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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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.

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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.

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What was the weather during the Titanic’s fateful voyage?

Posted on April 16, 2012 by Weather Guys Editor

There were northerly winds over North Atlantic in the months before the RMS Titanic left port. These winds likely played a role in pushing icebergs farther south than normal and into the Titanic’s path.

When the Titanic left port in Queenstown, Ireland, on Thursday, April 11, 1912, it sailed under brisk winds from the north-northwest at 15-20 knots and a temperature of about 50 degrees.

Two days earlier, well to the west in Boston, a few thousand fans shivered in the cold and snow flurries as the Red Sox beat Harvard University 2-0 in the first game ever played at Fenway Park.

On April 12, the winds were from the west-southwest at about 15 knots and the noon temperature was about 60 degrees. As the ship continued west, the skies got cloudier as a weak cold front approached. The noontime temperatures on Saturday, April 12, were still around 60 degrees, but another cold front (associated with the previous Fenway flurries) was to the west and north of the ship.

As the Titanic passed through the second cold front on Sunday, April 14, the winds switched to northwest at 20 knots. The noon temperature was around 50 degrees, but by 7:30 p.m., the temperature was 39 degrees.

On Sunday, nighttime temperatures dropped below freezing, and the skies cleared and the winds calmed. A large Arctic air mass was over the area on the clear, star-lit night with subfreezing temperatures and calm winds that resulted in a sea “like glass.”

Icebergs were known to be in the region, but the calm winds made spotting them difficult. To spot icebergs during the night, lookouts searched for wind-driven waves breaking around their bases.

The ship struck an iceberg at 11:40 p.m. Sunday, April 14.

On Monday morning, after the sinking, one survivor reported a breeze that came up around dawn to add to the morning chill. Photographs of the rescue that morning show small waves on the ocean surface, confirming that report.

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What is dual-polarization Radar?

Posted on April 9, 2012 by Weather Guys Editor

The next generation of weather radars, which are currently being installed throughout the United States, will improve observations of the interior of storm systems. These radars are called dual-polarization radars.

Radar, an acronym for Radio Detection And Ranging, consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern.

Precipitation scatters these radio waves, sending some energy back to the transmitting point where it is detected by the radar’s receiver. The intensity of this received signal, called the radar echo, indicates the intensity of the precipitation.

A radio wave is electromagnetic energy. That means it has electric and magnetic fields, which are oriented perpendicular to one another. The orientation of these oscillations is referred to as polarization.

It’s the same principle at work in a polarizing filter for a camera, or polarizing sunglasses. Rotate the filter, or glasses, while looking through them at a portion of the sky away from the sun; at a certain orientation, the intensity of the sky’s light will be reduced. This is because the filter is removing polarized light that is not aligned with the filter.

Typical weather radars transmit and receive radio waves with a single orientation of the electric field. Dual-polarization radars emit radio waves that alter their transmitted pulse between horizontal and vertical polarizations. The additional information on polarization improves the precipitation rate measurement as well as determining the type of precipitation (snow, rain, freezing rain and hail).

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