Alan Stahler: Two-faced storms
You’ve just met someone at a party and, naturally, you want to make a good impression.
It’s a warm evening, and you’re slightly sweaty. As sweat evaporates, it changes from liquid to gas, and wafts into the air.
It takes energy to evaporate water, from liquid to gas, so when sweat evaporates, it sucks up energy — it cools you off.
Your new acquaintance holds a glass of ice water. Water vapor in the air condenses into drops of water on the glass.
Some of that water vapor came from your sweat.
Adding insult to injury, even as evaporation cools you off, condensation — water turning from gas to liquid — releases heat, warming things up. What had been your sweat is now warming your new friend’s drink.
Dear Abby: Should you apologize, or pretend not to notice?
When water evaporates into the air above Earth’s surface, it carries off energy. If the air is warm, it rises. As the air rises, it cools. As air cools (as it does near a glass of ice water) water vapor in the air condenses to form cloud droplets.
As water vapor condenses, it releases energy, re-warming the air.
Re-warmed air rises yet higher. The rising air again cools; more water vapor condenses, forming more clouds, releasing more energy, re-warming the air yet again. The re-re-warmed air rises higher still. The cycle goes ‘round and ‘round; the cloud climbs higher and higher.
Updrafts — fast-rising air — carry raindrops to the higher, colder regions of the cloud. The raindrops freeze, and fall. But the updrafts blow them back up, to be coated with a fresh layer of ice. They fall, blow back up, over and over, growing larger and larger with more layers of ice. ‘Round and ‘round, until they fall to the ground as hail.
By processes still debated (though no doubt sharing some physics with doffing sweaters and petting cats on a dry winter’s day), these huge storm clouds become electrified. Air is a good insulator, but when the voltage (electrical “pressure”) rises high enough, a bolt of lightning crashes through the air.
The air carrying a lightning bolt can reach 55,000-plus degrees Fahrenheit — more than is needed to ignite wood.
The conditions that cause clouds to grow, to give birth to hail and lightning, also cause fire-quenching rain. But if the air beneath the cloud is dry, the rain may evaporate before it reaches the ground. Seen from a distance, evaporating rain — virga — creates a dark region beneath the cloud.
Evaporation sucks up heat, and cools the air. Cool air sinks. Virga can create powerful downdrafts. When such downdrafts hit the ground, they spread out sideways, becoming winds that fan the flames.
Summer in the Foothills is droughty; winter is (usually) wet. Spring and fall, conditions are right for thunderstorms. Rainy thunderstorms are amazing natural spectacles. Dry thunderstorms are another matter.
Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages — kid to adult — and may be reached at email@example.com.
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