Introduction

The weather in Acadia National Park is typical of the northern Maine coast. Generally the summer temperatures along the coast are a bit cooler than inland and the winter temperatures tend to be a bit warmer than inland. Although Mount Desert Island and other coastal regions receive abundant precipitation, there is generally less snowfall along the coast compared to areas just a few miles inland. There is usually a breeze blowing in Acadia National Park and strong winds are common.

Fog often settles on Mount Desert Island and the coast during the night and lingers for several hours after sunrise. In the afternoon a breeze will usually blow across Mount Desert Island from the ocean cooling the island in the summer and warming it in the winter. Evenings are normally cool or cold all year.

Sometimes the weather in the area of Acadia National Park can be extreme and dangerous. Residents of the area pay close attention to the weather forecasts and conditions. Be prepared for the most extreme forecast.

Weather Data

The following chart contains weather data for each month of the year. The mean (average) monthly water temperatures at Bar Harbor are also listed. The source for this information is the United States National Oceanic and Atmospheric Administration (NOAA).

One should remain aware that the weather in Acadia National Park can change dramatically in a short period of time and many parts of the park would be exposed to the full brunt of a storm. However the weather is predictable. The United States weather bureau operates a weather radio station in Ellsworth, Maine on a frequency of 162.40 MHz, which transmits a continuous weather forecast for coastal Hancock County that includes all of Acadia National Park and Mount Desert Island.

Temperatures in the following tables are in degrees Fahrenheit (F) and Celsius (C). Precipitation is in inches (") and centimeters (cm).

Month Mean
 Air Temperature F/C
Expected Maximum
Air Temperature F/C
Expected Minimum
Air Temperature F/C
Mean
Precipitation "/cm
Mean
Snow "/cm
January 24/-4 33/1 14/-10 4.7/12 17/43
February 25/-4 32/0 15/-9 3.9/10 19/48
March 33/1 41/5 23/-5 4.7/12 12/30
April 43/6 52/11 32/0 3.7/9 4/25
May 53/12 63/17 42/6 3.6/9 <1/<3
June 61/16 71/22 50/10 3.3/8 0/0

July

67/19 77/25 56/13 3.3/8 0/0
August 66/19 75/24 55/13 3.2/8 0/0
September 59/15 69/21 50/10 3.9/10 0/0
October 50/10 58/14 40/4 4.4/11 <1/<3
November 40/4 47/8 31/-1 4.9/12 4/10
December 28/-2 36/2 19/-7 4.5/11 10/25

 

Month

Mean Water Temperature

 Degrees Fahrenheit

Mean Water Temperature

 Degrees Celsius

January

38

3

February

36

2

March

38

3

April

44

7

May

49

9

June

55

13

July

59

15

August

62

17

September

58

14

October

54

12

November

52

11

December

44

7

 

For more weather data and information, click on this hyperlink to NOAA: www.noaa.gov.  When the NOAA home page is displayed, various sources of weather data and information can be selected. If the current weather and weather forecast  in Bar Harbor, Maine or any location in the United States is desired, click on this hyperlink to weather forecasts: www.weather.gov and input the location by city and state or zip code of the weather you wish to monitor. This website also has a map to which you can point and click for the weather forecast.

 

Precautions

Evenings tend to be cool in Acadia National Park. A coat or jacket may be required at any time of year. Although summer daytime temperatures may be warm, breezes may make the air feel cool to exposed skin. In winter the wind chill index will often be much lower than the ambient temperature. One should dress appropriately for these conditions.  Also, there is a danger of bad sunburn during the summer because the effects of the sun are not as apparent as in warmer climates.

Storms seldom appear without warning. Watch for gathering clouds. If a storm is forming, leave exposed areas like the tops of mountains and beaches. A number of people were killed while fleeing the tops of mountains during a storm. It is also possible to be washed out to sea from a beach or seaside cliff during a violent storm. Watching a storm can be fascinating, but do it from a safe location. Also, avoid shortcuts during a storm. Some paths and small streams can become raging torrents during and immediately after a storm.

