Daily Cycle

The most obvious tide cycle you’ll see in our data happens over 12.5 hours, spanning one high tide and one low tide.

Daily Cycle (Part 2)

This daily cycle is primarily caused by the Moon’s gravity, which pulls Earth’s ocean water toward the Moon, creating a tidal bulge of water.

Daily Cycle (Part 3)

When the rotation of the Earth brings San Francisco into the tidal bulge, then we get a high tide. When San Francisco is pointed 90 degrees away from the Moon, we get low tide.

BiWeekly Cycle

BiWeekly Cycle (Part 2)

The combined gravity of the Sun and the Moon creates this cycle.

When the Earth, Moon, and Sun are lined up, we get spring tides—very high high tides and very low low tides. And when Earth, Sun, and Moon make a 90-degree angle, we get more moderate neap tides.

Ebb and Flow

The water in San Francisco Bay never rests. As the tide rises, a current of water rushes in through the Golden Gate, swirling around islands and along shorelines, filling the Bay. Then as the tide falls, the current switches directions and surges back out again. 

Daily Cycle

Tides of Salt Water

Salty ocean water surges into the Bay with the rising tide, and rushes back out again with the ebbing tide. That’s why the salinity graph rises and falls together with the tide graph.

Daily Cycle (Part 2)

Meanwhile, fresh water pushes into the Bay from California’s largest river system—the Sacramento and the San Joaquin rivers. The salty ocean water and fresh river water meet and mix in the Bay, creating what’s known as an estuary.

Yearly Cycle

Fresh Water Pulses

In winter and spring, when rainstorms drench Northern California and mountain snowpacks begin to melt, fresh water from the rivers pushes farther into the Bay estuary, causing salinity to plummet.

During drier summer and early fall, salinity gradually increases as water evaporates from the Bay.

Who Cares About Salinity?

Fish do, that’s who. Especially certain types of fish like the Delta smelt.

Smelt thrive in the low salinity waters of the Sacramento–San Joaquin Delta. In dry years, the smelt struggle mightily to swim far upstream to reach water with the salinity levels they need. The fresh water that we pump out of the Delta to send to Southern California only makes matters worse.

Daily Cycle

Cold Ocean Tides

When the rising tide pushes colder, saltier ocean water into the Bay, we see water temperatures drop and salinity rise at Pier 15. At low tide, warmer, fresher water from farther into the Bay replaces the ocean water, so temperature rises and salinity falls.

Yearly Cycle

Warm Summer Water

Over the course of a year, the main influence on the Bay’s temperature is the amount of sunlight warming the surface of the Bay. So Bay water is warmest in August and coldest in December and January.

Cold Water Upwelling

The Bay gets warmer from January through August, but this steady rise in temperature seems to stall in April. Why?

In April, an ocean phenomenon called upwelling happens along the West Coast. Winds blowing away from the shore drive surface waters away from the coast, so colder water from below rises up to replace it. This cold, upwelling water gets drawn into the Bay by the tide, and stalls the Bay’s warming trend.