The Weather Guys

Why does the sun melt ice on sidewalks even when the temperature is below freezing?

Posted on July 28, 2017 by WeatherGuys Editor

If a sidewalk is salted, ice may absorb the salt, which lowers its freezing point and may lead to melting. (Photo credit: Baraboo News Republic archives)

All objects exchange energy with their environment. They do this via conduction, advection, convection and radiation. If water is involved, a change in the water phase (liquid, solid or gas) also will involve an exchange of energy.

Conduction moves energy by physical contact. Convection results from hot air rising. Advection by the wind moves heat horizontally.

Radiative processes transfer heat throughout the entire atmosphere and into space. Radiation can be absorbed, reflected or transmitted. Convection and latent heating transfer heat over great distances through vertical motions and phase changes of water.

In the case of ice on a sidewalk, assuming the sidewalk has not been salted and there is no wind, the important energy exchange mechanisms are conduction and radiation. The ice is exchanging energy with both the sidewalk and the air around it via conduction.

If the atmosphere is below freezing, this will not result in the ice melting. If it is night time, and the sidewalk is below freezing, then this will not result in the ice melting. But during the day, the story could be different.

If the sun is shining on the ice, some of that solar energy will be absorbed. Ice is clear at visible wavelengths, the energy to which our eyes are sensitive. But the sun emits radiation at other wavelengths which water will absorb, and thus increases its energy gain.

If these gains exceed the energy losses, the ice will warm. If it reaches the melting point, the ice will start to melt. The sun can also add energy and warm the sidewalk, increasing its temperature to above freezing. This will then warm the ice via conduction and lead to its melting.

If the sidewalk has been salted, the ice may absorb the salt, which lowers its freezing point and may lead to melting.

Category: Meteorology, Seasons

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Is moist air lighter than dry air?

Posted on July 17, 2017 by WeatherGuys Editor

UW-Madison student Jessica Bjerke contends with an unruly umbrella while making her way across campus amid March’s high winds and cold rains. Moist air is lighter than dry air, which is composed mostly of nitrogen and oxygen molecules that weigh more than water molecules. (Photo credit: John Hart, State Journal archives)

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.

The weight of a molecule is determined by summing the atomic weights of its atoms. A water molecule (H2O) has a molecular weight of 18 (1 + 1 + 16). Free nitrogen (N2) has a molecular weight of 28, and an oxygen molecule (O2) has an atomic weight of 32. Therefore, a water molecule is lighter than either a nitrogen or an oxygen molecule.

Any fixed volume of a gas at constant pressure and temperature has the same number of molecules. It does not matter what the gas is — the same number of molecules will exist in that volume.

To make a given volume of air moister, we need to add water vapor molecules to the volume. To add water molecules to the volume, we must remove other molecules to conserve the total number of molecules in the volume.

Dry air consists mostly of nitrogen and oxygen molecules, which weigh more than water molecules. This means that when a given volume of air is made more moist by adding water molecules, heavier molecules are replaced with lighter molecules. Therefore, moist air is lighter than dry air if both are at the same temperature and pressure.

Category: Meteorology

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Can weather fan the flames of revolution?

Posted on July 10, 2017 by WeatherGuys Editor

Thomas Jefferson, author of our Declaration of Independence and an avid weather observer, recorded a mild summer day with a high temperature of 76 degrees as we declared independence from Britain on July 4, 1776. (Official Presidential portrait of Thomas Jefferson by Rembrandt Peale, 1800)

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.

Not so for the French Revolution. The summer of 1788 had been exceptionally dry across the country and led to widespread crop failure. The hot, dry summer was followed by an unusually cold winter that made keeping warm more expensive. Food shortages brought on by the prior summer’s drought intensified in the spring of 1789 and left the populace, already frustrated with the opulence of Versailles, in a heightened state of agitation. Recent analysis by scholars at the London School of Economics has suggested that these conditions were a proximate cause to the civil unrest that led a mob of Parisians to storm the Bastille and, effectively, ignite the French Revolution.

Category: Meteorology, Severe Weather

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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

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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

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