WHEN A 40-ton, 50-foot whale pokes its head out of the waves and then rises up full length, almost as if to stand on its tail, the sight is spectacular, and it is drawing observers by the droves to the California coastline.

Whale specialists call the lunging antic "skyhopping." It is believed to be a navigating technique that helps these huge mammals find their way. If you spot it once, you will probably see it again, since the maneuver is repeated several times at intervals of a few seconds.

Winter is when the California gray whales, once nearly hunted to extinction but now numbering 12,000, make the 10,000-mile round-trip journey from the feeding waters of the Arctic to the breeding lagoons of Baja California in Mexico. The swim south begins about November, with the return beginning by late March.

The peak migration -- and best time to glimpse the whales -- is in January, says the California Office of Tourism, when they can be seen from onshore or from tour boats making one-day trips from several ports between San Diego and San Francisco.

"Nowhere in the world," says the tourism office, "is there a comparable population of large whales so close to the shores of metropolitan areas." Usually the whales travel in groups of two to five at a speed of five miles an hour, covering up to 80 miles a day without pausing to eat or sleep.

Since the whales must surface every three to five minutes to breathe -- their spout may shoot 50 feet into the air -- they are relatively easy to see. They can sometimes be spotted splashing and diving in a titanic courtship and mating ritual.

Many of the California state beach parks offer interpretive programs during the migration period, and several communities throw whale-watching festivals: The one at Dana Point in Orange County south of Los Angeles features parades, displays and whale-sighting boat trips each weekend in February. In March, the Fort Bragg-Mendocino Coast region, north of San Francisco, holds a similar celebration.

One of the best onshore vantage points, according to the tourism office, is the 65,000-acre Point Reyes National Seashore, just north of San Francisco. One-day whale-watching seminars are held at the lighthouse in January. In the south, there's a glassed-in observatory at Cabrillo National Monument on Point Loma in San Diego.

Bundle up, says the tourism office, because the weather can be drizzly and cold, though chances are it will be mild and sunny enough for a picnic. Don't forget the field glasses and camera.

For more information on whale-watching sites and a list of 24 boat tour companies, send a stamped, self-addressed, long-size envelope to: California Office of Tourism, Department WW, 1030 13th St, Suite 200, Sacramento, Calif. 95814.