Why is the water so cold?

Why is the water so cold?

It’s July. Or maybe, as you’re reading this, it’s June or August. The point is, it’s hot outside. Sweltering, even. You’ve been soaking up the sun for a few hours now and a dip in the ocean is starting to sound more and more appealing. You make a beeline for the water, moving quickly because the sand is so hot on your feet, and you can almost already feel the refreshingly cool waves providing some much-needed relief from the 90-degree day.

You dip your toes in, and your body becomes electrified in shock. You run backwards a few steps, feeling the goosebumps as they coat your skin. The water is freezing!

Why is this? It’s hot outside. Shouldn’t the Atlantic at least be lukewarm?

Unfortunately, not always.

Water temperatures differ from beach to beach

So while the ocean could feel like a nice, warm bath off the coast of Florida, it could still be numbingly cold in Ocean City. It is true that, typically, the water is colder in the winter—usually between 30 and 40 degrees—and warmer in the summer, sometimes reaching the upper 80s. As you’d expect, the sun beating down on the ocean does have a major effect on the water temperature—but wind plays a significant role in the temperature, too.

Blame it on the wind

Even when the sun is baking the beach, if there’s even a hint of wind, this could be making the ocean water much cooler than the air temperature. There’s a process called upwelling at play, where deep, cold water rises to the ocean’s surface. In the open ocean and along the shoreline, wind mixes the water around and brings water from the bottom of the ocean, which is cold and nutrient-rich, to the surface of the ocean.

The sun’s energy only reaches the surface of the ocean, and the deeper you get—from about 50-60 feet and onward—there’s what is known as a “cold pool.” The sun can’t penetrate the cold pool, so winter weather water remains in the deep ocean. When upwelling occurs, this cold, winter water is brought to the surface, so even on hot summer days, you could be swimming in an ocean that feels more like January than July.

Basically, it all comes down to the wind and where the wind is coming from. When the wind is blowing from the south and southwest, it pushes the warmer water offshore and away from the beach, and that water is replaced with water from the cold pool. That’s upwelling.

Upwelling graphic
It’s a simple graphic, but this is pretty much what’s happening.

Know before you go

Sometimes, when there’s a nice, summer breeze outside, that’s when the water will be the coldest. Or not. It’s pretty hard to know for sure until you go out and feel it for yourself.

There are apps like Surfline and Buoy Finder NOAA that can give you an idea of what the water temperature is like in your location on any given day. They’ll tell you the satellite measurements that have been recorded at your beach, usually a mile or two out from the shore.

If the water is freezing one day, don’t give up! It could be much warmer tomorrow. Just bring your sandcastle building supplies to the beach with you as a backup plan if you feel that the ocean is a little bit too frostbite-inducing for comfort today.

1 Comment on this Post

  1. Carole

    I am still confused why north wind vs. south wind changes the upwelling. Can you be more specific about how it works? I always thought upwelling happened when the wind blow off shore not parallel to the coast.

    Reply

Leave a Comment