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Category Archives: Meteorology
What causes a double rainbow?
The classic rainbow is a single, bright, colored arc. Red is the outermost color of this arc, and violet is always the innermost color.
On occasion, you may have seen two rainbows at once. The lower rainbow is the primary rainbow and the higher, fainter, colored arc is the secondary rainbow. The color sequence of the secondary rainbow is opposite to the primary; red is on the inside of the arc and violet on the outside. Continue reading
How do you measure how hot the summer is?
One reasonable way to gauge how hot a summer season was is to consider how many days that year reached 90F or above. It turns out that this number is extremely variable here in Madison.
From 1971 to 2016, the average number of days at or above 90F in Madison is 10.9. As is often the case with statistics, however, the average does not convey a sense of the variability. A better way to express that variability is by calculating the standard deviation, which, when added to or subtracted from the average, sets a range in which approximately 2/3 of the years will fall. Continue reading
What is a derecho?
A derecho (pronounced deh-RAY-cho, a Spanish word meaning “straight ahead”) is an hours-long windstorm associated with a line of severe thunderstorms.
It is a result of straight-line winds, not the rotary winds of a tornado — hence its name. Derechos in the United States are most common in the late spring and summer (May through August). Continue reading
Is the weather nicer on Memorial Day or Labor Day?
Memorial Day traditionally marks the beginning of the summer, while Labor Day marks the end.
People are bound to have their own recollections and opinions about which weekend more reliably delivers agreeable weather here in southern Wisconsin, and those opinions can be colored by many non-meteorological influences. Continue reading
Did someone predict the recent Wisconsin tornado?
We cannot yet forecast tornado occurrence with any accuracy. One problem is the small size of a tornado, which is a narrow column of strong winds that rotate around a center of low pressure.
Over the last 60 years, forecasts of the development of large-scale low-pressure systems, which often organize the ingredients needed to form a tornado, have steadily improved. Because of these advances, meteorologists are better able to predict those conditions a few days in advance, enabling forecasters to identify counties where there is a threat of severe weather sometimes as many as three days in advance. Two days in advance of the recent EF-2 tornado that hit southeastern Polk County, the National Weather Service’s Storm Prediction Center’s convective outlook issued a slight-risk for the area. Continue reading
Category: Meteorology, Severe Weather
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