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.

Category: Climate
Tags , , ,
Comments Off on Will May’s weather continue to be pleasant?

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.

Category: Climate
Tags ,
Comments Off on How long is the solar cycle?

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.

Category: Phenomena
Tags , ,
Comments Off on Why are the cloud streamers behind jets different sizes?

Why does it smell good after a rain?

Posted on April 30, 2012 by Weather Guys Editor

Many times after a rain, there is a distinctive odor in the air — a sort of musky smell. This pleasant fragrance is most common in rains that follow a dry spell. If you are a gardener, you may find this smell similar to the smell you sense when you turn over your soil. Good organic soils contain bacteria. One bacterium that is abundant in damp warm soils is actinomycete.

Actinomycetes are a key ingredient in the decomposition of organic materials in the soil. These bacteria thrive when the soil is moist. When the soil dries out, the actinomycetes produce tiny spores. These spores are part of their reproduction cycle. Rain kicks up these spores when the raindrops hit the ground and make them airborne. Air movements then disperse the spores and carry some of the spores to our nose, where we detect a pleasant aroma. Actinomycetes are very common, which is why you experience the pleasing after-the-rain smell in many locations.

Winds can transport odors from areas upwind of you. We also can experience bad after-the-rain smells, especially in urban regions. Rainwater tends to be slightly acidic and can break apart organic materials on the ground. The rain can release minerals in the soil that can also react with any chemicals on roadways, such as gasoline and oil, leading to unpleasant smells. These chemical smells are more noticeable with rains that follow a dry spell, as the chemicals have not been diluted or washed away.

Category: Phenomena
Tags , ,
Comments Off on Why does it smell good after a rain?

Was the recent tornado outbreak forecasted?

Posted on April 23, 2012 by Weather Guys Editor

The recent tornado outbreak in the Plains States was predicted by forecasters at the Storm Prediction Center in Norman, Okla., as far as seven days in advance. This kind of forecast undoubtedly contributed to a vigilance that resulted in only six lives being lost in this first major outbreak of the year.

Such forecasts are the product of an enterprise known as Numerical Weather Prediction (the forecasting of the weather by means of computer calculations). Though forecasting the weather has long been a dream of humankind, NWP is a very recent invention.

Perhaps the first thinker to consider the real possibility of forecasting the weather by calculation was the 14th century French mathematician, Nicole d’Oresme, who considered it possible if only the “proper rules” were discovered. These “rules” were the laws of physics discovered by Isaac Newton some 300 years later.

The first real attempt at calculating the future weather was made by Lewis F. Richardson, an English Quaker who, while between ambulance runs in France during WWI, wrote his great book, “Weather Prediction by Numerical Processes,” in which the first forecast by calculation was an utter failure.

His ideas were not taken up again until the early 1950s, after the invention of the computer. In fact, modern computers were arguably first tested on the problem of weather prediction through the efforts of professors Jule Charney (MIT) and John von Neumann (Princeton). They were the brains behind the so-called Joint Numerical Weather Prediction Unit, which issued its first daily forecast on May 6, 1955.

Through the efforts of many outstanding meteorologists and computer scientists in the intervening 57 years, weather prediction (informed by expert interpretation of the computer output at the core of NWP) has advanced enormously to the point where the three- to five-day forecast is nearly as accurate today as the one-day forecast was in 1975. Such advances certainly saved lives in the tornado outbreak of April 14.

Category: Meteorology
Tags , ,
Comments Off on Was the recent tornado outbreak forecasted?