Imagine blowing a soap bubble ... a rather large soap bubble, twenty miles across. In the center of our humongous bubble floats a grapefruit. Flying around the grapefruit – some flying close, some staying far out – are fruit flies ... perhaps just one, perhaps dozens.
Now let’s burst the soap bubble, leaving us with a grapefruit surrounded by fruit flies, flying near and far.
What we’ve made here is a scale model of an atom. The grapefruit is the atomic nucleus; it takes up very little space in the atom. The fruit flies represent electrons, surrounding the nucleus; they take up even less space.
Atomic nuclei and electrons are both electrically-charged: Nuclei are electrically positive; electrons, electrically negative.
Opposites attract , which is why nucleus and electrons hang together. The positive and negative charges balance, keeping the atom as a whole electrically neutral.
Some atoms hold onto their electrons more tightly than others. The atoms in wool hold onto electrons more tightly than the atoms in skin. When we wear a wool sweater, the electron-hungry atoms in the wool soak up electrons from our skin. Having lost electrons, our skin is no longer neutral ... it has a positive charge. When we doff the sweater, our positively-charged skin pulls hard on the electrons the wool has stolen. The wool cannot hang on to those electrons, and they jump back, back onto our skin. Flying through the air, the electrons smash into atoms in the air, making the atoms glow. We can see those glowing atoms of air, as sparks, in a dark room.
Those electrons really “want” to get back to our skin. If only we could harness them as they fly back, we could put them to work.
Harnessing sparks is a bit unrealistic, but we do harness the desire of atoms to snatch electrons: We put such atoms into a can, to make a battery. And we pull electrons off atoms in power plants and in solar panels, to generate electricity.
Nature, too, pulls electrons from atoms ... most dramatically, in a thunderstorm.
To build a thundercloud, the atmosphere must be unstable: Warm air must lie beneath cold. That warm air wants to rise; allowed to rise, it will build into a towering thundercloud.
All last weekend, the air was not unstable, but what meteorologists at the National Weather Service in Sacramento referred to as “conditionally unstable.” The air below was almost warm enough to rise into the cooler air above ... but conditions were not quite right. The air needed more energy – more warmth – if it were to rise.
There was white stuff falling, off and on, over the weekend.
Snow falls softly, silently. But most of what fell last weekend was not soft; it bounced and rattled noisily when it hit windows and clothing. It was not snow.
Snow forms slowly. Suspended in a cold cloud, ice crystals are “thirstier” than cloud droplets. Molecules of water evaporate off the droplets, and condense – molecule-by-molecule – on the tiny ice crystals, building intricate snowflakes.
If the temperature within a cloud falls slowly, the temperature of the cloud droplets may fall below freezing ... but the droplets don’t freeze – they become, instead, “super-cooled.” Super-cooled droplets are just waiting for an excuse to freeze.
Such an excuse comes when an ice crystal bashes into the droplets. Then the droplets freeze instantly, cementing themselves to the crystal. As more and more droplets freeze onto the icy mass, they create graupel … AKA soft hail ... AKA snow pellets. It was mostly snow pellets that were falling last weekend.
To melt ice, we pump energy – heat – into it. The melting ice turns from solid to liquid.
Same trick backwards: When liquid water – say, a cloud droplet – freezes, liquid to solid, it pumps energy – heat – out, into the air. When cloud droplets froze onto snow pellets, last Sunday, they pumped enough heat into the air to allow the air to rise and form thunderclouds.
Thunderclouds pull electrons off atoms; they send those negative electrons one way, and send the positive, electron-starved atoms another. Just how they do this has been debated for over a century.
When the electrons jump back onto their atoms (or jump to the ground), that spark is lightning. Lightning instantly heats the air around itself; the heated air expands, creating a shock wave: Thunder. Thunder rumbled over the foothills, Sunday afternoon.