The Weather Guys

What causes a double rainbow?

Posted on July 3, 2017 by WeatherGuys Editor

A rainbow, with just a hint of a double rainbow above it, arches across a Madison field after a storm moves away. The view is looking east toward Downtown from a hill near Mineral Point Road and Highway M. (Photo credit: State Journal archives)

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.

When sunlight passes through a triangular glass prism, it separates into the colors of the rainbow. This separation happens because different colors bend, or refract, by different amounts. The shortest (blue and violet) wavelengths refract the most; red light refracts the least.

The separation of colors is referred to as dispersion. Not only prisms but also water drops and ice crystals can cause dispersion. To form a rainbow you need large drops of water, the sun at your back and at the correct angle.

Raindrops act as prisms, bending and reflecting the sunlight that falls on them, just like a crystal hung in a sunny window.

As light enters water, the path it takes changes. How much the direction changes is a function of the color of the light.

You probably noticed that a smooth water surface can act like a mirror and reflect light. If the light beam entering the raindrop reaches the back of the drop at a certain angle, it undergoes a reflection and heads back toward the sun. As the light exits the raindrop and re-enters the air, its path bends an amount that again depends on the color. This bending of the light as it enters and leaves the drop disperses the light of the sun into its spectrum of colors that form the rainbow.

Sometimes the light reflects twice off the back of the raindrop; this leads to the secondary rainbow. The second reflection causes the order of the colors in the bow to reverse.

Category: Meteorology, Phenomena

Comments Off

How do you measure how hot the summer is?

Posted on June 26, 2017 by WeatherGuys Editor

One way to gauge how hot a summer season was is to count how many days reached 90 degrees F or above. It turns out that this number is extremely variable here in Madison. (Photo credit: State Journal archives)

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.

In this case the standard deviation is 9.0. Thus, we might expect that 2/3 of the years would range from having 19.9 to 1.9 days at or above 90F. As it turns out, 34 of the last 46 summers have been in that range!

It is interesting to note that six summers have had 20 or more hot days (1975, 1976, 1983, 1988, 1995 and 2012) — the recent scorching summer of 2012 had 39 days (one short of the record 40 of 1955)!

Notably cold summers (by this measure) include 1979, 1996, 1998, 2000, 2004, 2008 and 2014 with 2004 being the only summer in the last 46 years in which the temperature never reached 90F.

Broken down into decades, there had been a trend toward fewer hot days each summer with the averages being 15.8, 11.7, 8.2 and 7.3 days for the 1970s, 1980s, 1990s and 2000s, respectively. The summer of 2012 singlehandedly accounts for a departure from this trend as this decade has thus far averaged 11.42 days (only 6.83 without 2012).

These data remind us how complicated the interplay between weather and climate can be since the global average temperature has been trending the other way in these same decades.

Category: Climate, Meteorology, Seasons

Comments Off

When is the summer solstice?

Posted on June 19, 2017 by WeatherGuys Editor

This full disk image, one day before the 2017 summer solstice, illustrates the abundance of daylight in the northern hemisphere.

The summer solstice in the Northern Hemisphere is the day when the sun is farthest north.

In 2017, this occurs Tuesday at 11:24 p.m. Central Time.

As Earth orbits the Sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the Sun is always changing as Earth orbits around the Sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. The summer solstice is the day of the year with the most daylight.

On the Northern Hemisphere’s summer solstice, the northern spin axis is tilted toward the sun and latitudes north of the Arctic Circle (66.5 degrees N) have 24 hours of daylight.

At the summer solstice, the sun reaches its highest point in the sky and daylight is longest. However, our earliest sunrise in Madison occurs in mid-June while our latest sunset occurs in late June.

So, while the summer solstice has the longest daylight hours, that day does not correspond to the earliest sunrise or the latest sunset. The reason that the earliest sunrise and latest sunset do not occur on the summer solstice is a combined effect of the tilt of Earth’s axis and the elliptical path of Earth around the sun.

On our solstice, the sun rises and sets farthest north of due east and due west. The farther the sun sets from due west along the horizon, the shallower the angle of the setting sun. That means a longer duration for sunset at the solstices.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Seasons

Comments Off

What is a derecho?

Posted on June 12, 2017 by WeatherGuys Editor

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).

The extreme winds of a derecho — up to 150 mph in the strongest storms — come about in the following way. Derechos are often associated with a quasi-stationary front in mid-summer. If the atmosphere just north of the front is very unstable, the front may trigger rapidly developing thunderstorms. A line of thunderstorms that forms in the vicinity of the stationary front can, via its cold downdrafts, drag down high-speed air from above. This can cause the high winds of a derecho.

At the same time, the high winds push the line of thunderstorms outward, causing it to bend or “bow.” This results in a bow echo image on weather radar. Once they get going, derechos can cover lots of territory — up to 1,000 miles.

Derechos leave significant property damage in their wake, even flattening entire forests. In some cases, derechos wreak as much havoc as a hurricane or tornado.

The June 29, 2012, derecho swept across from U.S. from west of Chicago to the East Coast, leaving as many as 5 million households without power. The storm traveled at speeds of more than 60 mph, with wind gusts approaching 80 mph. At least 22 people were killed.

About 40 percent of all thunderstorm-related injuries and deaths occur because of derechos.

Category: Meteorology, Phenomena, Severe Weather

Comments Off

Why withdraw from the Paris Climate Accord?

Posted on June 5, 2017 by WeatherGuys Editor

President Trump’s decision to withdraw the U.S. from the Paris Climate Accord last Thursday is a disheartening blow to the longstanding idea that sound science should play a role in the formation of public policy in our country.

This notion, in fact, was central to the creation of the National Academy of Sciences by the first Republican president, Abraham Lincoln in 1863. Lincoln and the Congress charged the Academy with providing independent, objective advice to the nation on matters related to science and technology.

The National Academy and the overwhelming majority of climate scientists agree that human activity is a major contributing factor to global climate change – there is essentially no disputing this fact. Despite this robust consensus, which serves as the motivation for 195 other nations to join forces in committing to a cleaner energy future for the benefit of future generations, our own government has committed to skepticism, short-sightedness and ignorance (literally, “the act of ignoring”)..

It seems distressingly untenable to assert American exceptionalism on the one hand while on the other resigning to retreat from a unique and urgent global challenge. This is not a traditionally American position, it is an embarrassing shirking of responsibility, an unforced error motivated by ideology, not by reasoned judgment. Our country, in a complicated world where reason has long been one of its guides, has always done better than this.

Category: Climate, Uncategorized

Comments Off