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Global Scale and Westerlies
Global ScaleYou may have noticed yearly weather patterns -- it may be wet in the winter but very dry in the summer. Your area may have a wet season, when you get rain daily, but little precipitation the rest of the year. In fact, there are general patterns that weather systems follow. These flow patterns are enormous, covering tens of thousands of miles and affecting large parts of the world. Such a scale is called a planetary, or global scale. There are many patterns on the global scale which could be discussed. However, we will focus on two items that most influence weather over North America: westerly flow and jet streams.
WesterliesWind across the United States generally flows from west to east. This flow is called the prevailing westerlies, or more simply, the westerlies. Notice that westerlies flow eastward. Instead of describing the direction the wind vector points, we use the direction from where the wind comes, as discussed earlier. In some other areas of the world, the general flow is from east to west. But over the US, the wind carries lows and highs from the west coast to the east coast over the course of several days. This general flow may change slightly, but it does not reverse or shift dramatically to a north-south flow, for example. Because of this, you can generally count on weather systems over the United States to move in from the west.Confused? Have a question? If so, check out the Frequently Asked Questions (FAQ) page or send mail to the OS411 tutor (os411tutor@shodor.org) with your question! Report technical/content problems here |
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Meteorologists classify weather phenomena into four classes or scales based on horizontal coverage and duration. The four scales are: microscale, mesoscale, synoptic scale, and global scale. As you move up each class from microscale, you lose detail of motion in the previous scale. For example, small turbulence in the branches of a tree cannot be seen when looking at the entire city, though you may still see the tree. And the thunderstorm over the city cannot be seen when looking at a weather map of the entire nation. Likewise, one cannot see the cold front over the mid-west when looking at jet stream patterns over the Northern Hemisphere. Each scale has its own phenomena, which contain the phenomena in the scale below it. We start with global scale.