What location has the lowest average wind speed in the United States?

Posted on November 10, 2014 by WeatherGuys Editor

This question requires consideration of the number of factors that conspire to produce the wind.

Large-scale weather systems are perhaps the best-known features that impact the winds at a location.

Unsettled weather is nearly always associated with low sea-level pressure and relatively strong gradients of pressure (differences from one location to another in a region) that drive the winds associated with major storms.

Regions of high sea-level pressure often characterize fair weather and such weather systems are associated with small gradients in pressure and, consequently, light winds.

Aside from the influence of highs and lows, which can visit any location, proximity to a coast (whether it be the ocean or one of the Great Lakes) can also be a major influence on the windiness.

The local topography also exerts a major influence on average wind speeds with sheltered valleys being less windy than the open plains, for instance.

Thus, it is not surprising that inland locations, away from the coasts of large lakes and near or within valleys are among the least windy.

Among the largest 50 cities in the U.S., Phoenix has the lowest average annual wind speed at 6.2 mph.

For comparison, Milwaukee has an average breeze of 11.5 mph.

Oak Ridge, Tennessee, appears to be the least windy city in the U.S. with an average annual wind speed of 4.1 mph.

Interestingly, Anchorage, Fairbanks, and Valdez, Alaska, all have average annual wind speeds around 4 mph even though it can occasionally be very windy in those cities.

Light winds in interior Alaska are a consequence of the terrain.

Category: Meteorology

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Did we have an Indian summer this year?

Posted on November 3, 2014 by WeatherGuys Editor

We are not sure of the origin of this expression, but it has been used for over 200 years in reference to a weather phenomenon that occurs in the fall, usually in October for our state. It occurs when the autumn weather is characterized as sunny and warm.

Indian summer can occur only after the first frost but before the first snowfall. It occurs after the leaves have turned color and includes dry weather conditions with maximum temperatures greater than 65 degrees and minimum temperatures greater than 33 degrees.

We usually get these conditions when a high pressure system is over the eastern United States. This anticyclone pushes dry southwest winds into our area, resulting in what many consider pleasant weather conditions.

Given the above definition, Indian summer usually occurs in the first three weeks of October, and Wisconsin has, on average, eight days of Indian summer each fall. These days don’t have to be consecutive. We can have a year without an Indian summer as well as a year with more than one Indian summer.

This year, our October mean temperature was close to the average temperature for most of the state, and precipitation in southern Wisconsin was slightly above. We also have had a minimum temperature below freezing, and no snow fall. So, we are in a time when Indian summer can occur if the temperature is warm enough, like it was on Oct. 27 when the high temperature hit 75 degrees and the minimum was in the low 50s. This ties for the ninth-latest 75-degree or warmer day in a calendar year. On average, the last 75-degree day occurs on Oct. 9.

Category: Climate, Seasons

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Is human activity to blame for climate change?

Posted on October 20, 2014 by WeatherGuys Editor

A recent debate between candidates for Congress in the Wisconsin’s 1st District — U.S. Rep. Paul Ryan, R-Janesville, and Democratic challenger Rob Zerban — included questions about the role of human beings in producing discernible changes in the climate over the last 150 years.

Unfortunately, this question, which is a matter of evidence, analysis and conclusion as all scientific questions are, has become a source of partisan political divide.

The Intergovernmental Panel on Climate Change (IPCC), a scientific body created by the United Nations to inform the UN regarding “scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change,” has issued five reports on this question since 1990.

These reports are a synthesis of many hundreds of peer-reviewed scientific studies on the issue.

With each successive report — they have been issued in 1995, 2001, 2007 and 2014 — the IPCC has increased the certainty of its conclusions.

The language in these reports has changed from “the balance of evidence suggests a discernible human influence on global climate” (1995) to “most of the observed warming is likely (a greater than 66 percent chance) due to human activities” (2001) to warming “over the last 50 years is very likely (a greater than 90 percent chance) due to human activities” to “It is extremely likely (a 95-100 percent chance) that human influence was the dominant cause of global warming between 1951-2010.”

This makes two things quite clear.

