The Jet Stream
The jet stream is a narrow band of strong winds about 10 km above the Earth, marking the dividing line between warm and cold air masses. Because there are several types of air masses, there may be more than one jet stream on the map, each marking the change from one air mass to another. This river of air is a few hundred kilometres across with average speeds between 160 and 320 km/h. For a jet stream to exist at all, wind speeds must exceed 110 km/h. The speed of each jet depends on the difference in temperature between the 2 air masses--the greater the difference, the faster the wind blows!
Forecasters use maps like this one to monitor variations in pressure, temperature, moisture content, and--of course--wind speeds high in the atmosphere. In this example, the darkest areas indicate the strongest winds. The position of jet streams provides valuable information on the movement of air masses and weather systems that affect our day-to-day weather. Pilots are also interested in the speed and location of jet streams, so that they can plan their flights to either catch tail-winds or avoid the strongest head-winds.
Jet stream maps are posted to Environment Canada's Web site every day. The thick red line shows the location of the jet stream with red arrows to show the direction the wind is blowing--in general, from west to east. With your back to the wind, the cooler air mass is always on your left. Are you on the warm air side or the cooler air side in this example? Check out today's map and see which air mass you're in today.
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