MTP Applications - Forecasting Airport Fog Burn-off

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• Basic Definition of West Coast Fog
• Favored Locations for Fog in the U.S.
• Seasonal Variation of Fog in the U.S.
• Monthly Variation of West Coast Fog Frequency in the U.S.
• Diurnal Variation of Fog
• Comparison of Fog Frequency: California and Chile
• Summary of the West Coast Coastal Fog Phases and Life Cycle

Basic Definition of West Coast Fog

Not all fogs that occur in western coastal zones of continents are directly related to large-scale anticyclones. Fogs dominated by radiative and large-scale advective processes  also occur in these regions, although not as frequently as the type of fog episode that is presented in this module. In addition, west coast "hot spell" fogs are generally longer-lived than typical radiation fog episodes over land (exceptions include radiation-related valley fogs that can last for weeks in valleys such as the San Joaquin, Snake River, and Po Valley, Italy). Occasionally fogs occur in which all three processes are critical.

Fog is considered to have formed when the horizontal visibility is <1.0 km, although near airports fog is reported when the visibility is <8 km (5 miles). Heavy fog is reported when the horizontal visibility is <400 m (0.25 miles). Prior to 1932 this had been <1000 feet (305 meters).

Favored Locations for Fog in the U.S.

This graphic shows the average number of days each year with heavy fog in the U.S. Note the relatively high frequency (60+ days/year) of heavy fog along the western portions of California, Oregon, and Washington. In fact, some locations along California's coastline average more than 150 days per year of heavy fog. San Francisco International Airport (SFO) has the maximum average number of Instrument Flight Rules (IFR) events annually of all the major U.S. airports. During the warm season (May through October), SFO experiences ceilings of 3000 ft or lower on 30% of the days. On any given August morning, the probability that a low ceiling will exist at SFO is approximately 50%.

Seasonal Variation of Fog in the U.S.

The above table shows the average percentages of hours with visibility less than 2.5 km (~1.5 mi) for each season. Note in these charts that low visibilities are least frequent along the west coast in the spring. In summer, they are most frequent in southern CA and the coastal region of southern OR/northern CA. In autumn and      winter, low visibilities are more frequent in most regions along the west coast.

Monthly Variation of West Coast Fog Frequency in the U.S.

The graph to the left shows the average days per month with fog in selected cities along the West Coast. Fog frequencies in this region exhibit an autumn maximum. Monthly frequencies all along the West Coast begin to rise sharply by August, with a subsequent sharp decline beginning in the early winter months.  One study  found that in San Diego, Oceanside, and San Francisco, most dense fogs occur in winter from September through March. The AAF report also found that "most low ceilings occur in summer from June through September" in most regions along the west coast.

Diurnal Variation of Fog

Warm season fog and low stratus events along the U.S. West Coast exhibit a distinct diurnal cycle, with a maximum in low ceiling occurrences during the early- to mid-morning hours and a minimum during the late afternoon.

Comparison of Fog Frequency: California and Chile

Outside of the U.S., comparatively little research has been done on west coast fog formation; however, it is believed that the major processes discussed later in this module also occur in west coastal regions of South America and Africa. Some favored locations for fog outside of North America are the west coasts of South America (Chile, Peru, and Equador), northwestern Africa (Morroco, Western Sahara, Mauritania, and Senegal), and southwestern Africa (Namibia and South Africa). Here is a chart comparing grouped averages along the west coasts of California (from ~32°N to ~41°N) and Central Chile (from ~30°S to ~39°S). These data show that there is a cool season fog frequency maximum Similarity of Global Subtropical High Pressure Systems

Here is an animation showing global mean sea-level high pressure patterns for each month of the year. Note the positions and movements of the subtropical highs positioned off the west coasts of the U.S., South America, and southern Africa.
As discussed later in this module, short-term evolution of the subtropical high is an important synoptic feature associated with west coast fog episodes in both regions. (The cool season is March-September in the southern hemisphere.)

Summary of the West Coast Coastal Fog Phases and Life Cycle

Preconditioning

Onset/Formation

Growth/Maturity

Dissipation

U.S. West Coast fog episodes typically occur during the late spring, summer, and autumn. Each episode is characterized by several phases or stages: development of preconditioning, formation, growth and maturity, and dissipation (or stratus formation). This sequence is trackable on the synoptic scale and typically takes 5 to 15 days to complete.

During the preconditioning stage, the subtropical high pressure system moves inland and causes a 'hot spell' event characterized by warm, dry offshore winds that flow down the coastal ranges and out over the sea, clearing the marine boundary layer. Boundary-layer moistening eventually leads to haze and fog formation. The fog deepens and expands during the growth stage. During dissipation, the fog base lifts and the residual cloud mass is stratus in nature; subsequent offshore winds may dissipate the stratus.

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