Meteorology of Mount Desert Island and Acadia National Park

The following discussion provides more detailed information on the factors that influence the weather in downeast Maine. Topics covered are:

 The Atmosphere

The Stratosphere and the Ozone Layer

The Ionosphere and the Aurora

Air Masses

Circulation Cells

Jet Streams

Ridges, Troughs and Fronts

Fronts and Precipitation

Highs, Lows and Storms

Semi-permanent Highs and Lows

Weather Patterns

Storms

Clouds

High- level Clouds

Middle-level Clouds

Low-level Clouds

Cloud Formations 

 Seasons

 Radiational Cooling

 Fog

 Icing

 Predicting and Preparing for Weather Conditions

 

To proceed immediately to one of these topics, point at the highlighted word in the topic and double click.

The Atmosphere

Ninety nine percent of the atmosphere by volume consists of two gasses: nitrogen (78%) and oxygen (21%). There are also small quantities of argon, helium, carbon dioxide, carbon monoxide, sulfur dioxide and nitrogen dioxide. Meteorologists divide the atmosphere into layers with characteristic temperatures and activities. The lowest to at least five kilometers or three miles above sea level is called the "troposphere." The region from the top of the troposphere to about ten kilometers to or six miles above mean sea level is called the "tropopause." The tropopause marks the end of the activity that occurs in the troposphere and the beginning of the next layer called the "stratosphere."

Most weather occurs in the troposphere. Warm air rises through the troposphere to the tropopause where the air then cools and descends into the troposphere once again. The troposphere is a region of relatively wide temperature variations. The upper limit of the tropopause has more or less constant temperature and is rather cold.

The Stratosphere and the Ozone Layer

The stratosphere contains a large quantity of the form of oxygen called "ozone." Each ozone molecule contains three oxygen atoms as opposed to the two oxygen atoms found in the usual oxygen molecule. Ozone absorbs ultraviolet light radiation and releases the absorbed energy as heat. The lower part of the stratosphere to about 40 kilometers or 25 miles above mean sea level has a fairly constant cool temperature. Above this altitude the atmosphere begins to increase in temperature because of the ozone heating up to the "stratopause" about 48 kilometers or 30 miles above mean sea level.

The ozone in the stratosphere helps shield life on earth from the dangerous effects of ultraviolet radiation. These effects include eye damage and skin cancer. Unfortunately ozone is not a very stable molecule and easily combines with other molecules or breaks down to the more stable oxygen molecules under the influence of pollutants. These pollutants may be either natural, e.g. volcanic dust, or man made, e.g. fluorocarbons.

The mesosphere and mesopause are above the stratosphere to about 80 kilometers or 50 miles above mean sea level. Above that is the thermosphere.

The Ionosphere and the Aurora

The thermosphere ends about 190 kilometers or 120 miles above sea level. Above this altitude and to the vacuum of outer space is the region called the "ionosphere." In this region the energy from the sun causes the molecules of gas to become disassociated and electrically charged or "ionized." If the sun is particularly active, enough energy is imparted to the ions to excite the atoms to the point that they will emit light creating the "aurora borealis" or "northern lights." The magnetic field of the earth keeps this phenomena close to the poles of the earth but the northern lights can be seen from Mount Desert Island and Acadia National Park about five nights each year usually in the spring or fall.

Red Aurora 

The aurora can range from a diffuse glow in the sky to very distinct curtains of light. The colors may be yellow, green, red or (rarely) violet depending upon the amount of energy present. The above photograph shows a red aurora over Mount Desert Island in 2003. The streaks of white are stars shinning through the aurora. Note that the intensity of the aurora decreases from lower left to upper right. The mottling is a result of the aurora's scintillation during the time-lapse photograph. North is to the left of the photograph and east is at the bottom. This orientation is counterintuitive but keep in mind that looking up at the sky is like viewing a map from under the map. Think about laying on the ground with your head to the North. East is to your left under such circumstances. Communications and electrical transmission may be disrupted by the increase in high-energy electrons during an aurora display. The following photograph shows a green aurora over French Hill Pond. The streak of dots from the bottom center of the photograph to just above center is the pulsating lights of an airplane flying to the North Atlantic.

