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Navigating fog: A trick that could save your life

Tyler Hamilton

Friday, December 8, 2017, 5:01 PM - In the other corner of the continent, National Weather Service forecasters in New Orleans were grappling with the idea to put rare snow flurries into the forecast for portions of southern Louisiana in early December.

Seemingly unrelated to the Pacific Northwest and the recent bout of fog, the snow on the fringes of the Gulf of Mexico gives some important clues to our current state of the atmosphere; consequently, explaining the foggy, moist pattern in southern British Columbia.

But why is fog such an issue under a ridge of high pressure? Can a simple trick in your car potentially save your life? Find out below, but first, a breakdown on the weather pattern moving through the area.

Conditions favourable for fog

The ridge in the west has flavours of previous ridiculous resilient ridges of past years–a seemingly unstoppable ridge of high pressure that prevents typical Pacific storms to impact the region.

One of the primary conditions that produces the highest likelihood of fog is a stable environment which flourishes under a ridge of high pressure.

If this upper level pattern was present in the summer, the sun angle would be much higher above the horizon; therefore, morning fog would mix-out (rising air) and parts of the Pacific Northwest would see temperatures soar to near record-like summer temperatures.

This low-level cold air associated with high pressure paradox often creates below-seasonal temperatures in some of the valleys and regions at sea level blanketed in fog. Long nights with clear skies and diminished hours of daylight, paired with the sun low on the horizon, allows the cold pool of air to magnify and grow.

The longer the pattern remains stagnant, the cold pool can grow and expand in a blocking pattern. This cold air byproduct is not particularly intuitive.

It’s often called ‘fake cold’ when such a massive ridge moves over the Pacific Northwest, as it’s not originating from northern latitudes like the arctic air occasionally seen in the Pacific Northwest.

What’s the recipe card for fog?

The primary ingredients for fog are:

  • Moist, saturated air at the surface
  • Overnight clear skies
  • Recent precipitation/wet soils
  • Light winds

When all four of these ingredients are present, fog formation is extremely likely.

Fog is often downplayed as a nuisance to aircrafts, and is often the culprit of flight delays in the fall/winter months.

Fog is highly photogenic, so its beauty is often showcased when an inversion (warm air aloft) develops trapping the fog near the surface.


The dangers of fog: Staying safe on the road

Diminished visibility is certainly a risk to drivers, but a more dangerous and disturbing by-product of fog exists when temperatures approach the freezing mark.

Black ice.

There’s a major distinction to make between frost and black ice.

Frost has the capability to slicken up roads and sidewalks. It often creates locally slick conditions, but it has significantly less thickness and travel disruptions than black ice.

Black ice can close major roadways and make travel treacherous. When you look at the deadliest weather types in the Pacific Northwest, fog is one of the leading causes of weather related fatalities in many jurisdictions.

The slick roads cause numerous motor vehicle accidents.

What is black ice?

Don’t let the name fool you. Black ice is a misnomer. It’s not black, but rather clear and invisible to drivers who approach this serious hazard on the roadways. As the air reaches supersaturation (air unable to hold additional moisture) at ground level, moisture from the dense fog will condense on many of our roadways.

It’s not just a hazard at 0 degrees Celsius.

Most newer vehicles have outdoor temperature sensors. A word of caution: The sensors don’t measure the temperature directly at the surface and can be impacted by other nearby warm anomalies, such as the warmth of nearby car exhaust and engines.

An important rule of thumb is this: if your car thermometer reads 3°C or below, it’s time to be extra alert and attentive to the threat of black ice. A thin layer of air directly above the roadway can already be close to the freezing mark at this point. Valleys and bridges are relatively susceptible to the formation of black ice, which is often indicated by road signs—airflow under bridges can enhance the cooling of the pavement; consequently, the insulating factors of the ground are removed.

Safe driving this holiday season, especially in the presence of fog.

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