Supervolcano

The film begins with a group of hooded people in caribou parkas riding through snow on snowmobiles. Arriving at a nearly buried building, inside they find a video recorded journal of a man who appears to be dying. The man in the video reports that the Yellowstone Caldera eruption has affected nearly everything in the United States, burying much of the country under several feet of volcanic ash.The film then goes back to five years before the incident, where tourists are seen viewing Old Faithful and exploring the hydro volcanic features of the Yellowstone National Park. Inside the visitor's center, the same man from the video journal, Rick Lieberman, a USGS scientist in charge of the Yellowstone Volcano Observatory (YVO), is seen presenting to a crowd on Yellowstone's seismic activity. He states during the presentation, using a fictional holographic projector known as Virgil, that Yellowstone is on the verge of an eruption, as he inputs the current data into the holographic program, a horrific image appears. Instead of the one or two vent idea that the team had envisioned when the volcano explodes, 5 plus vents appear and the holographic program shuts off saying that it is overloaded. They only glimpsed the 5+ vents for a few seconds which leave them significantly unnerved wondering how much truth there really was to what they saw.Later throughout the film, more and more signs of seismic activity occur, all pointing towards the imminent eruption of Yellowstone (such as geyser explosions, earthquakes, Old Faithful going silent, and a mass exodus of wildlife from Yellowstone), though Rick and most of his colleagues try not to cause public panic by saying that these seismic activities do not necessarily indicate an imminent volcanic eruption. However, when Jock Galvin's sarcastic mockery of the situation sparks constant media speculation, it raises public alarm, helped in part by a woman from CNN constantly pressing Rick to confirm the eruption for her reports, and by Rick Lieberman's brother-in-law, who is selling a book on supervolcanoes called 'Super Bangs'. As seismic activity increases, a leaked government news report acknowledging a possible eruption causes widespread panic.Rick's team is caught by surprise while researching at the USGS field office next to Yellowstone when the volcano violently erupts, spewing tons of rock and pyroclastic material into the sky. While trying to escape, two of his colleagues, Nancy and Matt, are killed as a result of attempting to outrun the pyroclastic flow, and only an injured Jock survives the eruption, escaping via helicopter. Rick and his brother-in-law are flying back from a conference when their plane flies through the ash cloud, flaming-out the engines and are forced to do a dead stick landing in Cheyenne, Wyoming. When the ash starts raining down on them on the ground, they take shelter in a nearby U.S. airbase.Tension begins to rise at the Federal Emergency Management Agency (FEMA) as more and more vents open above the underlying magma chamber throughout the eruption (just as the holographic program had predicted in the beginning of the film). On Day Three the ash destroys the second Y.V.O. base in Bozeman, killing another colleague, Dave. People are literally being drowned in the volcanic ash and the death toll rises to hundreds of thousands. Slowly, thermal images begin to reveal a contour of the new caldera produced by the vents and authorities begin to realize the seriousness of this problem. Very soon, the authorities become desperate, trying to find a way to save the trapped Americans in the central and western half of the country. At FEMA, Jock raises tension by saying that even after 2,500 cubic km of magma had come out, that it could still go on because Yellowstone has a maximum capacity of 25,000 cubic kilometers, ten times the amount already ejected. And Rick, while in contact with F.E.M.A., mentions that the entire volcanic system could completely rupture after the collapse. Luckily, things take a better turn just as all hope is nearly lost: after 1 week of eruption the crust above the magma chamber collapses, as a result of -and therefore indicating- the decrease in pressure within the magma chamber. The crust collapse seals the chamber, and a titanic blast is prevented.As it turns out, the film ends with three-quarters of the United States covered in nearly one centimeter of volcanic ash on average as a looming cloud of sulfur dioxide gets carried over the globe, engulfing the northern hemisphere of Earth, and as a result, plunging it into a volcanic winter.The film ends the same way it started, only this time, the man is revealed to be Rick, shown with his brother-in-law and a USAF Airman. He later flies in a helicopter back to Yellowstone to see what has happened to the volcano that he has been studying all his life. What he sees is a frozen landscape resembling Antarctica, where no vegetation or animals are visible; but he optimistically says that although it is the ending of much life, life would also begin as a result of this event.Finally, the last scene shows the camera panning out from the area where Yellowstone erupted in a series of satellite images, eventually showing the North American continent now a tundra, and the cataclysmically large resulting landform caldera relative to the size of Wyoming.

Question: What weather event settles across the northern hemisphere of Earth?
Answer: Volcanic winter