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Understanding how tides change during storms

Harvey's torrential rain was causing flooding Saturday in Galveston, Texas. Photo by Scott Olson/Getty Images
Harvey's torrential rain was causing flooding Saturday in Galveston, Texas. Photo by Scott Olson/Getty Images

As hurricane season approaches, here’s what to know about tide levels and how they change during storms.

A normal tide also referred to as an astronomical tide, is the typical rise and fall of ocean water levels resulting from the gravitational forces of the sun and moon combined with the rotation of the earth.

Typically, a location will observe two high tides and two low tides within a day.

Normal tide
Normal tide (KPRC)

A storm surge is an abnormal rise in sea level accompanying a hurricane or other intense storm, caused by wind blowing seawater onshore.

Surge relates to the height difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone.

Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

Lastly, a storm tide is the actual level of seawater resulting from the astronomic tide combined with the storm surge.

Storm surge
Storm surge (KPRC)