Earth Science Question?

[~Just_A_Teen~]

Okay... i have a question.. its not hw.. i just want a better understanding of a topic..

Can some one explain why when you go to the beach in the afternoon the sand is hot, but the water is cool...but at night its vice versa... (cool sand)...i believe it has something to do with wind patterns

 

[Clay C]

The water is absorbing a lot of heat during the day but it is spread through the water column because the water is constantly moving and churning, so the water can take a lot of sunlight without heating up very much and warms slowly.

But the sand is not moving and as the sunlight heats the surface sand grains there is not much way for the heat to dissipate. If you dig into the sand it is hot on the surface but gets cool very quickly in the deeper layers.

At night the sea water retains the heat because it has heated deeply and takes a while to cool down. The sand cools very fast because there is not much heat coming up to it from below and it radiates heat into the sky from the surface layer.

 

[mark v]

It has zero to do with wind patterns. But this is what it *does* have to do with...

Water is an excellent heat sink. It accepts energy relatively slowly, but once that thermal energy is inside, it's very stingy about letting it out.

Beach sand, on the other hand, which is primarily silica/quartz is very poor. It heats up rapidly and can get quite hot, and it will stay that way as long as the Sun is pumping energy into it. But when the Sun goes down, it loses its thermal energy as fast as it gained it (which is why I life when someone on Star Trek uses a phases to heat up a rock for all-night warmth).

I can see the beach from my backyard, and I'm about 30 miles from the US - Mexican border. The water temperature averages about 72 F year-round. 72 degree water is pretty nice. Because water is such a good thermal battery, it won't change much between high noon and the dead of night.

When I go down during the day - especially when the sunlight is stronger, such as in the summer - the sand will have really sucked up the Sun's thermal energy. It can easily be 80 or 90 degrees, which is why you see people dancing around on the sand like hot coals. So the sand is about 85 F, but the water is only 72 F, so the relative temperature difference makes the water feel colder (because it is).

On the other hand, if I stay long enough with my wife and girls to watch the Sun go down, and make a little barbecue pit, I can walk into the surf and the water will feel warmer than the sand - because that silica has barfed up its energy as soon as it couldn't be maintained, so now is (I'll just pull out a number) only 60 degrees F. In this case, the water is warmer than the sand, so it feels warmer (because it is).

So even though the water temperature is constant at 72 F, the sand changes - so the *apparent* temperature of the water changes.

Many school kids prove this in 4th grade by getting one bowl of room temperature water, one of ice water, and one of warm water. You put one hand in the ice water, one hand in the warm water, give them time to change temperature, then put both hands into the room temperature water - the "hot" hand will feel cold, the "cold" hand will feel cool.

It's the same at the beach!

 

[kagmi]

Part of it has to do with how our body interacts with the sand and the water. Water often feels much cooler than a gas or solid, even if they're the same temperature, because water carries heat away from the body faster. What our bodies feel when they sense temperature is whether heat is being added or taken away from us. Water cooler than 98.6 degrees takes heat away from our body, so it feels cold even at the temperature of 70 or 80 degrees--even though air might feel warm at the same temperature!

Water also retains heat better than the tiny grains of sand. Water, even among liquids, is especially good at retaining heat. So when you warm it up during the day it takes a long time to warm up--but it also takes a long time to cool down at night. That's actually why coastal towns tend to have milder weather; water retains fairly stable temperatures between seasons, and it can warm or cool the air around it to keep temperatures stable.

Probably what you're feeling is that in the daytime, water feels cold in comparison to the sand, which may have been heated up quickly by the sunlight. But at night, water retains its daytime temperature, while the sand loses its heat as quickly as it gained it.

Hope this helps!