SkS Analogy 17 – Lotteries, evaporation, and superstorms
Posted on 24 January 2019 by Evan, jg
Evaporation is like a lottery. Lottery winners finance our storms.
Each day 10,000 people each put $100 into a million-dollar lottery where winner takes all. Because lottery winnings are taxed, the winner receives $600,000 and the government receives $400,000. This leaves the people who bought the lottery tickets poorer, one person richer, and the government more powerful.
A puddle of water loses water due to evaporation. Evaporation occurs when many neighboring molecules share a little of their energy with a single molecule that gains enough energy to leave the puddle and fly away into the atmosphere. This molecule wins the lottery. The high energy required to send a single water molecule into the atmosphere causes the energy of the other molecules in the puddle to decrease (i.e., the temperature of the puddle decreases), while the amount of water vapor in the atmosphere increases. A fraction of the water vapor in the atmosphere is available to storms, so that as the total amount of water vapor in the atmosphere increases, the amount available to storms also increases (i.e., the fraction of a larger number means more is available). Because storms get their power from condensing water vapor, as more water vapor is added to the atmosphere more water vapor is available to storms, which increases their power. Therefore, evaporation leaves the puddle with a less energy, the atmosphere with more water vapor, and storms with more power. This is how evaporation keeps Earth cool and supercharges storms.
For each 1°C increase in global temperatures the atmosphere holds about 7% more water vapor. It takes a lot of energy for 1 kg of water to go from the liquid to gas phase. With the energy it takes to vaporize 1 kg of water, you could heat 5.4 kg of water from 0 to 100°C! For perspective, if you weighed 150 lb, an extra 7% for each 1°C warming translates into the following weight gain.
Starting weight: 150
|150 + 7% increase (1°C warming)||161|
|161 + 7% increase (2°C warming)||172|
|172 + 7% increase (3°C warming)||184|
|184 + 7% increase (4°C warming)||197|
Think about how much energy it takes to vaporize an extra 7% more water vapor for each 1°C of global warming. And now all of that extra energy is floating around above your head, waiting to do the other thing that high-energy water vapor does well: supercharge storms. That is, the large amount of energy used to vaporize water is released when the water vapor condenses to the liquid phase in a cloud to form rain, snow, sleet, or hail. Higher-energy storms produce larger hail, stronger winds, more powerful tornadoes, and stronger hurricanes.
Here is another angle to the physics of evaporation. As noted, when water evaporates, overall the temperature of the system drops. By warming back up to the starting temperature, the system as a whole absorbs energy. There are at least three processes that allow Earth to absorb energy without any increase in air temperature:
- During a La-Nina cycle the temperature of the oceans can increase without increasing air temperature. Although subsequent El-Nino cycles dump the stored energy into the atmosphere, during a La-Nina cycle the temperature of the atmosphere can remain relatively stable or even decrease.
- Ice melts with no increase in the temperature of the local water-ice mixture.
- Water evaporates with a decrease in the temperature of the evaporating water.
The important point is that while each of these three processes occur, which do not result in the air temperature increasing, the total energy of the Earth continues to increase. Somehow, somewhere, at some time this extra energy will cause problems.