May 30, 2007
Jet Stream Forcast Explained
Many weather reports start with a jet stream forcast (forecast, actually). This gives us a look at the bigger picture before starting with the details. If you're really interested in understanding the day's situation, start with the national scene. Let's begin with a few definitions.A jet? It defines a stream of fluid moving through its environment like a train. Meteorological jet streams in the air fall into this category, since a continuous current of air moves (often quickly) through the surrounding air.
So what is a jet stream, then?
These currents of air flow through the atmosphere. They occur at varying elevations, but the largest ones flow near the top of the troposphere. And their size and strength depends on how sharply the air temperature below them changes with location, a horizontal temperature gradient. A jet stream diagram shows up as a large curvy line on the weathermap.In meteorology we have subtropical streams and equatorial easterly jets in the tropics, low-level jets above flatlands near mountains, and the major polar jet stream forcast professionals talk about. How long are they? Variable, up to thousands of kilometres or more. Then they can end quite abruptly and restart further downstream. They're fast, too. From 111 up to several hundred kilometres per hour, in winter. The big ones blow 10 km or so above the ground.
How the jet stream works
The jet stream flows horizontally, high up and lengthwise along the boundary between warm and colder air. Forecasters use the location of the jet stream to show where the boundary lies. By locating the up and down loops in the jet stream pattern, they easily identify warm and cool areas, and, as the jet stream sweeps sideways across the map, so does the incoming cold or warm air into any region. So, when you seek a jet stream forcast, what you want is expected temperature changes, and associated frontal stormy weather.A jet stream forcast also lets you know where surface highs and lows will likely travel, where it should become sunnier or cloudy and rainy. Surface based systems tend to travel downwind along the jetstream above, as if propelled by the winds aloft.