The Wind Machine
If you are into weather, then the past 24 hours have been as hectic as it gets. From wet and springy to wild winds and arctic cold, our region braved it all.
Last night’s peak winds mustered gusts as high as 55 miles per hour after midnight. In fact for a 6 hour period, the average wind was close to 25 miles per hour with many gusts exceeding 40. No wonder your windows rattled all night long and those wind chimes sang up a chorus. At the same time the temperature tumbled from near 55 at midnight to 25 by dawn, a not too shabby 30 degree drop in 6 hours.
All the while, the forecast for strong winds was maintained due to a rather complex meteorological truism, namely; as the pressure rises and falls, the winds increase.
On my Master’s comprehensive exam at Penn State back in 1981, there was a question that focused on defending the notion that winds on earth were a by-product of an unevenly heated planet earth. I recall deriving Jeffrey’s theorem and bagging the 10 points available for the question. Let me discuss this theory as it related to last night’s winds.
All week long, your Christmas barometer has been put thru quite a workout. Early in the week in the arctic air of mid winter, the needle soared to the high pressure end of the dial. Readings registered in the 30.30” range. Since cold air is heavy, and since pressure measures the weight of the air, it made sense that we were under HIGH PRESSURE OR HIGH WEIGHT AIR.
As winds turned to the south by Tuesday and Wednesday, temperatures recovered from their early week polar values. In fact highs made it well into the 50s on Wednesday. The converse of the cold, heavy air-ism is that WARM AIR WEIGHS LESS. We know that since when we are kids we are taught in a fire to crawl on all fours since smoke (warm air) rises (since it is a lightweight).
Last night, with readings in the springy 50s, the pressure tumbled as low as 29.25”. At 11pm I stated that is as low as it gets in our region and only in a hurricane would the pressure be lower. In fact, many hurricanes do not have central pressures that low.
I am surprise nobody called to tell me their ears were popping from the incredibly low pressures!
Now back to Jeffrey’s theorem. If the planet earth was heated uniformly (as in the poles were as hot as the equator), then no wind would blow. But the converse focuses on the fact that the larger the difference in temperature (we call that a gradient) across a region, the stronger the wind will blow.
Last night while we were at 55 degrees with thunder squalls, Chicago was down to 5 above with snow. That made for a 50 degree drop in temperature over a mere 300 miles. As the “hurricane”-like low pressure passed thru Michigan and into Ontario, it produced those powerful gusts that tried for awhile to blow us away.
This afternoon’s temperature difference from Chi-town to Charleston is down to 20 degrees (12 in the Windy City to 32 in Kanawha City). So the winds are down to 10-20 miles per hour. A testimony to Sir Jeffrey and his theory.
By the way, on Friday, Chicago will be snowing and 20 degrees while we have rain and 50. Guess what our forecast is calling for?