2005-07-22 / Columnists

Drawing On Science

by Stephen Yaeger

So far this summer there have been two or three reported deaths due to rip currents. Most people are familiar with the term, but not how and why they form. I have heard many reasons for their occurrence from rogue waves to groins and jetties. Let’s see if we can set the record straight.

Rip currents are very powerful, channeled “rivers” of water, which flow away from the shore. Such currents may reach speeds of 3+ miles per hour and carry masses of sand along with them. Their formation involves of number of things. As waves approach the shoreline they begin to break forming the surf. When the breaking water rises up on the surf zone it continues on to the upper beach carrying sand with it. This is called swash . The beach or other structures act as barriers and at some point water is prevented from going any further. The water will then move straight back toward the sea in a backwash gently moving below the following wave and carrying sand along with it. A very strong backwash is known as an undertow. But, of course, waves rarely approach the shore head-on. They will approach at an angle due to the fact that the winds that generated them move randomly in any direction out at sea.

When waves approach at an angle and break on the shore the swash will move up-beach at an angle too. When the barrier is reach the water will, again, return straight back. Now, consider the next series of waves, beyond the line of breakers . These waves are, naturally, pushing water toward the shore. But backwash is also pulling this water back into the ocean. This results in water moving somewhat parallel to the beach in what is called a longshore current.

When the surf brings in more water than the backwash can return a buildup of water can occur. This is the key to a rip current. Such a build-up of water may also form when two longshore currents meet head-on or when water is prevented from moving up-beach by a barrier such as a breakwater, headland, or underwater sandbar. If too much water builds up on the surf zone, and the backwash increases in speed, a portion of the longshore current may begin to move rapidly seaward by overcoming the incoming waves. This forms the rip current. Rip currents typically extend from the shoreline out past the incoming breakers. Any beach may experience rip currents whether it is located on the ocean or a lake.

It is possible to spot a rip current. Since they carry a great deal of sand they may appear whiter than the surrounding water. Also the waves in the vicinity of a rip current are usually steeper than the other waves. Once caught in a rip current it it best to “ride” it out. Allow it to carry you seaward and as it slows down begin to swim parallel to the shore. Once it stops swim directly back to the beach.

Since rip currents can occur at any time it is not advisable to go swimming unless a life guard is on duty. Check out the United States Lifesaving Association, NOAA’s National Weather Service, and the National Sea Grant for information on rip current safety and awareness.

Science News: On August 27, 2005 The Red Planet is about to be spectacular! This month and next, Earth is catching up with Mars in an encounter that will culminate in the closest approach between the two planets in recorded history. The next time Mars may come this close is in 2287. Due to the way Jupiter’s gravity tugs on Mars and perturbs its orbit, astronomers can only be certain that Mars has not come this close to Earth in the Last 5,000 years, but it may be as long as 60,000 years before it happens again.

The encounter will culminate on August 27th when Mars comes to within 34,649,589 miles of Earth and will be (next to the moon) the brightest object in the night sky. It will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification.

Mars will look as large as the full moon to the naked eye. Mars will be easy to spot. At the beginning of August it will rise in the east at 10 p.m. and reach its azimuth at about 3 a.m.

By the end of August when the two planets are closest, Mars will rise

at nightfall and reach its highest point in the sky at 12:30a.m. That’s pretty convenient to see something that no human being has seen in recorded history. So, mark your calendar at the beginning of August to see Mars grow progressively brighter and brighter throughout the month.

But not since the Neanderthals lived have Earth and Mars been quite as close as on August 27, 2003 at 9:51 Universal Time (the time in Greenwich, England) — 55,758,006 km (34,646,418 miles) from center to center. That was the nearest the two planets have been in almost 60,000 years!

Questions/comments? E-mail

Steve: Drawingonscience@aol.com

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