Green Aurora

Air Masses

The ionosphere supports the phenomena of the northern lights and the stratosphere contains the ozone layer that protects us from the harmful effects of ultraviolet radiation. The stratosphere also blankets the tropopause helping to confine extreme temperature variations and air movement to the troposphere. Of all the atmospheric zones the troposphere is of greatest interest. The troposphere can be divided into "air masses" that have characteristics determined by the directness of the sun's radiant energy and the topography of the earth beneath the air masses. The weather in and around Acadia National Park is influenced most directly by four air masses: the Arctic, Continental Polar, Maritime Polar Atlantic and Maritime Tropical Atlantic.

The Arctic air mass is located over the North Pole and northern Canada. The Continental Polar air mass is south of the Arctic air mass over the North American continent nearly as far south as the Great Lakes. Both of these air masses are cold and dry. The Maritime Polar Atlantic air mass over the North Atlantic is cool and moist. The Maritime Tropical Atlantic air mass over the mid-Atlantic is warm and moist. The interactions of these air masses help determine the temperature and precipitation in downeast Maine.

Air Masses 

Circulation Cells

The sun heats the land and air causing warm air to rise in the troposphere where it cools near the tropopause and then falls into the lower levels of the troposphere to repeat the process. The rotation of the earth causes these air movements to be skewed east or west creating "circulation cells." The circulation cells at the latitude of Acadia National Park have air movement from west to east at upper altitudes. Meteorologists call these cells "Ferrel Cells." The resulting winds are called "westerlies." These cells have a major influence on the paths of storms. Storms and other weather events will generally move from west to east or southwest to northeast in the area of Acadia National Park.

Jet Streams

All the air rising in the Ferrel Cells and the difference in temperature between the air masses creates large pressure differences in the area just below the tropopause. These differences create very high-speed winds aloft called "jet streams." These wind streams blow from west to east in the Northern Hemisphere and in erratic patterns determined by the prevailing atmospheric conditions. The jet stream strongly influences the development and path of storms.

Ridges, Troughs and Fronts

The high winds of a jet stream create waves in the troposphere that cause the flow of a jet stream in the Northern Hemisphere to shift north or south. The north flowing portions of a jet stream are called "ridges" and the south-flowing portions are called "troughs." Ridges usually mark the edges of warm air masses and these boundaries are called "warm fronts." Troughs usually mark the southern edges of cold air masses and these boundaries are called "cold fronts."

Fronts and Precipitation

Warm fronts are normally associated with steady precipitation over a wide area. Cold fronts are associated with brief but heavy precipitation like thunderstorms. In both cases there is a clash of cold, dry air with warm, moist air. Warm fronts are formed by an abundance of warm moist air pouring into a cold area. This warm air provides a renewing source of moisture in the same area until the warm air completely displaces the cold air. The steady precipitation will continue until the cold air is displaced. When a cold and dry air mass moves into a warm and moist air mass, the available moisture is released suddenly in a narrow band of precipitation along the leading edge of the cold front. As the cold air moves further into the warm air mass, precipitation falls on areas further into the warm air mass. Precipitation stops suddenly as the cold air displaces the warm air. Warm fronts generally bring warmer temperatures and higher humidity. Cold fronts generally lower the temperature and humidity.

Highs, Lows and Storms

When cold and warm air masses mix, the air moves in circular paths. If this circular motion is counterclockwise (as viewed from space), the resulting phenomena is called a "cyclone" (not to be confused with a tornado-like storm) or "surface low" pressure area. Cyclones usually develop west of a ridge and east of a trough, that is, west of a warm air mass and east of a cold air mass. This association is logical because the warm air mass to the east is air moving north and the cold air to the west of the cyclone is moving south. The combination of these motions would cause the air in a cyclone to move counterclockwise. In addition, precipitation would be drawn to the center of the cyclone as warm air is drawn to cold air. The air pressure drops at the earth's surface. Thus cyclones or surface lows develop heavy precipitation and strong winds commonly known as "storms." These storms follow the jet streams on paths called "storm tracks" until they dissipate.

"Anticyclones" or "surface highs" usually develop east of a ridge and west of a trough. Their air motion is clockwise. Thus, they encourage the movement of cold air to the south and warm air to the north that prevents the warm and cold air from mixing in the region of the anticyclone. Air pressure at the surface of the earth increases. Surface highs are associated with clear weather.

