In the early hours of Wednesday morning, with the temperature probably hovering around the freezing point, a team of engineers and military experts were gathered North Korea's South Pyongan province. Just before 3 a.m. they launched a missile into the sky. It flew for 54 minutes and reached a height of about 2,800 miles before crashing down into waters in Japan's exclusive economic zone.

The missile test, the North's first in 74 days, quickly raised a number of questions for the United States and its allies. But for many observers — particularly those in East Asia — a more immediate query sprang to mind: Why would Pyongyang test a ballistic missile in the middle of a bitterly cold night?

According to data compiled by Shea Cotton, a research associate at the James Martin Center for Nonproliferation Studies, North Korean missile tests tend to take place mid-morning. Between 9 and 11 a.m. has been most popular, shows Cotton's data, which goes back to April 15, 2016.

There have, however, been a variety of tests at different times. North Korea has tested missiles a number of times during the afternoon, as well as in the evening.

No one knows exactly why North Korea spreads out its missile test timings like this. Cotton said that his best guess was that North Korea favored the mid-mornings because “that way, in case something goes wrong (like maybe the truck breaks down or a hose doesn’t work or whatever) they have a full day to try and make it right and get it to work.”

If a missile test fails and the missile blows up, “they have a full day of sun to try and collect the pieces and conduct some early failure analysis,” Cotton added.

Nighttime launches would seem less logical; working in the dark or the cold is clearly not ideal.

But these late tests may serve a broader strategy instead — perhaps showing that North Korea can launch a missile anytime and anywhere with little warning, as it would have to in a real wartime scenario, said Kingston Reif, director for disarmament and threat reduction policy at the Arms Control Association.

In particular, the timing of the missile test could also have been deliberately designed to surprise North Korea's neighbors and show how unpredictable a Pyongyang attack could be. “In terms of this test taking place at night, they might be trying to obscure the final launch prep from overhead surveillance to maximize surprise,” said Ian Williams, associate director of the Center for Strategic and International Studies Missile Defense Project.

More worryingly still, Reif said that North Korea's last two missile tests appeared to be designed at least in part to goad U.S. missile defense systems. The United States' primary system for defending the country from a North Korean ballistic missile attack is a ground-based midcourse system mostly based in Alaska and partially in California. "Among other shortcomings with the system, it has never been successfully intercept tested at night," Reif said. "In fact, there has been only one intercept test that has taken place at night.”

Whether North Korea's use of nighttime testing is deliberate is ultimately unclear, however. Domestic factors unrelated to a broader strategy do appear to affect Pyongyang's weapons testing — North Korean missile testing notably slows down in autumn when the country is preoccupied with a harvest, for example. But the fact that North Korea can pull off these tests at nighttime is probably one more sign of sophistication and the flexibility of its weapons program.

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