours, 26 minutes later, at 7:45 PM, while on June 21, the shortest day of the year, it rises at 7:40 AM and sets 9 hours, 53 minutes later, at 5:33 PM.
Sunrise & Sunset with Twilight in December
The solar day over the course of December. From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray.
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 in Riversdale is increasing during December, rising from 2% to 8% over the course of the month.
For reference, on February 11, the muggiest day of the year, there are muggy conditions 14% of the time, while on May 26, the least muggy day of the year, there are muggy conditions 0% of the time.
The percentage of time spent at various humidity comfort levels, categorized by dew point.
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 in Riversdale is essentially constant during December, remaining within 0.1 miles per hour of 11.1 miles per hour throughout.
For reference, on November 26, the windiest day of the year, the daily average wind speed is 11.2 miles per hour, while on April 27, the calmest day of the year, the daily average wind speed is 8.7 miles per hour.
Average Wind Speed in December
The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.
The hourly average wind direction in Riversdale throughout December is predominantly from the south, with a peak proportion of 43% on December 28.
Wind Direction in December
The percentage of hours in which the mean wind direction is from each of the four cardinal wind directions, excluding hours in which the mean wind speed is less than 1.0 mph. The lightly tinted areas at the boundaries are the percentage of hours spent in the implied intermediate directions (northeast, southeast, southwest, and northwest).
Riversdale is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average surface water temperature in Riversdale is gradually increasing during December, rising by 3°F, from 66°F to 69°F, over the course of the month.
Average Water Temperature in December
The daily average water temperature (purple line), with 25th to 75th and 10th to 90th percentile bands.
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).
Temperatures in Riversdale are sufficiently warm year round that it is not entirely meaningful to discuss the growing season in these terms. We nevertheless include the chart below as an illustration of the distribution of temperatures experienced throughout the year.
Time Spent in Various Temperature Bands and the Growing Season in December
The percentage of time spent in various temperature bands. The black line is the percentage chance that a given day is within 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.
The average accumulated growing degree days in Riversdale are rapidly increasing during December, increasing by 560°F, from 1,489°F to 2,048°F, over the course of the month.
Growing Degree Days in December
The average growing degree days accumulated over the course of December, with 25th to 75th and 10th to 90th percentile bands.
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 in Riversdale is essentially constant during December, remaining within 0.2 kWh of 8.3 kWh throughout.
Average Daily Incident Shortwave Solar Energy in December
The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.
For the purposes of this report, the geographical coordinates of Riversdale are -34.093 deg latitude, 21.257 deg longitude, and 486 ft elevation.
The topography within 2 miles of Riversdale contains significant variations in elevation, with a maximum elevation change of 509 feet and an average elevation above sea level of 469 feet. Within 10 miles contains significant variations in elevation (4,373 feet). Within 50 miles also contains extreme variations in elevation (7,592 feet).
The area within 2 miles of Riversdale is covered by cropland (73%), artificial surfaces (15%), and shrubs (11%), within 10 miles by cropland (73%) and shrubs (26%), and within 50 miles by shrubs (41%) and water (32%).
This report illustrates the typical weather in Riversdale 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
There is only a single weather station, Mossel Bay Cape Saint Blaize, in our network suitable to be used as a proxy for the historical temperature and dew point records of Riversdale.
At a distance of 83 kilometers from Riversdale, closer than our threshold of 150 kilometers, this station is deemed sufficiently nearby to be relied upon as our primary source for temperature and dew point records.
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.
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.