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Category Archives: Climate
How can the Upper Midwest get such high dew points every year?
The dew point temperature is a measure of the amount of water vapor in the atmosphere and is also a good way to tell how uncomfortable you might feel on a hot day. Hot days and high water vapor contents are a serious health risk for some. Also, high water vapor content is a key fuel for severe thunderstorms.
There are three primary sources of water vapor that address this question: advection of water vapor largely from the Gulf of Mexico, evaporation over the Great Lakes, and transpiration from corn fields. The contribution from the Great Lakes in summer is minimal in comparison to the other factors, except maybe near the shorelines. Continue reading
What happens to the wintertime cold pool in summertime?
We have commented a number of times in the past few years about the areal extent of the hemispheric cold pool of air at 850 millibars (about a mile above the surface) during the winter. As one might expect, that pool expands dramatically from October through February and then begins to contract as we move toward spring and summer.
Our analysis uses the minus 5 Celsius isotherm (line of constant temperature) and has shown that the average winter cold pool area has systematically shrunk for at least the past 76 years. One might reasonably wonder if this cold pool survives at all during the height of Northern Hemisphere summer. As it turns out, some summers have a number of days in mid-July on which there is absolutely no air at 850 mb that is as cold as minus 5 Celsius. Roughly half of the last 76 years have had such a “vanishing” cold pool, with the pool getting very close to vanishing many of the other years. Continue reading
Why is Memorial Day weekend weather highly variable?
Memorial Day weekend weather can be absolutely glorious in the state of Wisconsin. But it can be rainy and cold as well. Perhaps no other major holiday suffers such a Jekyll and Hyde split, and there are really good scientific reasons that underlie this duality.
By the end of May, the Northern Hemisphere is just about completely over the prior winter and the cold air that characterized it is almost completely left to very high latitudes, where the longer days act quickly to erode what is left even near the North Pole. The process of “shedding” the cold air from winter sometimes involves the excursion southward of regional cold air vortices in the mid-troposphere, which meteorologists refer to as “cut off” low pressure systems. Continue reading
Category: Climate, Meteorology, Seasons
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Has this year’s tornado activity been abnormal?
Preliminary reports from the Storm Prediction Center tally higher than the historical averages though May 2024. Continue reading
Category: Climate, Meteorology, Severe Weather
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Has our plant hardiness zone changed?
If you are involved with gardening, you probably are aware of the Plant Hardiness Zone Map, or PHZM, often listed on seed packets. The U.S. Department of Agriculture developed the zones and first published them in 1960. The USDA updated them in 2012 and more recently in November 2023.
A hardiness zone provides information on the type of plants capable of surviving certain climatic conditions. The designations are based on the “average annual extreme minimum temperature” at a given location during a particular 30-year period. The climate zones are determined from temperature records kept by National Atmospheric and Oceanic Administration. Continue reading
