Evaporation: If Water Were Olive Oil, We’d All Be Fried

Lysichiton_americanumCroppedOn Friday my dad had to go to the clinic. I took him there. They said his blood pressure was low–really low, and I should get him an ambulance for the ride to the emergency room half a mile away. But I drove him instead.  I parked, slammed my door, ran for a wheelchair, rotated him into it, slammed his door, ran him to the ER’s front desk, and began the wait. By then his blood pressure was up to 90 over something, closer to acceptable, and so comparatively we weren’t an emergency anymore. They put us in an overflow room with a door we couldn’t close because it would lock–it was for psychiatric cases. From there we saw dramas file by. I even saw people I knew.  Dad and I had nice talks, dad lying there, me sitting, noise pouring in our open door. After about 2 hours I went outside where there was cell reception to call a brother in another state. As I paced, I saw that I’d left the engine running and the air conditioning on–I, who turn the engine off to save’s Earth’s resources if the engine will be idling for more than a half-minute (I’ve read 10 seconds is the threshold, actually).  And smack under the car was a huge puddle from the AC.

And that’s brings me to the point: evaporation. Have you ever been amazed, like me, at how quickly rainwater goes away? It can pour all day and all night. Only some of that water infiltrates or runs off, but next thing you know, the road is dry, the sidewalk is dry, every surface is dry. Or you put a swimsuit on a line–it dries. You take a shower–and your hair is dry in a few hours, the shower is dry when you get home. You sponge a countertop and it dries.  If it had rained olive oil, if you’d showered with olive oil, or sponged the countertop with olive oil, there’d be goop for days to come.  The air has an amazing capacity to accept water (we say “it has a large vapor pressure deficit”) and we depend on it every day.

An aside to my aside is that when water evaporates, the object that dried gets cooled. (Your skin surface has sweat, there’s wind, the sweat dries up, and your skin surface becomes cooler).  Liquid water molecules hold on to one another with hydrogen bonds. Imagine you’re a water molecule (two hydrogens and an oxygen), and you’re sitting on the ground with your legs outstretched for an exercise.  Your butt is a big oxygen atom, and your feet are these tiny hydrogens. The hydrogens have positive charges on them by their nature. You, as a whole, are neutral, so that leaves your butt with some negative charge. Hydrogen bonds are the weak bonds where the feet of one water molecule are attracted to the butt of another: feet touching butts, butts touching feet, in a disorganized tangle that’s liquid water. Because of the atmosphere’s vapor pressure deficit, it can accept water (evaporate it). Energy has to be absorbed by the liquid to break those hydrogen bonds, which is why a surface cools as its water evaporates.  Go into a darkened restaurant that’s decorated with plants. Are they real?  Grab a leaf (I do it all the time). If it’s cool, the plant’s real (it’s transpiring, meaning evaporating water).  If it’s not, it’s almost certainly not a real plant.

Plants rely on drying to cool themselves off. That’s why plants with huge leaves (think skunk cabbage–pictured, water lilies, Monsteras . . . ) have to live in very wet places: without water to cool off, they’d overheat and fry. (Smaller leaves have smaller ‘boundary layers’ so when a slight breeze blows they cool off.)  And the Earth relies on evaporation to cool off, too–from oceans, which are 70% of the surface, and from land including the plants. If oceans were olive oil, we’d all be fried.

But the point of my story is that when I rolled dad (with antibiotics) out in the wheelchair to take him home, I didn’t have to admit I’d left the engine running and the keys in the ignition. The puddle was gone.

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