WRAL WeatherCenter Blog

As I watched Greg's weather spot one afternoon, he pointed to an "H" (for "high") on the weather map and then referred to the counter-clockwise rotation about the high. The arrows on the map looked like a clockwise rotation to me, so I tuned in again for the evening coverage at which time he referred to the exact same map using the term "clockwise" rotation. I find wind references confusing anyway, since a "northerly wind" refers (I think) to a wind out of the north instead of a wind pointing to the north. So I have to wonder if the same backwards reference might apply to the rotation description. And, while I'm at it, I would also ask whether the rotations about a high are the same or opposite in the southern hemisphere. THANKS.

Posted October 2, 2007

MIKE MOSS SAYS:        Robert,     I can readily attest that it's all too easy to make the occasional "verbal typo" in front of the camera when it comes to the words "clockwise" and "counter-clockwise," and for that matter "left/right" and "east/west" cause trouble once in a while as well! So, it sounds like Greg just misspoke during the first of the two weathercasts you mentioned. In fact, for the northern hemisphere, the general sense of flow around a center of high pressure is clockwise.

You make an interesting point in picking up on the potential that the names might be backwards in the same sense as wind directions. That can indeed be confusing, but as you noted meteorological convention is to refer to winds by the direction from which they are blowing. This may have originated with the idea that the wind direction in some cases provides a clue as to the weather that will ensue, i.e. for our area north winds are often cool and dry, south winds are often warm and humid, etc. Of course, the type of weather associated with a particular direction depends on the geography of a given area, but the "north = cool" and "south=warm" convention works pretty well for the midlatitudes of the northern hemisphere.

The same rule would be reversed in the southern hemisphere, and so are winds around pressure centers, at least if described in terms of clock rotation. Winds flow around highs in the southern hemisphere in a counter-clockwise manner, which one can consider "opposite" from our hemisphere.

Not to toss more confusion into this, but there is also a sense in which the rotation about highs and lows is the same in both hemispheres. That is because in both cases, flow around low pressure systems is considered "cyclonic" while flow around a high is "anticyclonic." You can think of cyclonic winds as those that circulate with the same sense as the earth's rotation about the nearest pole. At the North Pole, the earth rotates about that pole in a counterclockwise manner as seen by an observer standing at that location, while to someone standing at the South Pole the earth would be rotating clockwise.

To summarize, Low pressure centers are cyclones, in which the air flows around the center counterclockwise for the northern hemisphere and clockwise in the south. High pressure centers are anticyclones, around which the flow is clockwise in the the northern hemisphere and vice versa.


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