Exploring the Causes of Fog in the Northeast and Canadian Maritimes
Cara Czech, Meteorologist
As warm, moist air over the SE'rn U.S. lifts N'ward and interacts with the cooler New England waters, foggy conditions start to form. A voyage across the New England and Canadian Maritimes region can often be hindered with poor visibility, particularly during the spring and early summer months.
Figure 1. Courtesy of Jeremy Davis, image showing warm air advection fog blanketing the outport of Francois, Newfoundland, in June of 2019.
When it comes to forecasting fog, it's important to know what ingredients are needed for fog to develop and what type of factors play a dissipating/persisting role. Listed below are the key ingredients for fog development:
1. Calm to light winds - There must be a mixing component of different air masses for fog to generate. Stronger winds would inhibit the formation process.
2. Temperature and humidity - Warm, moist air from the south shifts over a cold sea surface, resulting in advection fog.
Listed below are factors that play a dissipating/persisting role:
1. Weather pattern - A broad high over an area typically offers a stable environment across the region, offering a steadier light wind profile. Air behind fronts can assist in the fog formation; warm and more humid air behind a warm front moves over cooler waters.
2. Daytime heating - The air will warm as the sun rises, resulting in the air temperature increasing from the dew point temperature and fog will start to dissipate.
3. Higher winds - Stronger winds create a larger mixing of air at the surface and in higher altitudes (winds higher up are generally more dry), resulting in the inability for fog to form or assisting in the dissipation of fog.
During the spring and early summer, the days start to warm up from the winter months where warm, moist air from the southern states region starts to accumulate. Fog starts to develop as calm/light S’ly winds draw up the warm air from the south, passing over cooler sea surface temperatures to the north over the New England and Canadian Maritimes area. This region is prone to fog development during this time of year given the optimal geographic setup. This fog usually develops overnight and tends to last through much of the morning until the sun breaks out of the low cloud deck and/or the winds start to increase.
However, there are occasions where the coastal fog bank will linger throughout the entire day due to a broad high pressure near the region. By later July through August, the foggy season gradually diminishes as sea surface temperatures start to rise along the New England Coast/Gulf of Maine, though cold air behind cold fronts typically then becomes the main catalyst for fog development more so during the winter with arctic air/sea smoke.
Figure 2. GIF of GOES-16 ABI Band 2 (0.64 µm) from 0902-1312 UTC on 9 May 2018. Showing dense fog over the Maine and Nova Scotia region gradually dissipating through the morning.
Not only do offshore regions get impacted by fog, but also bay and inlet regions that locate more inland. Radiation fog is a type of fog that often forms overnight as the sun sets and the air near the ground is able to cool, allowing for the air temperature to reach the dewpoint. If there had been precipitation over the region from the past day or so, this would assist in creating a higher dewpoint, therefore making it easier for the air temperature to become saturated. This type of fog often prevails as a patchy state and dissipates in the morning as the air temperature rises due to the rising sun.
Fog can be a major hindrance for vessels out at sea, whether you're routing through the North Atlantic iceberg limits or just about to depart from a port farther inland. WRI can assist with your next voyage in recommending best windows, advising of when fog is likely to dissipate, and/or construct a route to best avoid reduced visibility. Contact us for a forecast update.
