Hot Water and Hurricanes
Part A: Earth's Ocean: A Storage Unit for Heat
Three-quarters of Earth's surface is covered with a substance that is a very effective heat-storage materialwater. Liquid H2O can absorb and store a tremendous amount of heat energy without becoming too hot itself. The effectiveness of a substance at storing heat energy depends on a parameter called specific heat.
Specific heat is a measure of how much energy something absorbs compared to how hot it gets. More precisely, the specific heat of a substance is the amount of energy it takes to raise the temperature of 1 gram of that substance by 1 degree Celsius.
Quantifying Specific Heat
Quantitative experiments show that 4.18 Joules of heat energy are required to raise the temperature of 1g of water by 1°C. Thus, a liter (1000g) of water that increased from 24 to 25°C has absorbed 4.18 J/g°C x 1000g x 1°C or 4180 Joules of energy. For comparison, alcohol (ethanol) has a lower specific heat: it takes only 2.2 Joules of energy to increase the temperature of one gram of ethanol by one degree Celsius.
To calculate the amount of heat energy gained or lost by a substance, multiply the mass of the substance by its specific heat constant multiplied by the change in temperature.
q = m x C x ΔT
q = heat energy in Joules (J)
m = mass of the substance in grams (g)
C = specific heat for that substance in Joules per gram per degree Celsius (J/g°C)
ΔT = change in temperature in degrees Celsius (°C) Change is calculated by subtracting initial temperature from final temperature (Tf - Ti)
Stop and Think1. Consider that the tropical ocean is exposed to direct sunlight for many hours every day. If the top 1 cm of a 100km2 area of water warms from 25 to 27 degrees, how much heat energy has the water absorbed?
mass of water = 1g/cm3 x (1cm x 10cm/m x 1000m/km x 100 km) x (1cm x 10cm/m x 1000m/km x 100 km) x 1 cm
Use your answer in the heat energy equation.
Where does all that heat energy go?
The ocean is constantly moving. Currents of water move through the ocean powered by global winds and differences in density. Generally, heat energy gained by water in the the tropical regions moves toward the poles.
Though we can't see currents in photographic images, we can detect them in sea surface temperature (SST) images like the one on the right. Instruments on several Earth-orbiting satellites gather data that is processed to show the surface temperature of water. Animating a series of these images allows us to visualize the moving current.
Click this link to see an animation of Sea Surface Temperatures (will open in a new window). Note the Gulf Stream Current moving north in the Atlantic Ocean and the entire Atlantic Ocean warming up as the seasons change. As you watch the animation, mentally visualize the current as a river of heat energy.