8.3: Westerlies and the Jet Stream
- Page ID
- 45361
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While the 500 millibar level provides excellent insight into the specifics of upper-level wind flow, it does not reveal the overall wind pattern, notably the Jet Stream. To do that, we need to reach the 300 millibar level. Figure 8.3.1 is taken at 1200 UTC on February 28, 2025, the same time as Figure 8.1.1. It is a map of the 300 mb winds, heights, and isotachs. Isotachs are lines of constant wind speed, analogous to isobars but for wind rather than pressure. All guidelines for height lines and station models are identical to those on the 500 mb map.
Before we jump into the key features of the 300 mb map, let’s make a few comparisons with the 500 mb map:
- Compared to the 500 mb map (Figure 8.1.1), wind speeds at the 300 mb level are generally:
- Higher than the 500 mb level
- Lower than the 500 mb level
- Heights in the 300 mb level are ______ than the 500 mb level. This is reasonable because air pressure decreases with altitude.
- higher
- lower
- The relationship between latitude and height at the 300 mb level (that warmer air corresponds to higher heights) is _________ the 500 mb level.
- similar to
- different from
Let’s examine a few key features of the 300 mb map.
Zonal vs. Meridional Flow
The Jet Stream is present when the blue height contours are close together, resulting in higher wind speeds. This creates a band of strong winds that travels from West to East. We can identify the presence and shape of that band by looking at the pattern of blue lines. If the blue lines are relatively straight, traveling from West to East, we call that Zonal Flow. In this case, winds are predominantly from the West to the East (Figure 8.3.2a). On the other hand, if there are large ridges/troughs present (the jet stream is wavy), we call this Meridional Flow (Figure 8.3.2b)
a
b- The general wind flow present in the Jet Stream on Figure 8.3.1 is primarily:
- Zonal across the entire United States, with a very weak trough near North/South Dakota and Minnesota
- Meridional across the western 2/3 of the United States, and Zonal near the East Coast
- Zonal on the West Coast and Meridional across the Eastern 2/3 of the United States
- Winds from the North/Northwest, which prevail over the eastern half of the ridges and the western half of the troughs, are likely to bring ______ advection.
- warm air
- cold air
- Such advection is likely occurring ________.
- on the West Coast (California, Oregon, Washington)
- across the Midwest (Iowa, Wisconsin, and eastern Nebraska)
- along the US East Coast (New York, Massachusetts, and Pennsylvania)