Semi-permanent Highs and Lows

The polar and subtropical jet streams (yes, there is more than one jet stream) tend to establish two lows to the north and two highs to the south in the Western Hemisphere that are somewhat permanent. The "Bermuda High" is located in the Atlantic Ocean near Bermuda. This high moves north in the summer and south in the winter as the Northern Hemisphere tilts toward or away from the sun. It may also move east or west. The Bermuda High steers storms in the Atlantic Ocean. The corresponding low is the "Icelandic Low" centered between Greenland and Iceland. Storms traveling over North America are attracted to the Icelandic Low. Many European storms originate in the Icelandic Low.

The other semi-permanent high is the "Pacific High" north-northeast of the Hawaiian Islands. Its corresponding low is the "Aleutian Low" centered between Siberia and Alaska. This low produces a large number of storms that move across North America toward the Icelandic Low.

Weather Patterns

Now that the basics of tropospheric dynamics are discussed and if the reader is not completely bewildered by the discussion, weather patterns can be understood. In general, storms move across North America toward the Icelandic Low. Acadia National Park is often under this storm path. In addition, tropical storms originating near the equator will often be steered by the Bermuda High up the coast of North America toward the Icelandic Low. Acadia National Park is frequently in this path as well. The position of the Bermuda High and Icelandic Low will often determine whether or not a storm passes over Acadia National Park. Predictions concerning the path of a storm are difficult to make and frequently inaccurate because changes in the positions of the jet streams, Icelandic Low and Bermuda High can occur quite suddenly and change the path of the storm dramatically. The weather resulting from frontal movements are more predictable and are related to airflow patterns.

Weather Patterns - Westerly Air Flow

The most basic and least complicated airflow pattern is the westerly pattern where fronts and storms move from west to east across the United States. Weak storms and relatively mild winds are associated with this pattern. A front or storm on the West Coast of the United States will take about three days to cross the country and be felt in downeast Maine. There may be some cloudiness or precipitation with this pattern but not very much. Quite often storms associated with this pattern will pass north of Mount Desert Island.

Weather Patterns - Western Trough/Eastern Ridge

A low-pressure area west of the Rocky Mountains and a high-pressure area to the east characterize this airflow pattern. The area of Acadia National Park will have warm weather and winds from the southwest. Any precipitation will normally occur to the north. Although there may be major storms west of the Mississippi, they will not normally migrate to downeast Maine.

Weather Patterns - Western Ridge/Eastern Trough

A strong high-pressure area west of the Rocky Mountains and a low-pressure area east of the Mississippi River characterize this airflow pattern. The area of Acadia National Park will normally have cloudy skies and cold temperatures. If a cold front passes over the Eastern Seaboard, strong storms often develop and move northeast up the coast bringing significant cold rain or snow to downeast Maine. The more powerful of these storms are called "northeasters" because the associated winds blow from the northeast.

Weather Patterns - Western Ridge/Central Trough/Eastern Ridge

High-pressure areas lie west of the Rocky Mountains and over the eastern states for this weather pattern. In addition, the central part of the United States is under a low-pressure area. Storms developed in the central trough will move northeast to bring heavy precipitation to Maine. In the summer this precipitation can take the form of severe thunderstorms and (rarely) tornadoes. Freezing rain or blizzards are expected in the winter.

Weather Patterns - Western Trough/Central Ridge/Eastern Trough

Low-pressure areas west of the Rocky Mountains and along the Atlantic Seaboard and a high-pressure area in the central part of the country characterize this airflow pattern. Maine may receive some precipitation. However, major storms are normally associated with this pattern only during the months from September to May and will predominate in winter.

Storms

Precipitation in the area of Acadia National Park tends to be light to moderate with most storms. Several times during the year storms will produce heavy precipitation flooding roads or blocking them with snow. Winter storms are more likely to produce freezing rain than heavy snow. This generalization does not apply to the interior of Maine.

Thunderstorms are infrequent but may occur at any time of year. They may develop rapidly. Shelter should be sought upon the approach of a thunderstorm. Tornadoes are rare in New England but are very destructive and dangerous when they occur. The Acadia National Park area is not prone to tornadoes and their development would be most unusual.

