Did you hear about the 148 mph record wind gust at Monarch Pass in Colorado this week? The Monarch Pass wind record is likely associated with a combination of strong winds, convective activity and the unique aerodynamics of the mountainside. At this link NOAA discusses the background on this event and the validity of the measurement. While this wind extreme somewhat associated with convection. Another interesting wind event was playing out in Chicago, and it is an opportunity to discuss the really fascinating Venturi Effect. Wind can be quite the hazard even when it is not associated with a convection, a tornadic storm or a hurricane. For a review of non-convective wind hazards I refer you to an excellent discussion by Dr. John Knox and colleagues.
I mentioned Venturi Effect earlier. It is not weird if you are saying, “what in the heck is that?” Recently my colleague Tom Niziol was talking about the Venturi Effect on Weather Underground TV. I realized that many people may not be familiar with the term. Here is a quick “Meteorology and Physics 101″ on the Venturi Effect.
Some people are familiar with me from the TV show WxGeeks or my Forbes contributions, but my day job is tenured faculty member and research meteorologist at University of Georgia. One of my primary areas of scholarly research is urban climatology/meteorology so the Venturi Effect is right at home to me.
Our atmosphere is a fluid and is governed by the basic principles and science of fluid dynamics. The Venturi Effect is common in fluid dynamics. In an excellent discussion, Caty Fairclough describes the Venturi Effect this way,
When flowing through a constricted area of a pipe, a fluid’s velocity increases and its static pressure decreases……..The Venturi effect (see animated diagram) states that in a situation with constant mechanical energy, the velocity of a fluid passing through a constricted area will increase and its static pressure will decrease. The effect utilizes both the principle of continuity as well as the principle of conservation of mechanical energy.
This principle actually has many applications in scientific analysis, industrial processes, and even an pre-fuel injection automobiles with a carburetor. This effect carries the name of Italian scientist Giovanni Battista Venturi (1746-1822), a professor at the University of Modena (Italy). Venturi studied color, sound, and hydraulics but is most noted for his pioneering work on fluid movement and constricted channels.
But how is it relevant to weather? You may have noticed that it may be more windy in the “urban canyons” between large buildings or skyscrapers. When buildings are close together, the flow is constricted and the wind (fluid) velocity must increase as described above.
Chicago is a notably windy city, and it is not surprising that the Venturi Effect would be an issue there because of the combination of wind and skyscrapers. Tom Niziol was talking about Chicago on the evening of February 19th. News reports discussed building evacuations and wind gusts over 60 mph on that day. In the figure below, Weather Underground TV is displaying a wind rose. A wind rose is a succinct graphical way of displaying the climatological distribution of wind speeds and directions for a location. Chicago’s wind rose suggests that the dominate wind direction is from approximately “west.” Such an orientation is ideal for the Venturi Effect if you know anything about how Chicago’s building grid is aligned. However, any city with narrow urban canyons and the optimal wind direction can experience this effect. This effect is partly why it is dangerous to shelter from a tornado under a highway overpass