December Weather in Terceira Island Portugal

Daily high temperatures are around 63°F, rarely falling below 58°F or exceeding 68°F.

Daily low temperatures are around 56°F, rarely falling below 49°F or exceeding 62°F.

For reference, on August 12, the hottest day of the year, temperatures in Terceira Island typically range from 67°F to 78°F, while on February 13, the coldest day of the year, they range from 54°F to 62°F.

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.

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The month of December in Terceira Island experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 63% throughout the month. The highest chance of overcast or mostly cloudy conditions is 63% on December 16.

The clearest day of the month is December 31, with clear, mostly clear, or partly cloudy conditions 38% of the time.

For reference, on December 16, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 63%, while on July 18, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 75%.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Terceira Island, the chance of a wet day over the course of December is essentially constant, remaining around 34% throughout.

For reference, the year's highest daily chance of a wet day is 36% on December 8, and its lowest chance is 5% on July 11.

Rainfall

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 Terceira Island is gradually decreasing, starting the month at 3.3 inches, when it rarely exceeds 5.5 inches or falls below 1.0 inches, and ending the month at 2.9 inches, when it rarely exceeds 5.5 inches or falls below 0.7 inches.

The highest average 31-day accumulation is 3.4 inches on December 11.

Over the course of December in Terceira Island, the length of the day is essentially constant. The shortest day of the month is December 21, with 9 hours, 27 minutes of daylight and the longest day is December 1, with 9 hours, 40 minutes of daylight.

The earliest sunrise of the month in Terceira Island is 7:48 AM on December 1 and the latest sunrise is 19 minutes later at 8:06 AM on December 31.

The earliest sunset is 5:27 PM on December 7 and the latest sunset is 10 minutes later at 5:37 PM on December 31.

Daylight saving time is observed in Terceira Island during 2025, but it neither starts nor ends during December, so the entire month is in standard time.

For reference, on June 21, the longest day of the year, the Sun rises at 6:24 AM and sets 14 hours, 53 minutes later, at 9:17 PM, while on December 21, the shortest day of the year, it rises at 8:03 AM and sets 9 hours, 27 minutes later, at 5:30 PM.

The figure below presents a compact representation of the sun's elevation (the angle of the sun above the horizon) and azimuth (its compass bearing) for every hour of every day in the reporting period. The horizontal axis is the day of the year and the vertical axis is the hour of the day. For a given day and hour of that day, the background color indicates the azimuth of the sun at that moment. The black isolines are contours of constant solar elevation.

The figure below presents a compact representation of key lunar data for December 2025. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases. The label associated with each bar indicates the date and time that the phase is obtained, and the companion time labels indicate the rise and set times of the Moon for the nearest time interval in which the moon is above the horizon.

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 Terceira Island is essentially constant during December, remaining around 0% throughout.

For reference, on August 13, the muggiest day of the year, there are muggy conditions 51% of the time, while on March 4, the least muggy day of the year, there are muggy conditions 0% of the time.

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 Terceira Island is increasing during December, increasing from 17.4 miles per hour to 18.5 miles per hour over the course of the month.

For reference, on December 27, the windiest day of the year, the daily average wind speed is 18.7 miles per hour, while on July 17, the calmest day of the year, the daily average wind speed is 10.0 miles per hour.

The highest daily average wind speed during December is 18.7 miles per hour on December 27.

The wind direction in Terceira Island during December is predominantly out of the south from December 1 to December 23 and the west from December 23 to December 31.

Terceira Island 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 Terceira Island is gradually decreasing during December, falling by 2°F, from 64°F to 62°F, over the course of the month.

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 Terceira Island 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.

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 Terceira Island are increasing during December, increasing by 288°F, from 4,716°F to 5,004°F, over the course of the month.

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 Terceira Island is essentially constant during December, remaining within 0.1 kWh of 1.9 kWh throughout.

The lowest average daily incident shortwave solar energy during December is 1.8 kWh on December 16.

For the purposes of this report, the geographical coordinates of Terceira Island are 38.717 deg latitude, -27.217 deg longitude, and 190 ft elevation.

The topography within 2 miles of Terceira Island contains very significant variations in elevation, with a maximum elevation change of 1,260 feet and an average elevation above sea level of 1,758 feet. Within 10 miles contains very significant variations in elevation (3,363 feet). Within 50 miles also contains extreme variations in elevation (3,409 feet).

The area within 2 miles of Terceira Island is covered by grassland (100%), within 10 miles by water (55%) and grassland (32%), and within 50 miles by water (98%).