Category Archives: Meteorology

Is moist air lighter than dry air?

Yes. Our air is primarily composed of nitrogen and oxygen, but it always has some water molecules.

The weight of an individual atom is represented by its atomic weight. The (rounded) atomic weight of hydrogen (H) is 1, oxygen (O) is 16, nitrogen (N) is 14, and carbon (C) is 12. Continue reading

Category: Meteorology

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Can weather fan the flames of revolution?

July was the month of revolution in both America and France in the late 18th century as we declared independence from Britain on July 4, 1776, and the French Revolution began with the assault on the Bastille in Paris on July 14, 1789. It is interesting to examine the extent to which weather may have influenced the passions that led to these seismic events.

The author of our Declaration, Thomas Jefferson, was such an avid weather observer that he brought his instruments with him from Monticello to Philadelphia that summer. He recorded a mild day on July 4 with a high temperature of 76 degrees. Phineas Pemberton, a prominent citizen, independently recorded the same high temperature – nearly 10 degrees Fahrenheit below normal. Pemberton also noted a wind shift from northerly to southwesterly with a falling pressure as often accompanies passage of a surface high-pressure system. Thus, the great revolutionary act in America was birthed in benevolent weather conditions. Continue reading

Category: Meteorology, Severe Weather

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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

Category: Meteorology, Phenomena

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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

Category: Climate, Meteorology, Seasons

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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

Category: Meteorology, Phenomena, Severe Weather

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