ASTRO BOB: Dorian versus Jupiter’s red spot

Enormous by earthly standards, Dorian spans a small fraction of its big brother, the Great Red Spot.

Hurricane Dorian became a monster Category 5 hurricane. When it perched over the Bahamas sporting with crisp eye and a diameter of 280 miles, the storm sustained winds of 185 mph with gusts to 220 mph!

That is wider than most of Minnesota. Photos showing the storm parked off the Florida coast give one pause. It’s a BIG DEAL. Still, other storms that have ravaged the western hemisphere have been larger, like Irma in Sept. 2017, which swelled to 420 miles across, wider than any state except Texas, Alaska and Montana. That set a Florida record for size.

But when it comes to record storms, Jupiter is still king. It’s the proud owner of the Great Red Spot (GRS), an enormous anticyclonic storm in the planet’s southern hemisphere where the winds flow in the opposite direction of those found in southern hemisphere hurricanes. Hurricane winds spin counterclockwise in the northern hemisphere and clockwise in the southern. Winds in the GRS can reach speeds up to 300 mph and are strongest not in the spot’s center but around its edge.

As for size, you may have read that the GRS, an iconic feature that appears to have been around for some 350 years (based on observations with some of the earliest telescopes), has been shrinking in the past few decades. While true, it’s still 1.3 times the size of the Earth or 10,290 miles across. That’s big enough to see in a 4-inch telescope on a good night at Jupiter’s average distance of 484 million miles.

But compared to Hurricane Dorian, the Big Red Hurricane is nearly 37 times larger. There are several reasons it has been around so long. The spot sucks wind energy from jet streams running along either side of it and taps into vertical air flows that bring warmer air up from below and remove colder air on top. It also eats up other smaller oval storms like it, absorbing their energy.


And unlike a hurricane, which sooner or later makes landfall and loses energy, the Great Red Spot lacks the option — there’s no land on Jupiter to bump into! Like the Rolling Stones song “Jumpin’ Jack Flash,” Jupiter’s just a “gas, gas, gas!” With a whole bunch of clouds.

If there is something solid, it’s deep down in the planet’s core.

Despite its smaller size, Dorian is large not only because it’s close and present, but it’s a big storm for a small planet!

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