Do wind farms modify climate?

Posted on May 27, 2014 by WeatherGuys Editor

The turbines on wind farms convert the kinetic energy of the wind into electricity. So, we might expect to see some changes in the region due to this energy conversion. The impact will be a function of how big the wind farm is and how long the turbines operate.

One way to estimate changes is through observations. The best method is to compare the observation of a region before and after wind farms are installed. Unfortunately, there are rarely enough observations before the wind farm installation to make strong conclusions about any observed relationships.

So, another approach is to measure weather conditions around the turbines with a similar area without turbines. This measurement approach has indicated that the soil around the turbines is drier and warmer than the surrounding region. This soil temperature and moisture is a very local effect, impacting the wind farm area, not the global temperatures.

In particular, nighttime temperatures are warmer, on the order of half a degree. This is because without the turbine the ground cools down faster than the air above. The turbine mixes up the atmosphere and adds energy to the ground from the warmer air above. Fruit growers use this mixing theory to prevent early autumn frosts from damaging plants.

Since wind turbines extract kinetic energy from the air that flows through them, we should expect a change in the local wind. This will be a local, not a global impact. The wind slows down and is more turbulent behind the wind turbine. That way turbines are not lined up one behind another. You might predict this from your experiences with wind blowing around an obstacle like a building. The wind downwind of the building is gustier then the air in a nearby open field.

Category: Climate, Meteorology

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When is the last frost?

Posted on May 19, 2014 by WeatherGuys Editor

The growing season is often defined as the number of days between the last time the temperature was 32 degrees in spring and the first time the temperature falls to 32 in fall.

The latest frost in spring is important to gardeners as they seek to plant after a long winter, and for most plants it is wise to do so when the temperatures are likely to remain warmer than 32 degrees.

For our region, based on temperature observations between 1981 and 2010, the median date of the last frost is between May 1 and May 10. That is why many gardeners consider Mother’s Day a good day to plant.

One reason for the variation in the last frost date is how the first frost date is defined.

Giving the median date of last frost means that there is still a 50 percent chance that a frost will occur after this date. Some definitions require that there is only a 10 percent chance of a 32 or lower temperature.

Some sites use a different 30-year period, for example, between 1951 to 1980. As it turns out, our nighttime minimum temperatures have been getting warmer, so the last frost date has been moving earlier in the spring.

The change is not much for Dane County, but in northwestern Wisconsin, the last frost date now occurs about two weeks earlier than it did in 1950.

According to the Wisconsin State Climatology Office, for Madison, based again on the 1981-2010 time period, the median date for a temperature of 32 is May 2, and it isn’t until May 20 that the probability of an occurrence of 32 has dropped to 10 percent. On April 18, there’s a 90 percent chance of a later occurrence of 32 degrees.

So, if you have not yet done your planting, now would be a good time.

Category: Climate, Seasons

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How does carbon dioxide affect global warming?

Posted on May 14, 2014 by WeatherGuys Editor

Since 1958, a continuous measurement of the carbon dioxide content of the atmosphere has been made at Mauna Loa Observatory in Hawaii. These observations were initiated by Charles Keeling, who died in 2005, and have been maintained by his son Ralph ever since.

Sunshine is a manifestation of solar radiation and when it is absorbed by the surface of the Earth, the surface heats up and emits a different kind of radiation, known as infrared radiation. Carbon dioxide is a special chemical in that it is transparent to solar radiation and yet it absorbs infrared radiation. Thus, the presence of carbon dioxode in our atmosphere allows sunshine to penetrate to the surface but inhibits the emission of infrared radiation to space.

The consequence of the absorption of infrared radiation by carbon dioxide in the atmosphere is that Earth is much warmer than it has any right to expect based upon its distance from the Sun. In fact, Earth’s average surface temperature is 59 degrees Fahrenheit when it would be 0 if carbon dioxide and other such greenhouse gases (like water vapor and methane) did not exist in our atmosphere.

When Keeling began his measurements in 1958, the atmosphere contained 315 carbon dioxide molecules for every million molecules of gaseous atmosphere. April 2014 was the first month in 56 years in which the monthly average carbon dioxide fraction topped 400 molecules per million (it was 401.33).

The continual increase in this carbon dioxide fraction is considered to be the main contributor to the global temperature increase known as global warming. Such values are a first in human history and likely represent the highest carbon dioxide fraction in our atmosphere in at least the last 800,000 years. It is high time that we had a sober, data-driven discussion about the hazards presented by this dangerous trend. Analytical, skeptical science has to be central to this discussion.

Category: Climate, Meteorology

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Ask the Weather Guys: Like it or not, numerical technique vastly improved forecasting

Posted on May 6, 2014 by WeatherGuys Editor

Pretty good forecast last week, wasn’t it?

Though last week’s persistent cloudiness and intermittent rain was almost surely met with derision by all, it is hard to imagine that many people were surprised by it, as the forecasts very accurately painted the gloomy picture several days in advance.

This advance warning was provided by the National Weather Service through the triumph of numerical weather prediction (NWP). Tuesday May 6th is the 59th anniversary of the issuance of the first NWP forecast in world history.

That forecast was issued in 1955 by the Joint Numerical Weather Prediction Unit headed by Jule Charney, a theoretical meteorologist from MIT, and John von Neumann, a computer scientist from Princeton. It was a dismal failure. In fact, a number of noted atmospheric scientists who had, to that point, offered only polite support for the development of NWP, were quick to use this initial failure as proof of the intractable nature of the challenge of numerically forecasting the weather.

The skeptics were on the wrong side of an amazing history. By 1965, the accuracy of a one-day forecast was substantially improved.

Today, we enjoy the fruits of the persistence of the pioneers who never wavered in their confidence that NWP was a worthy endeavor.

Last week’s persistent clouds and rain, forecast several days in advance, are just the latest example of the remarkable success of NWP. It may not be an exaggeration to claim that NWP is among the most underrated scientific advancements of the second half of the 20th century.

Category: Meteorology

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Are the jet stream and tornadoes related?

Posted on April 28, 2014 by WeatherGuys Editor

Wintertime weather is driven largely by the parade of upper-level weather systems carried rapidly eastward by the high level winds known as the jet stream.

The jet stream is a ribbon of high-speed wind (located at nearly 6 miles above the surface) that is related to the strength, and location, of the pole-to-equator temperature contrast.

As we all know too well after this past winter, cold polar air can spill as far south as Mexico during our winter. This drags the jet stream as far south as southern Texas during the depth of winter.

As we enter the middle of spring, the southern latitudes of our country begin to warm up rapidly and our attention turns to the return of severe weather season.

Though severe weather includes more than just tornadoes, these storms are the season’s highest-profile events. But the climatology of tornadoes varies depending on geography.

We are already past the tornado frequency maximum over the Gulf Coast states (Louisiana, Alabama and Mississippi) that occurs in mid- to late March. From April to May, the tornado frequency maximum occurs over the Southern Plains states of Oklahoma, Texas and Kansas. By June through August, it moves to the Great Lakes states and High Plains.

This northward migration of tornadoes reflects the northward migration of the jet stream. Since the jet stream is tied to the southern edge of the cold air centered on the Pole, as the seasons change from late winter to late summer, the polar cold air retreats to the north dragging the jet stream, and the tornado season, with it.

Category: Seasons, Severe Weather

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