Chicago Window Expert

Feb
15

Envision the Wind in a Blizzard

Filed under: Energy Efficient Windows;
Chicago Ground Hog Day Blizzard – Envision the Wind
On February 1, 2011 at about 3pm, a somewhat rare occurrence took place in Chicago. A real-life blizzard! It continued for 20 hours.

Contrary to the misconceptions of some, a blizzard is not distinguished by a large volume of snow, but rather by an intense and pervasive wind.

In order to achieve the coveted blizzard status, a storm must have winds in excess of 35 mph for a duration of 3 hours or more. This results in low visibility, 1/4 mile or less.

One of the problems we have in trying to understand the wind is that it is usually invisible. A blizzard makes the wind visible. The millions of tiny snowflakes move along with the wind, expressing the wind’s every huff, puff and nuance.

Now it’s time to don our special glasses and understand exactly what we are seeing when we see snow blown around by the wind. In real time and 3D, we are actually seeing the three variables that we love to speak about when wind is concerned: direction, speed and pressure. The wind speed and direction are easily seen when snowflakes are suspended in the air:
Snowflakes suspended in the air allow us to visualize wind speed and direction

Less intuitive is the idea that we can also see the wind pressure, too. It’s easy when you know how. When you see a group of snowflakes crowded together, they are in an area of higher pressure. In areas where they are spaced far apart, the pressure is relatively lower. The snowflakes can be thought of as very large air molecules. How do we know this? Elementary, dear Watson.

If you begin with an assumption that in a neutral (no wind) state, snow particles would be evenly distributed in the air as they fall, then we take any cubic foot of that snowy air and compress it (this requires increased pressure on all sides of the cube) to half its volume, the pressure inside the shrunken cube would now be greater. The same number of snowflakes would occupy much less space, and would now be closer together. This closeness would form a cloudlike appearance as seen in fast-moving snow cloud in the image above.

If you took another cubic foot of air and stretched it to double its volume (this requires reduced pressure to pull at at the boundary of the cube) creating a reduced pressure inside the cube, the visual impression of the snowflakes would be more dispersed. You might not even see the snowflakes.