What is the summer solstice?

Posted on June 18, 2012 by Weather Guys Editor

The summer solstice (in Latin, sol, “sun,” and stice, “come to a stop”) is the day of the year with the most daylight. The first day of the astronomical Northern Hemisphere summer is the day of the year when the sun is farthest north (on June 20 or 21). In 2012, this occurs on June 20 at 6:09 pm CDT.

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 this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

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 light. A common misconception is that Earth is closest to the sun in our summer. Actually, Earth is closest to the sun in December, which is winter in the Northern Hemisphere.

The summer solstice is often referred to as the first day of summer but there are other definitions of summer. For example, culturally we can consider summer to begin during the Memorial Day holiday weekend or after school gets out.

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What are pyrocumulus clouds?

Posted on June 11, 2012 by Weather Guys Editor

A pyrocumulus cloud forms from rising air that results from intense heating of the surface by phenomena such as wildfires or volcanic eruptions. The fires that generate these clouds can be man-made or natural. A big fire produces strong upward moving air currents that carry water vapor and ash upward. The water vapor can condense on the ash forming cloud drops. The vigorous upward motions produce these pyrocumulus clouds that look similar to thunderstorm clouds, which also form due to strong upward moving air.

A pyrocumulus is a fire cloud. In Latin, pyro means “fire” and cumulus means “pile up.” Cumulus is a type of cloud that is common in Wisconsin, particularly in summer. Cumulus clouds are those puffy-white clouds with tops that have a cauliflower appearance. Pyrocumulus clouds are grayish or brown in color because of the ashes and smoke of the fire. The tops of these clouds can reach as high as 30,000 feet. It is difficult to locate the bottom of a pyrocumulus cloud as it is often obscure by the ash generated by the fire or the volcanic eruption.

If lots of water vapor is available, the pyrocumulus can develop into a cumulonimbus, or thunderstorm. When a thundercloud forms, it is called pyrocumulonimbus. Like thunderstorms, pyrocumulonimbus can produce lightning because of the strong updrafts. Rain can also fall from these clouds, which could help extinguish the fire generating the cloud. Of course, the lightning might cause another fire.

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When does Hurricane Season begin?

Posted on June 4, 2012 by Weather Guys Editor

The Atlantic hurricane season officially began Friday, although there already has been some tropical cyclone activity in the Atlantic this season.

Throughout much of last week, the Southeast dealt with Tropical Storm Beryl. Typical for early season storms, Beryl was not very well organized and did not pose the kind of threat storms can later in the season.

The hurricane season stretches from June 1 to Nov. 30, but early storms like Beryl are not uncommon. In fact, the earliest hurricane ever in the Atlantic was Hurricane Alice, which formed on Dec. 31, 1954, with maximum winds of 80 mph intermittently from Jan. 1-6, 1955.

Statistically, the peak of the Atlantic hurricane season occurs about Sept. 10 with a dramatic increase in the frequency of storms occurring after Aug. 1, followed by a slightly less dramatic decrease through the end of November.

In the early fall, the ocean surface temperatures are as warm as they will be all year (a result of absorbing summer’s heat), and the jet stream is as far north as it will be all year. These two circumstances make it easier for clusters of tropical thunderstorms not only to grow but also to organize first into tropical storms and then, sometimes, into more ferocious hurricanes.

It is interesting to consider the fact Christopher Columbus crossed the tropical Atlantic at the climatological peak of the hurricane season and yet, in 1492 anyway, his particular route luckily was spared an encounter with one of these storms.

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When was the last time we had a colder-than-normal month?

Posted on May 28, 2012 by Weather Guys Editor

With our mild winter, incredibly warm March, and reasonably warm May coming in sequence, it seems as if it has been a long time since Madison experienced a below-normal month.

In reality, it is not that long ago – September 2011 was 2.0 degrees below normal and the six months from December 2010 through May 2011 were all below normal, though often only slightly so. The average departure over that period was only 1.16 degrees below normal. Since Oct. 1, we have averaged 8.3 degrees above normal — a remarkably warm stretch.

The current seasonal outlook for our summer from the National Centers for Environmental Prediction suggest “equal chances” for us to have a normal summer. This forecast amounts to saying that there are no clear indicators that the summer will be warmer or colder than normal.

One physical factor that has changed since we embarked upon our now eight-month warm spell is that the tropical Pacific sea-surface temperatures have returned to their normal state after having demonstrated anomalous cooling (La Nina conditions) in the eastern Pacific during our warmth.

Though such forecasts are more statistical than physical in nature (quite different from the more physical one- to five- day forecasts of the weather) such a switch from La Nina conditions to more neutral conditions often heralds a return to normal seasonal conditions for the north central United States. It appears more likely than not, however, that the southern tier of states will have a warmer-than-normal summer.

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Will May’s weather continue to be pleasant?

Posted on May 21, 2012 by Weather Guys Editor

As we head from early to late spring during the month of May, there are a number of ways to measure this progress. One way is to consider how often we experience a temperature 90 degrees during May. The last time Madison reached 90 degrees in May was just two years ago — on May 24, 2010. This is a relatively rare occurrence, however, as Madison has reached 90 degrees in May only 10 times since 1971 (once each in 2006, 1991, and 1988; twice each in 1978 and 1977 and three times in 1975).

A 90-degree temperature in May is not a particularly good indicator of the expected intensity of the heat during the subsequent summer either as 1988 had 36 days over 90 degrees while 2006 had only eight.

Another less parochial way to measure the advance of spring into summer is to consider the inevitable shrinkage of the pool of frigid air that rings the North Pole. We have recently calculated the daily average area over the Northern Hemisphere covered by air (at one mile above the surface) with temperatures below various thresholds.

It turns out that air at that level that is colder than minus-13 degrees has, on average since 1960, disappeared from the Northern Hemisphere on May 25 and does not rear its chilly head again until (on average) Sept. 13. So, by this rather obscure but precise measure, the Northern Hemisphere warm season begins in earnest on May 25.

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