Wind advisory, Sunday Dec 14, 2008
The National Weather Service will issue a wind advisory for sustained winds 31 to 39 mph for at least 1 hour or any gusts 46 to 57 mph. However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance a wind advisory.
Lexington recorded sustained winds of 39 mph with gusts around 45 mph. This created difficult walking conditions at times around downtown. Wind speeds can be faster in isolated parts of the built environment because of turbulence. Though cities typically decrease average wind speeds near ground level, during periods of high wind speed tall buildings interrupt flow causing them to ‘pile up’ on windward sides and then rush off with increased velocity in channels between buildings. Wind eddies on the leeward side of buildings will also have strong gusts.
A comparison to a simple stream might be helpful. Water flows fastest in the stream’s center. An obstruction blocking the stream will divert the flow into two channels, each with increased velocity. Unlike water though, air can be highly compressed. When air piles up on the windward or upstream side, it will move in a more dynamic manner around the obstruction. Turbulence.
The mechanics of how all this plays out is beyond my understanding, but it is sufficient to say that when a wind advisory is issued for Lexington, we can experience strong and erratic winds downtown. I noticed many flocks of pigeons remaining on their protective perch, and no soaring hawks or vultures, so that’s probably a good gauge of strong turbulence.
The last time the area was under a wind advisory was during the passing of the remnants of Hurricane Ike. I think winds gusted to near-hurricane strength. Winds this weekend are associated with a strong low that passed to our north. This low pressure system created blizzard conditions and wind chill warnings across the northern states. The system also brought the cold punch of air that created our first major winter storm a day later.
The chart below is from the Beaufort Scale of wind speed. So next time you’re wondering, “What’s that wind speed?” here’s your chart.
| Description | Speed | Visual Clues and Damage Effects |
|---|---|---|
| Calm | Calm | Calm wind. Smoke rises vertically with little if any drift. |
| Light Air | 1 to 3 mph | Direction of wind shown by smoke drift, not by wind vanes. Little if any movement with flags. Wind barely moves tree leaves. |
| Light Breeze | 4 to 7 mph | Wind felt on face. Leaves rustle and small twigs move. Ordinary wind vanes move. |
| Gentle Breeze | 8 to 12 mph | Leaves and small twigs in constant motion. Wind blows up dry leaves from the ground. Flags are extended out. |
| Moderate Breeze | 13 to 18 mph | Wind moves small branches. Wind raises dust and loose paper from the ground and drives them along. |
| Fresh Breeze | 19 to 24 mph | Large branches and small trees in leaf begin to sway. Crested wavelets form on inland lakes and large rivers. |
| Strong Breeze | 25 to 31 mph | Large branches in continuous motion. Whistling sounds heard in overhead or nearby power and telephone lines. Umbrellas used with difficulty. |
| Near Gale | 32 to 38 mph | Whole trees in motion. Inconvenience felt when walking against the wind. |
| Gale | 39 to 46 mph | Wind breaks twigs and small branches. Wind generally impedes walking. |
| Strong Gale | 47 to 54 mph | Structural damage occurs, such as chimney covers, roofing tiles blown off, and television antennas damaged. Ground is littered with many small twigs and broken branches. |
| Whole Gale | 55 to 63 mph | Considerable structural damage occurs, especially on roofs. Small trees may be blown over and uprooted. |
| Storm Force | 64 to 75 mph | Widespread damage occurs. Larger trees blown over and uprooted. |
| Hurricane Force | over 75 mph | Severe and extensive damage. Roofs can be peeled off. Windows broken. Trees uprooted. RVs and small mobile homes overturned. Moving automobiles can be pushed off the roadways. |
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