XKCD's Randall Munroe is publishing a book of science explainers! What If?: Serious Scientific Answers to Absurd Hypothetical Questions is due out September 2. To celebrate its imminent arrival, here's the first chapter from the book – an explainer about global devastation. Fun!
Q. What would happen if the Earth and all terrestrial objects suddenly stopped spinning, but the atmosphere retained its velocity?
A. NEARLY EVERYONE WOULD DIE. Then things would get interesting. At the equator, the Earth's surface is moving at about 470 meters per sec- ond —a little over a thousand miles per hour —relative to its axis. If the Earth stopped and the air didn't, the result would be a sudden thousand-mile-per-hour wind.
The wind would be highest at the equator, but everyone and everything living between 42 degrees north and 42 degrees south—which includes about 85 percent of the world's population —would suddenly experience supersonic winds.
The highest winds would last for only a few minutes near the surface; friction with the ground would slow them down. However, those few minutes would belong enough to reduce virtually all human structures to ruins.
My home in Boston is far enough north to be just barely outside the supersonic wind zone, but the winds there would still be twice as strong as those in the most powerful tornadoes. Buildings, from sheds to skyscrapers, would be smashed flat, torn from their foundations, and sent tumbling across the landscape.
Winds would be lower near the poles, but no human cities are far enough from the equator to escape devastation. Longyearbyen, on the island of Svalbard in Norway — the highest-latitude city on the planet — would be devastated by winds equal to those in the planet's strongest tropical cyclones.
If you're going to wait it out, one of the best places to do it might be Helsinki, Finland. While its high latitude — above 60°N — wouldn't be enough to keep it from being scoured clean by the winds, the bedrock below Helsinki contains a sophisticated network of tunnels, along with a subterranean shopping mall, hockey rink, swimming complex, and more.
No buildings would be safe; even structures strong enough to survive the winds would be in trouble. As comedian Ron White said about hurricanes, "It's not that the wind is blowing, it's what the wind is blowing."
Say you're in a massive bunker made out of some material that can withstand thousand-mile-per-hour winds.
That's good, and you'd be fine . . . if you were the only one with a bunker. Unfortunately, you probably have neighbors, and if the neighbor upwind of you has a less-well-anchored bunker, your bunker will have to withstand a thousand mile-per-hour impact by their bunker.
The human race wouldn't go extinct.  In general, very few people above the surface would survive; the flying debris would pulverize anything that wasn't nuclear-hardened. However, a lot of people below the surface of the ground would survive just fine. If you were in a deep basement (or, better yet, a subway tunnel) when it happened, you would stand a good chance of surviving.
There would be other lucky survivors. The dozens of scientists and staff at the Amundsen–Scott research station at the South Pole would be safe from the winds. For them, the first sign of trouble would be that the outside world had suddenly gone silent.
The mysterious silence would probably distract them for a while, but eventually someone would notice something even stranger:
As the surface winds died down, things would get weirder.
The wind blast would translate to a heat blast. Normally, the kinetic energy of rushing wind is small enough to be negligible, but this would not be normal wind. As it tumbled to a turbulent stop, the air would heat up.
Over land, this would lead to scorching temperature increases and — in areas where the air is moist — global thunderstorms.
At the same time, wind sweeping over the oceans would churn up and atomize the surface layer of the water. For a while, the ocean would cease to have a surface at all; it would be impossible to tell where the spray ended and the sea began.
Oceans are cold. Below the thin surface layer, they're a fairly uniform 4°C. The tempest would churn up cold water from the depths. The influx of cold spray into superheated air would create a type of weather never before seen on Earth — a roiling mix of wind, spray, fog, and rapid temperature changes.
This upwelling would lead to blooms of life, as fresh nutrients flooded the upper layers. At the same time, it would lead to huge die-offs of fish, crabs, sea turtles, and animals unable to cope with the influx of low-oxygen water from the depths. Any animal that needs to breathe — such as whales and dolphins — would be hard-pressed to survive in the turbulent sea-air interface.
The waves would sweep around the globe, east to west, and every east-facing shore would encounter the largest storm surge in world history. A blinding cloud of sea spray would sweep inland, and behind it, a turbulent, roiling wall of water would advance like a tsunami. In some places, the waves would reach many miles inland.
The windstorms would inject huge amounts of dust and debris into the atmosphere. At the same time, a dense blanket of fog would form over the cold ocean surfaces. Normally, this would cause global temperatures to plummet. And they would.
At least, on one side of the Earth.
If the Earth stopped spinning, the normal cycle of day and night would end. The Sun wouldn't completely stop moving across the sky, but instead of rising and setting once a day, it would rise and set once a year.
Day and night would each be six months long, even at the equator. On the day side, the surface would bake under the constant sunlight, while on the night side the temperature would plummet. Convection on the day side would lead to massive storms in the area directly beneath the Sun. 
In some ways, this Earth would resemble one of the tidally locked exoplanets commonly found in a red dwarf star's habitable zone, but a better comparison might be a very early Venus. Due to its rotation, Venus — like our stopped Earth — keeps the same face pointed toward the Sun for months at a time. However, its thick atmosphere circulates quite quickly, which results in the day and the night side having about the same temperature.
Although the length of the day would change, the length of the month would not! The Moon hasn't stopped rotating around the Earth. However, without the Earth's rotation feeding it tidal energy, the Moon would stop drifting away from the Earth (as it is doing currently) and would start to slowly drift back toward us.
In fact, the Moon — our faithful companion — would act to undo the damage Andrew's scenario caused. Right now, the Earth spins faster than the Moon, and our tides slow down the Earth's rotation while pushing the Moon away from us.  If we stopped rotating, the Moon would stop drifting away from us. Instead of slowing us down, its tides would accelerate our spin. Quietly, gently, the Moon's gravity would tug on our planet . . .
. . . and Earth would start turning again.