Science at the beach: Hot sand, cool water

Have you ever walked across the sand at the beach on a hot summer day? It’s hot, right? Good thing you can cool off your feet by dipping them in the water.

(Photo via Shutterstock)

But wait? Why is the sand so much hotter than the water? Try this simple at-home experiment to find out.

Materials  

• 3 plastic containers (bonus if they are upcycled!)  

• A measuring cup  

• Room temperature water  

• Soil or sand  

• A thermometer  

Directions  

(Photo by Brittany Schaller)

1. Measure ½ cup of water and pour it in one container. Measure ½ cup of soil or sand and pour it in the other container. Then carefully measure out ½ cup of air and transfer it into the last container. Wait, measure air?! Okay, you are going to leave the last cup empty. But is it really empty? No, it is full of air!   

2. Place all three containers in the refrigerator to cool. After 1 or 2 hours, take the containers out of the refrigerator and record the temperature of each material on a sheet of paper.   

3. Place the containers in the sunlight.

4. Record the temperature of each material after 15 minutes.

5. Record the temperature of each material after 30 minutes.   

In real life 

Sunlight heats soil and sand faster than water. This is because land is darker than water and retains heat. Heat is able to travel through water more easily and can spread out farther. Soil keeps the heat on its surface.   

Think about a hot day at the beach. When you take off your shoes, the sand might burn your feet. But when you start digging a deep hole — the deepest hole anyone has ever dug on that beach — how does the temperature change inside the hole? You probably discovered that it is much cooler and damper than the hot, dry surface. That's because sunlight can't pass through it.   

But what about water? The specific heat of water is higher than sand or soil. This means it takes more heat to raise the temperature of water than it takes for the same amount of soil or sand. So, on a hot summer day at the beach, you can cool your feet off in the water after walking on the scorching hot sand.   

The "empty" glass, of course filled with air, loses its heat more rapidly than water. The container with water keeps the cold longer than the container of air.   

Think about a hot day in early spring. The first day when you get to leave your winter coat at home and wear a pair of shorts. The sun is shining, and it feels so good on your face. Would this be a good day for swimming? Absolutely not! Swimming on a day like this is very dangerous.  That's because water temperatures increase much more slowly than air temperatures.