Estuarine Science

2. How do we measure the tides?

Tides have been measured many different ways. The National Oceanic and Atmospheric Administration, NOAA, is responsible for measuring tides at a series of stations all around the United States. There are 6 tide stations in Narragansett Bay: Providence, Conimicut Light, Fall River, Prudence Island, Quonset Point, and Newport.

In the past, tides were measured with a floating ball in a pipe open to the water. As the water level moved up and down, the floating ball moved with the water level. The ball was connected to a recording device to record the tides.

Low tide		High tide
Tide guage at low and high tide in the Gulf of Alaska. Photos from NOAA Photo Library. http://www.photolib.noaa.gov (Click for larger images)

Today, NOAA uses plastic pipes open to the water and sound waves to measure the water height. The travel time of the sound waves in water can be determined very precisely, giving very accurate measurements of the water height over very short times (many times a second). In deep water, NOAA uses pressure sensors on the bottom of the ocean to measure the water height. More water above the pressure sensor results in more pressure on the sensor. Pressure can also be measured very precisely.

Installing a pressure-sensing tide guage. The tide guage sits on the bottom and measures the pressure of the water above it. Photo from NOAA Photo Library. http://www.photolib.noaa.gov(Click for larger image)