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Step 1: What kind of measurement do you want to compare?
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Equivalents in other units

How fast is 391 meters per hour?

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It's about one-nine-hundred-thousandth as fast as a bolt of Lightning.
In other words, 391 meters per hour is 0.0000011 times the speed of a bolt of Lightning, and the speed of a bolt of Lightning is 910,000 times that amount.
(atmospheric electrical discharge)
A bolt of lightning can travel at up to 350,000,000 meters per hour when arcing from the atmosphere to the Earth. An estimated 280 million lightning strikes strikes occur each year.
It's about 2,500,000 times as fast as Grass growing.
In other words, the speed of Grass growing is 0.00000037 times 391 meters per hour.
(for Kentucky Bluegrass, Poa pratensis) (a.k.a. Smooth Meadow grass, a.k.a. Common Meadow grass) (Summer months, ideal growing conditions)
Kentucky Bluegrass, one of the most common grasses in North America, grows about 0.00015 meters per hour in the summer months under ideal conditions. When left un-mowed, Kentucky Bluegrass grows to heights of 60 to 90 cm (2 to 3 ft) and blossoms with a blue flower.
It's about 0.0000000004 times as fast as The Speed of Light.
In other words, 391 meters per hour is 0.00000000036228767 times the speed of The Speed of Light, and the speed of The Speed of Light is 2,760,237,470 times that amount.
(C) (in a vacuum)
The speed of light (and all other electromagnetic radiation) in a perfect vacuum is known as "c" and measures exactly 1,079,252,850,000 meters per hour, per the formal definition of the meter. While visible light travels close to this speed in air, it slows to 719,501,899,000 meters per hour when passing through the glass of a typical window.