09/25 Atmospheric dynamics: Fire, then ice

Wild weather spans the world

ON SEPTEMBER 7TH in Denver, Colorado, the temperature reached 34°C (93°F), 6°C above what is normal for the time of year. The city was sitting under the dome of hot air encouraging record fires across the American West. The next day snow started to fall. By midnight the temperature was below freezing. What happened?

The immediate cause was the polar jet stream, a world-girdling high-altitude wind driven by temperature differences between Arctic air to the north and warmer air to the south. Its meandering path is set by patterns of high and low pressure known as Rossby waves. And because fluid dynamics are never simple, the jet stream exerts its own influence in turn upon these guiding waves.

As the jet stream passed to the north of the high pressure over America’s west coast, the big temperature difference between that hot air and what lay farther to the north added…