Fog at Boundary Bay: Why Visibility Can Change Across the Lower Mainland

On an autumn morning in Delta, fog can sit over the farmland and shoreline while other parts of Metro Vancouver are clear. Some mornings it burns off soon after sunrise. On others it lingers for hours. For pilots at Boundary Bay Airport (CZBB), the difference usually comes down to how the fog formed and where it came from.

Radiation Fog and Marine Fog

NAV CANADA’s The Weather of British Columbia, a local-area manual written for flight service specialists and pilots, separates Lower Mainland airports into two groups. In autumn, radiation fog is common around Langley and Pitt Meadows. Vancouver and Boundary Bay are particularly susceptible to marine stratus and fog moving in from the straits.

Radiation fog forms on clear nights with light winds, when the ground cools the air above it to saturation. Wind speed matters more than many students expect. Too much wind mixes the moist layer away, and near-calm air tends to leave dew or frost instead. NAV CANADA’s Aviation Meteorology Reference notes that a surface wind forecast off by only a few knots can decide whether radiation fog forms at all. Cirrus moving in overnight can have the same effect by limiting how far the temperature falls.

Marine fog and stratus don’t need to form over the airport. According to the manual, they can be carried inland with early-morning inflow, often under a ridge of high pressure along the coast. Conditions can deteriorate quickly, the low cloud can hang on into mid-afternoon, and in a persistent pattern it may take longer to clear each day. The manual doesn’t tie this pattern to a particular season, so a foggy morning at Boundary Bay isn’t necessarily radiation fog that will burn off once the sun is up.

Why Visibility Can Drop After Sunrise

Radiation fog clears from its edges inward. As the sun warms the surrounding ground, mixing along the edge of the fog can briefly cause more condensation and lower visibility slightly before it starts evaporating the droplets. Places near the edge of a fog bank often clear well before those in the middle. In autumn the weaker sun slows the process, and NAV CANADA notes that fog tends to be thicker and slower to burn off as winter approaches.

Marine fog doesn’t follow the same pattern. If moist air keeps moving in, daytime heating may not be enough to clear it, and fog lying over water takes longer to dissipate than fog over land. When a forecast calls for improving visibility, the question worth asking is what change in the atmosphere is expected to bring that improvement, and whether current observations show it actually taking place.

Reading the Weather When CZBB Has No METAR

CZBB doesn’t have its own routinely published METAR or TAF. Vancouver International (CYVR) provides the nearest observations and forecasts, but a CYVR METAR doesn’t necessarily describe the visibility or ceiling at Boundary Bay. Boundary Bay’s ATIS provides local information during its operating hours, and the airport’s pilot information page points pilots to NAV CANADA’s flight planning website and weather briefing service.

From there, pilots build a picture from GFAs, nearby METARs, weather cameras, satellite imagery and PIREPs. A narrowing temperature and dew point spread shows the air approaching saturation, but it doesn’t confirm fog or tell you the actual visibility or ceiling. Weather cameras are useful for seeing what is happening, though they don’t replace official reports.