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Anyone who lives in or drives through the San Joaquin Valley during the late fall or winter months will inevitably experience Tule fog. Not only does Tule, or ground, fog develop in the Great Central Valley of California but also in the coastal and inland valleys — especially after it rains. The first rains of the season moisten the bone-dry ground of summer. When much of the Earth’s heat is radiated out to space, usually on clear, windless nights, it cools the moist and denser layer of air near the valley’s floor.

Fog is water vapor that has condensed onto microscopic particles in the air and formed a cloud at ground level. When air temperature decreases, relative-humidity levels increase and the valley begins to take on a grayish tint. When the air has reached its dew-point temperature, the relative humidity is at 100 percent. Water droplets become visible to the naked eye. At that point, Tule fog can rapidly develop. Each cubic meter of fog contains between .05 and 0.5 grams of liquid water.

Under these conditions, water droplets have little time to grow. They become too heavy for the weak air currents to support, and they fall to the ground as drizzle. Drizzle is defined as water drops with diameters less than 0.02 inches. That’s very small. Mist water drops are so tiny they remain suspended in the atmosphere.

Heavy drizzle occurs when visibility is less than one-fourth of a mile. Moderate drizzle happens when visibility ranges between one-fourth and one-half mile. And light drizzle occurs when visibility is greater than one-half mile.

Rain is composed of water drops with diameters greater than 0.02 inches. Rain falls along the Central Coast when cold fronts move down the California coastline, producing rapid, upward-moving air currents. These upward air currents keep the water droplets suspended in the air column, where they combine and grow. Raindrops can reach sizes of up to 0.25 inches before they fall to the ground.

The intensity of rain is based on the amount that falls in one hour. Light rain is classified as 0.1 inch or less per hour. Moderate rain ranges from 0.11 to 0.3 inches per hour. Heavy rain is greater than 0.3 inches per hour.

This leads to the question of the difference between rain and rain showers.

Rain is persistent precipitation over a large area associated with a warm or cold front. In other words, a rain episode lasts for a longer time. On the other hand, rain showers occur in a much smaller area of coverage and usually have an abrupt start and end and are often associated with convective types of clouds caused by cold and unstable air that often filter in behind cold fronts.

Tips for driving in foggy conditions:

  • Drive with lights on low beam. High beams will reflect off the fog, creating a “white wall” effect.
  • Reduce your speed, and watch your speedometer. Fog creates a visual illusion of slow motion when you might actually be speeding.
  • Avoid crossing traffic lanes.
  • Travel with the driver’s window partially open. Listen for traffic.
  • Watch for CHP pace cars to guide you.
  • If your car is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your vehicle. Consider postponing your trip until the fog lifts.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative.

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