8.3: Westerlies and the Jet Stream
- Page ID
- 45361
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While the 500 millibar level gives us excellent insight into the specifics of upper-level wind flow, it does not allow us to view the general wind pattern, notably the Jet Stream. In order to do that, we need to rise to the 300 millibar level. Figure 8.3.1 is a map of the 300 mb winds, heights, and isotachs (lines of constant wind speed, similar to isobars but for wind instead of pressure) taken at 1200 UTC on February 28, 2025, the same time as Figure 8.1.1. 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), winds 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 completely _____________________ than the 500 mb level. This should make sense because air pressure decreases with height.
- Higher
- Lower
- The relationship between latitude and height at the 300 mb level (that warmer air corresponds with heigher heights) is ___________________ the 500 mb level.
- Very similar to
- Very different from
Now let’s talk about a few key features of the 300 mb map.
Zonal vs. Meridional Flow
The Jet Stream is present when the blue height lines are close together, causing 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 mostly traveling from West to East (Figure 8.3.2a). On the other hand, if there are large ridges/troughs present (hence the jet stream is wavy), we call this Meridional Flow (Figure 8.3.2b)
- The 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 are present on the eastern half of Ridges and the western half of troughs) are likely to bring warm advection with them.
- Warm Air
- Cold Air
- Such advection is likely occurring _________________________________.
- In the Northern Rockies (Idaho, Montana, and Wyoming)
- Across the Midwest of the United States (Iowa, Wisconsin, and eastern Nebraska)
- Along the US East Coast (New York, Massachusetts, and Pennsylvania)