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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How long is the solar cycle?

Posted on May 14, 2012 by Weather Guys Editor

Since the invention of the telescope in the 1600s, observers have recorded variations in the numbers of dark spots, or sunspots, on the sun’s surface. Observations show that the sun exhibits a periodic change in the number of sunspots that normally follow a regular cycle with peaks 11 years apart.

NASA and the National Oceanic and Atmospheric Administration have continually monitored the sun from space since 1979. The solar cycle is coincident with an oscillation in solar energy output, with the sun’s output being slightly higher during periods with large numbers of sunspots.

Variations in solar output may affect climate on the time scales of decades to centuries. Between the years 1645 to 1715 the number of sunspots was dramatically lower than observed before or since. This period is known as the Maunder Minimum. It is hypothesized that the reduction in solar energy output during this period could have cooled the Earth. The historical record supplies some evidence supporting this hypothesis.

The period between about 1400 and 1850 is called the Little Ice Age in Europe, though it was not a true ice age. Around 1570 Europe was 2 to 4 degrees Farenheit cooler than it is today. The coldest portion of this period, accompanied by the greatest advance of mountain glaciers, occurred around 1750.

During a solar cycle, sunspots, solar flares and solar storms move from intense activity to relative calm and back again. Strong solar storms can disrupt communication, navigation systems and satellite instruments.

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Why are the cloud streamers behind jets different sizes?

Posted on May 7, 2012 by Weather Guys Editor

The white condensation trails left behind jet aircraft are called contrails (condensation trails). Contrails usually form higher than 26,000 feet above the ground. Contrails form when hot, humid air from jet exhaust mixes with surrounding air of low water vapor content and low temperature. The clouds that form are similar to the cloud you see when you exhale in cold air and “see your breath.”

If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact depends on the humidity level and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for some time. On the other hand, if the atmosphere is dry, the contrail mixes with the surrounding air and dissipates quickly.

Contrails are a concern in climate studies as increased jet traffic may result in an increase in cloud cover. It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20 percent.

An increase in clouds can change a region’s radiation balance. For example, solar energy reaching the surface may be reduced, resulting in surface cooling. Contrails also reduce energy loss from the Earth’s surface, resulting in warming. Averaged over a year, contrails tend to cause a cooling over the regions they inhabit.

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