First, that scientists are a skeptical bunch and will move toward increased certainty only as evidence accumulates in favor of that conclusion.

Second, that human-induced global warming is a reality with which we must reckon.

During the debate, when asked if humans have a role in global warming, Ryan answered, “I don’t know the answer to that question. I don’t think science does either.”

He may well be correct in his first response, but he is certainly wrong in his second.

Category: Climate

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What happened during last week’s lunar eclipse?

Posted on October 15, 2014 by WeatherGuys Editor

What happened during last week’s lunar eclipse?

Our recent lunar eclipse, visible in Madison at 5:25 a.m. Wednesday, resulted from the Earth being directly between the sun and the moon, thereby casting a shadow on the moon.

The degree of directness of the shadow determines the degree of completeness of the eclipse.

We were lucky enough to see a total lunar eclipse in this part of the country. In addition, we were fortunate that the sky was perfectly clear as it has been quite often during our recent beautiful autumn weather.

The timing of the eclipse, coupled with the perfectly clear skies, also delivered a rare selenelion – an event characterized by the simultaneous appearance of the sun and the lunar eclipse.

This was visible in Madison at 7:03 a.m. and was captured by the rooftop cameras on the Atmospheric, Oceanic and Space Sciences building on UW-Madison’s campus.

One might wonder how both the sun and the eclipse can be visible at the same time given that the eclipse is a result of the moon being directly in the shadow of the Earth. The answer is that the Earth’s atmosphere refracts, or bends, light that is traveling through outer space because the atmosphere is more dense than the vacuum of deep space.

This bending of light is the same type of phenomena that renders the illusion of a spoon being discontinuous at the fluid-air boundary in a clear glass of water.

The refraction in the selenelion case “lifted” the images of the sun and the eclipsed moon slightly above the horizons.

Had the sunrise been at a slightly different time, or had there been low clouds on either horizon at sunrise on Wednesday, we would not have been lucky enough to catch the rare selenelion in the act.

 

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: Meteorology, Phenomena

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What is the status of the ozone hole?

Posted on October 6, 2014 by WeatherGuys Editor

Ozone occurs about 18 miles above the surface. Ozone is both caused by and provides protection from damaging ultraviolet energy emitted by the sun. The development of an atmospheric “ozone layer” allowed life to move out of the oceans and onto land.

The ozone hole occurs high over the continent of Antarctica. It is not actually a hole, but rather the appearance of very low values of ozone in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October.

The Antarctic ozone hole varies in size each year, from nearly zero in 1980 to an area larger than North America in 2000. The amount of ozone in the atmosphere is now routinely measured from instruments flying on satellites.

The size of this year’s ozone hole reached a maximum size in September of about 7 million square miles. It is about the same size as the ozone hole in 2011 and 2012.

The ozone hole forms through the destruction of ozone over Antarctica. The winter atmosphere above that continent is very cold. The cold temperatures result in a temperature gradient between the South Pole and the Southern Hemisphere middle latitudes, which results in strong westerly stratospheric winds that encircle the South Pole region.

These strong winds prevent warm air from the equator from reaching these polar latitudes. These extremely cold temperatures inside the strong winds help to form unique types of clouds called polar stratospheric clouds, or PSCs.

PSCs begin to form during June, which is winter time at the South Pole. Chemicals on the surface of the particles composing PSCs result in chemical reactions that remove the chlorine from the atmospheric compounds. When the sun returns to the Antarctic stratosphere in the spring (our fall), sunlight splits the chlorine molecules into highly reactive chlorine atoms which rapidly deplete ozone. The depletion is so rapid that it has been termed a “hole in the ozone layer.”

Thanks to the Montreal Protocol’s phased global ban on chlorofluorocarbon (CFC) use and the natural decay of these chlorine compounds, the stratosphere will be CFC-free near the end of the 21st century. In their absence, the ozone layer will repair itself naturally.

The good news is that the size of this ozone hole is showing signs of shrinking. This recovery is a prime example of the power of employing science research in the shaping of public policy.

We would be wise to learn from this example to inform our collective approach to climate change.

Category: Climate, Meteorology

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