Define jet stream

James Partain, science and operations officer at the Marine Prediction Center (part of the National Centers for Environmental Prediction) in Camp Springs, Md., replies: "The earth's atmosphere contains two major 'jet streams' (one in each hemisphere). They have considerable impact on human affairs. As we often hear during the weather segment of the nightly news, the jet streams are related to weather patterns of high and low pressure. And airline pilots are well aware of the consequences of being in or near the jet stream in an aircraft. Detailed knowledge of the jet stream--its location, altitude and strength--is therefore critical to modern-day weather forecasting, as well as to more specific applications such as the safe and efficient routing of aircraft. "By way of definition, a jet in fluid dynamics is simply a core (or 'stream') of fluid moving at a higher velocity than the surrounding fluid. Although they are complicated to describe mathematically, the jet streams in the atmosphere are a straightforward, natural result of the meridional (that is, equator-to-pole) temperature gradient in the earth's atmosphere. Analogous flows exist on other planets with substantial atmospheres having similar temperature gradients. "The temperature gradient derives from the differential solar heating of the spherical surface of a planet: the surface is generally warmest at the equator and grows progressively cooler as one moves poleward. The centrifugal effects of the earth's rotatio...

The past few months have seen some remarkable weather, from the UK’s Key to this unseasonable weather are persistent high-pressure “blocking” weather systems, which bring clear, dry conditions on the ground below for many days or weeks. Blocking events bat away oncoming low-pressure systems that would bring the prospect of clouds and rain. They are particularly synonymous with heatwaves and drought in summer and bitterly cold conditions in winter. But what are the prospects for blocking events in a warming climate? And could a rapidly warming Arctic also have a role to play? In this Q&A, Carbon Brief takes a closer look at the causes of blocking events and the potential changes in the future. What is ‘blocking’? Around the mid-latitudes of the northern hemisphere, low-pressure weather fronts, which bring cloudy, windy and potentially wet weather, generally move from west to east. These are carried along by the The jet generally keeps a steady stream of weather systems moving across the Earth’s surface. This means that any low-pressure system – or intervening high-pressure system that brings clear, still and sunny conditions – will generally only linger for a matter of days before being shunted on by the next system. However, sometimes weather systems can get stuck in place for an extended period of time. This is known as “blocking”, explains “Blocking is a stationary and persistent weather pattern, most often an anticyclone [high-pressure system], that blocks the oncoming ...