Hurricanes are a problem in downeast Maine. They develop from June through November but are most numerous in August and September. Generally they weaken before arriving in Maine and are more of a nuisance than a threat to life and property if precautions are taken. Loose objects should be secured, windows boarded and people should stay in a substantial shelter during the storm. Boat damage, shore erosion, fallen trees and downed power lines are common problems associated with hurricanes. Stay away from the seashore until the hurricane has passed well out to sea.

Major snow or ice storms can paralyze Maine for a day or two. These storms are infrequent but travel should be avoided when they occur. Be alert for storm warnings. The term "heavy snow warning" is associated with different levels of snowfall in different parts of the country. Heavy snow usually means six or more inches of snow during a 24-hour period in Maine. Blizzards can be very severe in Maine. Stay in doors during a blizzard.

Clouds

Clouds can indicate when a storm is approaching or when the sky will clear. Type, formation and height generally classify them. High clouds begin at an altitude of 18,000 feet (5,500 meters) above mean sea level. Middle-level clouds begin about 6,000 feet (2,000 meters) above mean sea level. Low-level clouds begin below 6,000 feet (2,000 meters) above mean sea level.

Cumulus humilis Clouds 

The types of clouds are cumulus, stratus, and nimbus. Cumulus or "heaped" clouds have flat bases and puffy domes. These clouds may produce showers or thunderstorms. Stratus or "layered" clouds appear as large white sheets of no discernible height or thickness. Nimbus clouds generally have no clear shape, often cover the sky or have ragged edges and are usually dark. They usually produce rain or snow. These type identifiers may be combined to describe hybrid clouds, e.g. "cumulonimbus." An altitude descriptor may also be added to the cloud type, e.g. "altostratus."

High-level Clouds

Cirrus Clouds 

High-level clouds are identified by the prefix "Cirro." When used alone, the word "cirrus" describes high-altitude, wispy clouds that look like strands of white threads or filaments. These clouds often precede a warm front and associated storms.

 

Cirrostratus Clouds 

Cirrostratus clouds are thin sheets that may cover a part or the whole of the sky. The sun and moon will shine through these thin sheets. These clouds may produce a halo around the sun or moon. Like the cirrus clouds, the cirrostratus clouds often indicate an approaching warm front.

Cirrocumulus clouds are small cumulus clouds that gather in a patchwork formation. They develop from cirrus or cirrostratus clouds and will eventually reform into these cloud formations again. Precipitation is not normally associated with cirrocumulus clouds.

Middle-level Clouds

Altocumulus clouds are rounded clouds that may be either patches or sheets with shading. They often gather into regular arrangements. Generally they do not produce precipitation but may coexist with other clouds that will produce rain or snow.

Altostratus clouds are gray to blue sheets that may cover the sky. The sun cannot be seen through these clouds except through a few breaks in the clouds or an occasional thin layer. Although altostratus clouds do not produce rain or snow, they may form nimbostratus clouds that do.

Nimbostratus Clouds 

Nimbostratus clouds are dark sheets that cover the sky. They normally produce rain or snow and are often called "rain clouds." Smaller shreds of these clouds sometimes form beneath the main layer forewarning heavy precipitation and high winds.

Low-level Clouds

Stratocumulus 

Stratocumulus clouds are gray or white, rounded patches that are very distinct. The winds at the same altitude will often arrange these clouds into bands or rolls across the direction of the winds. Low-level stratus clouds may produce light precipitation or fog on the sides of hills or mountains. Low-level cumulus clouds are usually associated with fair weather.

Cumulonimbus clouds have flat, dark bases and rise in puffy masses high into the tropopause. The tops will often become anvil-shaped. These clouds usually indicate that a severe storm is coming.

Cloud Formations

There are dozens of identifiable cloud formations. The following formations are common in the skies of Acadia National Park or have important associations with changes in the weather.

Castellanus are cirrus, cirrocumulus, altocumulus or stratocumulus cloud formations whose upper parts look like turrets emerging from a common base.

Congestus are cumulus cloud formations that are very tall and vertical. They precede bad weather.

