Over the ocean and through the air
By Joy Moses-Hall
Sunday, September 30, 2018
Greenville has dodged some latent heat.
Hurricane season is always dicey, and a near miss like the recent grazing by Florence, still bearing heavily on the lives of those a few rain bands away, brings a sense of how does this happen?
It comes down to the physics of latent heat.
Latent heat is the energy stored by things that are in a warm phase.
Vapors, such as steam, are warmer than their liquids, such as water, all else being equal. The same is true for liquids relative to their solids. Some of that warmth drives the molecules to behave differently.
Molecules of water flit about independently, tumbling over each other when poured in a glass, unlike the more orderly particles in ice. In water vapor, the molecules are as independent as liquid, following the shape of the container instead of keeping a rigid shape like cubes, but unlike the liquid phase, vapor particles are far apart, and moving much faster. That molecular speed is responsible for the warmth, and that is the crux of latent heat.
It’s no coincidence that hurricanes are a summer phenomenon. Hurricane molecules get their speeds from warm oceans, oceans with temperatures above 80 degrees, the warmer the better. Warm oceans kick off a cycle that feeds on itself: warm water evaporates.
In any ocean, all of the molecules are zipping around, some faster than others. In a warm ocean, more of them are going fast enough to spread apart and become vapor, and when they do, they leave the ocean behind and become water vapor in the atmosphere. Their speed energy rides with them, transferring from ocean to atmosphere.
Once in the air, those water vapor molecules float up. Turns out, water molecules weigh less than air molecules, and they float up, up, up. Up top, the breathable atmosphere is cool, so cool that water molecules convert back from vapor to liquid. So cool that the water condenses, or even freezes, onto tiny specks, of pollen, of dust, or of sand, such as fine grains lifting off the Sahara in northern Africa. The vapor condenses into clouds.
But with the evaporation reversal, with the slowing of the particles during condensation, comes a reversal of the latent heat. The speed energy is released from the vapor molecules as they slow down and return to liquid water. So the energy that started in the toasty ocean as fast water, becomes toasty air, moving the air particles farther, faster, warmer.
And warm air warms and evaporates more water, into more vapor, which floats up to add to more clouds, more rain, and heats up the air still more.
This cycle of increasing evaporation increasing clouds increasing evaporation increasing clouds builds up into a huge, spinning cloud dripping with condensation. As long as it stays over warm ocean, the handoff between evaporation and cloud building grows. And it’s a big ocean, big enough to grow a Florence.
Blizzards grow much the same way, but colder. Instead of wind and rain they grow snow and drifts out of warm ocean waters.
On Thanksgiving Eve in New York State in 1971, it was snowing when school let out, it was snowing as we packed the car, it was snowing as we followed the snow plow down the road in the dark. We could have waited for better conditions, but that wouldn’t be until about the third week in April. So we kept going.
The snow came thick and fast, crystals of hurricane rain. Like meteors of mashed potatoes, wads of snow latched on to the windshield and wipers. The headlights dimmed from the hurry of flurries, and the wipers’ frozen arms clumped uselessly over the slushy gravy on the glass. We couldn’t see a thing.
We could have stopped and brushed off the windshield, but then we would be snowed in until the third week in April. Instead, Dad opened the window.
“Hey!” Snow flew in the window like an amphibious assault by cold mac and cheese.
“Pass me the brush!” Dad called, peering around snowflakes the size of biscuits. Mom handed him the ice scraper, a long wooden handle with some tufts of broom on one end. He steered and veered with one hand and reached the brush out the window with the other, parrying the brush like a butter knife at the whippy buildup on the wipers. He still couldn’t see anything but an eyehole of onrushing marshmallow fluff.
The good news was that nobody else was on the road as we skidded and swerved and blindly found our way over the shiver and through the foods to Thanksgiving dinner.
Follow @jmoseshall on Facebook. Joy Moses-Hall teaches physics and astronomy at Pitt Community College. She has a PhD in oceanography and is the author of the novel Wretched Refuge.