Monday, July 18, 2011

Heat, humidity, and fog? Chicago weather strikes again!

Aside from flying planes, the next best thing about being a pilot is having an understanding of weather. And after living in Chicago for almost a decade, I’ve come to appreciate the variety and speed at which the weather changes. Chicago’s weather is predictable in its unpredictability. In terms of aviation, it can be challenging, but having this knowledge simply makes me a pilot with variety of experience. I'm not just a fair-weather weekend warrior.

Predictable Weather Patterns

The past several days have been hot and humid. This is a cause for concern to a pilot as a plane’s flight performance will suffer (hot, humid air is less dense than cold, dry air so engines and wings don’t perform optimally). If that weren’t enough, the likelihood of clouds that can grow into thunderstorms increase, and, under the right conditions, fog can form. Yup, fog can form on a hot day in Chicago.

So, how does it form?

Lake Michigan is east of Chicago, and the prevailing winds often come from the northwest or southwest. Winds from the northeast (a lake breeze) is less common.

After several unseasonably hot days, the lake water is warmed and saturates the air with water vapor in a manner similar to steam rising from a boiling pot of water. When winds shifts and a lake breeze is established, the air over lake cools to the dew point and the water vapor condenses to form low clouds or fog.

When conditions are right, the northeast wind will blow fog banks over parts of the city’s lake front. How long the fog lasts is determined by the temperature, humidity, and wind. Sometimes the fog barely lasts an hour, whereas other days (especially in the spring and fall) it can last a whole day.

Dr. Dave

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