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On a bright summer day in California, locals and tourists pack the beach, setting up on the sand and making sure to use lotion or the shade of an umbrella to protect from the sun’s powerful rays.

But on that same beach, it’s not uncommon to see kids shivering as they run out of the water, or a surfer wearing a partial wetsuit as they jog out for a session.

California’s ocean water is pretty cold, even in the summer, and it often catches visitors off guard. What causes this phenomenon, and why do water temperatures on the West Coast differ so much from the ocean at a beach in say, Florida?

Experts say two key factors chill the sea even when the sun is shining: The cold California Current, which flows south along the coast from the Gulf of Alaska, and a concept called “upwelling.”

In a phone interview this week, San Diego-based meteorologist Miguel Miller, who works for the National Weather Service, helped explain the two forces.

Incoming from Alaska

Miller describes the California Current as a “river of cool air that’s transporting Alaskan water down to California.”

This basic map demonstrates how ocean currents create a northerly wind on the West Coast of the U.S., and a southerly wind in the East. (Photo: National Weather Service)

The air is driven, in part, by high pressure that sets up over the Eastern Pacific for long portions of the year. Scientists have found that air moves around high pressure in a clockwise direction, which results in wind that flows from north to south off the West Coast of the U.S.

Miller says the breeze is typically light, but it’s consistent. As the moving air comes into contact with the Pacific Ocean’s surface, it pulls the water along in the same direction.

That sends the frigid waters of Alaska down past Washington, Oregon and then California en route to Mexico. If you’ve swum or surfed at a vacation spot in western Baja California, like Ensenada or Rosarito, you know the water’s chilly there, too.

Swapping warm water for cold

Understanding the California Current can help a shivering swimmer grasp the second factor at play here: upwelling. Miller personally credits this phenomenon for chilling our beach day even more than the current.

Upwelling occurs when winds push aside the top layer of ocean water — which is warmer from sitting directly in the sun. The water getting shoved away creates a vacuum that has to be filled with something, so colder water rises from the depths to the surface.

The same strong winds driving the California Current contribute to frequent upwelling at the coast in California. As air moves south, the Earth’s rotation creates rightward pressure, pulling the wind and water away from the coast. This phenomenon is called the “Coriolis effect.” It moves objects right of their intended path in the Northern Hemisphere and left in the Southern Hemisphere.

Meteorological terms aside, the end result is pretty straightforward: Warmer water at the surface keeps getting swapped for cold water from below, and that means you’ll be bundling up after a swim on your beach day.

Same latitude, different temps

The East Coast doesn’t have the same conditions that contribute to our chilly ocean temperatures. The warm Gulf Stream current flows from south to north, bringing warm waters from the Gulf of Mexico and Caribbean Sea up past Florida and along the coast.

That warm channel isn’t disrupted by the same degree of upwelling, either, letting those who take a dip in the Atlantic Ocean in the eastern U.S. enjoy generally warmer temperatures.

From coast-to-coast, swimmers on the same latitude can see differences of 5 or 10 degrees — or even more.

For example: San Diego’s average September water temperature is about 67 degrees. But follow the same line of latitude (32.7 N) to Charleston, South Carolina, and the average water temperature that month is around 82 degrees, according to the Fleet Science Center.

While this trend generally remains true, Miller says the conditions influencing water temperatures are not ironclad. Storm systems and any number of other forces can influence the weather on both coasts.

Sometimes upwelling is reversed entirely off the West Coast, for example, leading to unseasonably warm water temps there.

Generally speaking, though, visitors should plan to cool off in the water and warm up on the sand when they come to California — even during summer. Tourists, you’ve been warned.