Alan Stahler: Hurricane Harvey
We build mostly with solids: wood and metal and stone. Only occasionally do we build structures — decorative water fountains, for instance — with liquids.
Rarely do we build things with gases.
Nature feels no such constraint. Nearly 300 miles across when it first made landfall, Hurricane Harvey was almost all gas — all air — with a bit of liquid water mixed in.
Look down on Earth from above the north pole to see Earth turning counter-clockwise (like the hands of a clock going backwards, five to four to three). This creates a “preferred” direction for air to spin: counter-clockwise.
When air spins in Earth’s preferred, counter-clockwise direction (seen from above the north pole), it is said to be cyclonic.
A storm swirling counter-clockwise in the northern hemisphere is, by definition, a cyclone. California’s winter storms are extra-tropical (“outside-the-tropics”) cyclones.
Hurricanes are tropical cyclones.
The sun shines bright on the tropics, warming the rocks, the soil, the water. The higher latitudes are warmed less by the sun; the poles are warmed least of all.
Pile anything too high and the pile collapses, spreads itself out.
As heat “piles up” in the tropics, some can escape by radiation (what you feel coming off a sun-warmed wall). Some heat flows out by conduction (the way the handle of a fry pan grows warm). But most tropical heat must be carried away, toward the poles, by air and water.
Cyclones have a job to do: Cyclones carry heat away from the tropics, toward the poles.
Picture a cyclone spinning counter-clockwise. Air on the western (left-hand) side of the cyclone blows from north to south, carrying colder air down from the north, down toward the tropics. Air on the eastern (right-hand) side blows south-to-north, carrying warmer air — carrying heat — up toward the poles.
And cyclones have another way to move heat.
Converting water from liquid to gas requires energy. When sweat evaporates, it sucks energy from our skin, and cools us down.
When seawater and soil water evaporate, the process pulls energy from sea and soil, cooling them down.
The gaseous water vapor that rises from sea and soil holds within itself the energy of evaporation. When the vapor moves north with the air, when it cools and condenses to become cloud droplets … that energy is released once more, warming the air around each droplet, injecting energy into the storm.
A category 3 hurricane has winds of around 120 miles per hour. Few people have experienced a 120 mile-per-hour wind, but many have experienced winds of sixty miles per hour, simply by putting a hand out the window of a car doing sixty.
One-hundred twenty miles per hour (category 3) is twice sixty. But winds of a hundred-twenty miles an hour wouldn’t push on your hand just twice as hard. When wind speed doubles, its energy increases, not by two, but by two times two, equals four. There is four times as much energy — a four-times-harder push — in a category 3, 120-mile-per-hour wind than there is in a wind of 60 mph.
A hurricane is steered by the air around it. After Harvey made landfall, its steering winds didn’t push it much of anywhere. The storm stalled — south Texas flooded. As I write on Monday, the winds are steering Harvey back toward the Gulf, where it can again absorb warmth and water vapor. Harvey is then forecast to make landfall once again.
Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at firstname.lastname@example.org
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