Fall Weather at Attu Heliport Greenland

Daily high temperatures decrease by 21°F, from 45°F to 24°F, rarely falling below 13°F or exceeding 50°F.

Daily low temperatures decrease by 23°F, from 39°F to 16°F, rarely falling below 5°F or exceeding 42°F.

For reference, on July 21, the hottest day of the year, temperatures at Attu Heliport typically range from 40°F to 49°F, while on February 26, the coldest day of the year, they range from -1°F to 8°F.

The figure below shows you a compact characterization of the hourly average fall temperatures. 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 fall at Attu Heliport experiences very rapidly increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 62% to 79%. The lowest chance of overcast or mostly cloudy conditions is 59% on September 22.

The clearest day of the fall is September 22, with clear, mostly clear, or partly cloudy conditions 41% of the time.

For reference, on January 19, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 86%, while on May 27, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 42%.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. At Attu Heliport, the chance of a wet day over the course of the fall is rapidly decreasing, starting the season at 18% and ending it at 11%.

For reference, the year's highest daily chance of a wet day is 23% on August 9, and its lowest chance is 4% on January 6.

Over the course of the fall at Attu Heliport, the chance of a day with only rain decreases from 18% to 3%, the chance of a day with mixed snow and rain increases from 0% to 2%, and the chance of a day with only snow increases from 0% to 6%.

Rainfall

To show variation within the season and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day rainfall during the fall at Attu Heliport is rapidly decreasing, starting the season at 1.3 inches, when it rarely exceeds 2.6 inches or falls below 0.4 inches, and ending the season at 0.2 inches, when it rarely exceeds 0.7 inches or falls below -0.0 inches.

Snowfall

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day snowfall during the fall at Attu Heliport is increasing, starting the season at 0.0 inches, when it rarely exceeds 0.0 inches or falls below -0.0 inches, and ending the season at 2.9 inches, when it rarely exceeds 6.5 inches or falls below 0.3 inches.

The highest average 31-day accumulation is 3.5 inches on November 14.

Due to its extreme latitude, Attu Heliport experiences polar day (also known as the midnight Sun) during the summer and polar night during the winter. These are periods of time in which the sun is continuously above or below the horizon for more than one day. The precise start and end dates of polar day and night vary from year to year and depend on the precise location and elevation of the observer, and the local topography.

Neither polar day nor polar night occur during the fall.

Over the course of the fall at Attu Heliport, the length of the day is very rapidly decreasing. From the start to the end of the season, the length of the day decreases by 12 hours, 27 minutes, implying an average daily decrease of 8 minutes, 18 seconds, and weekly decrease of 58 minutes, 5 seconds.

The shortest day of the fall is November 30, with 2 hours, 33 minutes of daylight and the longest day is September 1, with 15 hours, 0 minutes of daylight.

The earliest sunrise of the fall at Attu Heliport is 7:03 AM on September 1 and the latest sunrise is 5 hours, 3 minutes later at 12:06 PM on November 30.

The latest sunset is 10:03 PM on September 1 and the earliest sunset is 7 hours, 23 minutes earlier at 2:39 PM on November 30.

Daylight saving time (DST) starts at 11:00 PM on October 26, 2024, shifting sunrise and sunset to be an hour later.

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 the fall of 2024. 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 at Attu Heliport is essentially constant during the fall, remaining around 0% throughout.

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 Attu Heliport is rapidly increasing during the fall, increasing from 8.5 miles per hour to 12.3 miles per hour over the course of the season.

For reference, on December 2, the windiest day of the year, the daily average wind speed is 12.4 miles per hour, while on July 12, the calmest day of the year, the daily average wind speed is 7.3 miles per hour.

The hourly average wind direction at Attu Heliport throughout the fall is predominantly from the north, with a peak proportion of 39% on September 17.

Attu Heliport 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 at Attu Heliport is rapidly decreasing during the fall, falling by 7°F, from 40°F to 32°F, over the course of the season.

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 Attu Heliport typically lasts for 3.2 months (99 days), from around June 16 to around September 23, rarely starting before May 27 or after July 8, and rarely ending before September 4 or after October 9.

During the fall at Attu Heliport, the chance that a given day is within the growing season is very rapidly decreasing falling from 93% to -0% over the course of the 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 at Attu Heliport are essentially constant during the fall, remaining around 15°F throughout.

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 Attu Heliport is very rapidly decreasing during the fall, falling by 2.8 kWh, from 2.8 kWh to 0.0 kWh, over the course of the season.

For the purposes of this report, the geographical coordinates of Attu Heliport are 67.940 deg latitude, -53.623 deg longitude, and 3 ft elevation.

The topography within 2 miles of Attu Heliport contains only modest variations in elevation, with a maximum elevation change of 341 feet and an average elevation above sea level of 34 feet. Within 10 miles contains only modest variations in elevation (1,010 feet). Within 50 miles contains very significant variations in elevation (4,856 feet).

The area within 2 miles of Attu Heliport is covered by water (55%) and sparse vegetation (32%), within 10 miles by water (59%) and sparse vegetation (22%), and within 50 miles by water (56%) and grassland (13%).