High above our heads, an unseen traffic jam is occurring as meandering jet streams cause eastward atmospheric circulations to become blocked, a process that can in turn create extreme events such as heat waves. This phenomenon closely mimics the dynamics of traffic congestion on a highway, according to a new study. Jet streams that propagate at high altitudes strongly influence weather patterns. If a jet stream develops large and persistent meandering, it can create a condition called atmospheric blocking. The onset of blocking is still poorly understood, as is the potential for climate change to affect blocking, and the related uncertainty remains a challenging problem for accurate weather prediction. Now, Noboru Nakamura and Clare Huang have developed a new framework for understanding the nature of atmospheric blocking. They had previously developed a metric for assessing jet streams (which, for example, can distinguish between jet stream waves propagating, and true blocking), and, here, they combined this metric with mathematical theory to understand the processes underlying blocking. Based on their work, jet streams that become blocked appear to be very similar to traffic jams, in terms of flow and density of vehicles. In light traffic, most cars travel at or near the speed limit so the traffic flow is proportional to traffic density; however, as the traffic becomes heavier, this reduces the flow of traffic - a scenario analogous to atmospheric blocking. Blocking can do more than just slow eastward winds, such as in 2012 when a highly meandering jet stream steered Superstorm Sandy on an unexpected, westward path towards New Jersey. Towards understanding the impacts of climate change on blocking, the authors suggest climate change has the potential to alter the frequency of atmospheric blocking, specifically by altering the jet stream's tendency to hit capacity and become "jammed;" indeed, a decrease of blocking may already be occurring in the Pacific, where increased eastward wind may have rendered the jet stream less saturated.