Christopher J. Petrone, Virginia Sea Grant, Virginia Institute of Marine Science
The weekend finally arrives and you find yourself in your bathing suit standing inches from the breaking waves. The air temperature is in the mid-70s, but you’re at the beach, so you must brave the coldest of water temperatures to get your money’s-worth out of the trip. Finally, you take a deep breath, grit your teeth and run full speed into the water expecting it to feel like the Arctic the instant it touches your skin.
Once submerged, you come up for air and are ready to run out just as fast as you ran in, and curl up in your beach towel. But wait! You soon realize that the water is not cold at all, but instead, actually warmer than the air all around you. As you continue to splash around, riding waves and swimming, you start to ponder how this is even possible. How could the water actually be warmer than the air after it’s been so cool out the past few days? The difference has everything to do with the heat capacities of the two substances.
Heat capacity is the amount of heat required to raise the temperature of a object by 1 degreeC without changing the state of matter. It is measured in Joules/degreesC and its value is proportional to the amount of material in the object; for example, a lake has a greater heat capacity than a puddle.
The specific heat is the actual quantity of heat energy required to raise 1 gram of a substance 1° C and it is typically measured in J/g degreesC. Water has a much higher heat capacity, and specific heat, than air, meaning it takes more energy to heat water than it does to heat air. Water has a specific heat of 4.186 J/g degreesC, versus air, which has a specific heat of 1.005 J/g degreesC.
In the beach scenario above, the water was actually warmer than the air, despite the recent lower air temperatures. This is because of water’s much higher heat capacity than air; and because of its higher heat capacity, it takes longer for water to gain and lose heat (cool), than it does for air. In both cases, either heating or cooling, there will be a lag between the air and water temperatures. Because of this, you may also find chilly water temperatures in early summer, even though the air temperature has been in the 80s and 90s for weeks.
The heat capacity of water has tremendous effects on the climate of the surrounding area. Because the water buffers the air temperature, the range of air temperature near water bodies is often smaller than the air temperature range further from large bodies of water. On a greater scale, because the ocean occupies over 70% of the Earth’s surface, it buffers the atmospheric temperature, providing a livable climate.
In addition to keeping the Earth’s atmospheric temperature in check, water’s high heat capacity has numerous practical applications for humans. We use water to prevent engines from overheating in automobiles, boats and power plants. This is also why water is used in fire fighting; it absorbs the heat of the material it comes in contact with, dissipates the heat as it changes from liquid to gas, and actually lowers the temperature of the fire. At the same time, the water increases the heat capacity of the material, making it harder for the fire to burn the material. The human body even benefits from water's high heat capacity when we sweat!
You often come in contact with materials that have different heat capacities. Perhaps you have walked home from the beach on a hot, sunny day without wearing shoes. The sand is scorching, so you quickly walk to the street, which you find is also hot, so you move to the sidewalk, which may be only slightly cooler, so you end up on the grass, which is the coolest. These materials each have very different heat capacities. Although they are all subject to the same sun exposure, they all store the thermal energy at different rates and thus radiate different temperatures to your bare feet.
The heat capacity of a material is very carefully considered in the construction of houses and other buildings. The ability of a material to collect and tolerate heat and then effectively dissipate it is critical to ensuring the durability and safety of a structure, and the comfort of its inhabitants.
Using the information learned above, you will now explore air and water temperature data from four monitoring stations in Virginia along an inland-to-offshore gradient. Two of the stations are monitored by NOAA National Centers for Environmental Information. These are located in Amelia, VA and Petersburg, VA and students will use only air temperature data from these stations. The other two stations are ocean observing system buoys; one is located in Virginia's James River and is a part of the Chesapeake Bay Interpretive Buoy System, which is a part of the NOAA Chesapeake Bay Office. The second buoy is a NOAA National Data Buoy Center entity. From these two buoys, students will use both air and water temperatures.
A. General Analysis
B. Graphing and Graph Analysis
Using Figure 2, a blank graph, graph the temperature range data (Table 1, columns 7 and 8) by hand and then discuss trends as a class or in small groups.
*Note to teachers: If students are graphing by hand, instructions on how to create a floating stacked column graph, as seen in Figure 2a, may need to be given. If students do not need graphing practice, use the completed graph found here to discuss trends as a class or in small groups.
C. Additional Analysis
Answer the following questions after viewing Figure 3, air and water temperature from the Chesapeake Bay Interpretive Buoy System (CBIBS) buoy at Jamestown, VA from April 13-20, 2008.
D. Real Time Data Analysis: www.buoybay.org
Visit the CBIBS website and click on Get Data for one of the buoys. On the buoy page, click on Buoy Data to view the data.
Now click on the air temperature data for the past seven days at this buoy ("7" in the Graphs column at far right).
Repeat for water temperature data.
Discussion and Application Questions