Fibratus are cirrus or cirrostratus clouds formations consisting of straight filaments or filaments that do not form regular curves.

Fractus are stratus or cumulus cloud formations that are shredded.

Humilis are cumulus clouds whose tops are somewhat flattened (see the photograph above).

Incus are cumulonimbus cloud formations whose tops are anvil shaped. These clouds precede a storm.

Mediocris are short cumulus clouds whose tops are somewhat stable.

Nebulosus are cirrostratus and stratus cloud formations that have no discernable form.

Opacus are cloud formations that obscure the sun or moon.

Stratiformis are altocumulus, stratocumulus or cirrocumulus cloud formations that are horizontal sheets.

Translucidus are altocumulus, altostratus, stratocumulus and stratus cloud formations through which the sun or moon can be seen.

Undulatus are cloud formations with an undulating (wavy) surface.

Cloud formations can be of multiple forms, e.g. stratus opacus nebulosus or cumulus humilis fractus. Combinations of the terms introduced thus far should describe any cloud formation of downeast Maine in detail.

Seasons

The astronomical and meteorological seasons begin at different dates. Astronomical spring begins at the vernal equinox around March 21st. Meteorological spring begins when the mean air temperature for each day stays above the freezing point of water. This event occurs in late March or early April in Acadia National Park. Astronomical summer begins about June 21st, the summer solstice. Meteorological summer begins when the average daily temperature exceeds 68 degrees Fahrenheit (20 degrees Celsius). Meteorological summer begins late June in Acadia National Park. Meteorological summer ends when the average daily temperature drops below 68 degrees Fahrenheit. This event normally happens in late August in Acadia National Park well before the autumnal equinox (the beginning of astronomical fall) around September 20th. Astronomical winter begins around December 21st, the winter solstice. Meteorological winter begins when the daily mean air temperature drops below freezing. This event begins about mid-November in Acadia National Park.

Radiational Cooling

Warm air at the earth's surface will rise into the upper reaches of the troposphere during clear nights. This convection will bring cold air into the area of Acadia National Park. The ambient air temperature will drop dramatically by morning even during the summer. If there is enough moisture in the air, this cooling will generate fog. Some heat, about 3 percent, will be radiated by the surface of the earth through a clear atmosphere to outer space. The combined effects of convection and radiation are sometimes called "radiational cooling." A cloud cover reduces radiational cooling.

Fog

Fog is a significant problem on the islands and coasts of Acadia National Park at any time of year. It will usually appear at night and lift sometime in the early morning. Low stratus clouds often create foggy conditions at higher elevations in the park at any time of day. Care should be taken while traveling in fog.

Icing

Icing can be a problem in downeast Maine in the winter. Moisture from the warmer ocean will blow over the land and freeze on roads, rocks, trees and man-made objects. Exercise appropriate care when traveling near the ocean or other body of water. Look for "black ice" (thin sheets of ice difficult to see) on roadways whenever the temperature drops below the freezing point of water. Even Maine natives have problems with black ice.

Predicting and Preparing for Weather Conditions

Bad weather can ruin carefully planned activities. Weather can change quickly in downeast Maine. Prepare alternative plans that allow for bad weather. Monitor the weather bureau radio station or local radio or television weather reports. Keep an eye on the sky as you engage in your planned activities. Be prepared to seek shelter away from bodies of water if a storm approaches.

Be aware of the national airflow pattern. Remember that western ridge/eastern trough, western ridge/central trough/eastern ridge and winter western trough/central ridge/eastern trough patterns usually generate stormy weather in downeast Maine.

High-level clouds usually indicate the approach of a warm front and light but steady precipitation. Nimbostratus clouds normally produce rain or snow. Cumulonimbus clouds usually indicate that a severe storm is forming.

Pay close attention to warnings concerning hurricanes and heavy snow. Be prepared to postpone travel and secure possessions if necessary.

Acadia National Park is an excellent place to observe weather but take reasonable precautions to do this safely.

References

1. Ludlum, David M., The Audubon Society Field Guide to North American Weather, 1991, Alfred A. Knopf, Inc., New York.

2. Williams, Jack, The USA Today Weather Almanac 1995, 1994, Vintage Books, New York.

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