July 2Ø11 Newsletter

The next SKYWARN meeting is 19-Jul @ 7:00 PM, the third Tuesday of the month.

Newsletter Articles
Do you have something you want to share? We are always interested in contributions from the SKYWARN team to share information that is of interested to themselves. If you have an short article or news item, send it to the SKYWARN e-mail address.

Gulf Dead Zone From Several Sources
The Gulf of Mexico Dead Zone is a large region of water that is very low in oxygen (hypoxic). Up to 8,000 square miles of Gulf waters develop a lack of oxygen each year, making it difficult to support life. USGS identified commercial fertilizers and animal manure from farmland in 9* states as the cause of over 70 percent of the Dead Zone pollution. NOAA scientists predict Dead Zone in the Gulf of Mexico may reach record size this year because of the volume of discharge from the flooded Mississippi River. It is estimated to exceed 9400 square miles later this summer.

Low Level Jet Stream From Several Sources
When the "Jet Stream" is mentioned in any discussion, most individuals relate to the ribbon of air circulating about 18,000 feet up. This is otherwise known as the "Polar Front Jet Stream" and helps direct storm systems. There is another jet stream that races across the central Plains only 5,000 feet up. The sequence starts with temperature inversion, when a layer of warm gets trapped between two cold layers. The result is a rapid change in wind speed with height. The "Low Level Jet" (LLJ) produces 35-50 mph winds and can create windy days in the summer. It may trigger "nocturnal" thunderstorms when it intersects with warm frontal boundaries late at night in summer. The LLJ will dissipate when the sun returns. The layer of cold air near the surface is heated and mixes with the trapped layer of warmer air.

Humidity From Multiple Sources
Humidity indicates the amount of water vapor in air. Relative humidity is a measurement of the amount of water in air compared to the maximum amount the air can hold. Condensation or rain form when there is more water vapor than the air can hold. One might think that the humidity might be 100% when it is raining. That may be the case at the upper levels where the rain forms in the clouds. Once the rain drops start to fall, the moisture may not be absorbed by the air surrounding the rain drop. Sometimes the air between the cloud and ground is so dry that the raindrops evaporate entirely before reaching the ground. You might recognize this when radar indicates precipitation, but it it is not raining. Precipitation that evaporates before reaching the ground is called virga. Fog forming after a rainfall, would be a indication of relative humidity approaching 100% at the surface.

Rain, Showers, and Drizzle From Multiple Sources
When the forecast indicates "rain" or "showers", the two terms may sound like the same thing. The two terms are used, however, to refine the type of precipitation that will be experienced. Precipitation that forms in stratus clouds and has a falls consistently over a period of time is referred to as "rain". Precipitation forming due to convection in cumuliform clouds will be more inconsistent. These "showers" form intermittently over a small area and can be heavy or light. "Drizzle" consists of very small raindrops, generally less than 0.02-inch in diameter. Drizzle forms when updrafts in clouds are not quite strong enough to produce rain.

Lightning Video From Multiple Sources
A lightning bolt's downward leader (the initial discharge) speeds toward its target at 136,000 mph, while the luminous return stroke into the clouds moves at 62 million mph. The National Severe Storms Laboratory (NSSL) began studying storms with high speed video in the early 1990s. Technology has improved on the 1,000 frames per second cameras at that time to over 100,000 frames per second today. Not all pictures of lightning need to be high speed to be impressive. This web page show a vehicle being hit by lightning during a 20-second exposure with a still camera.

Wind Shear From Multiple Sources
Wind shear is defined as a difference in wind speed and direction over a relatively short distance in the atmosphere. Shear can result due to winds changing direction, speed, or both. It is one of the more significant reasons for turbulence on an airplane.

Shear can also affect how a storm may develop. A developing storm with significant wind shear may blow the updraft away from its base. The compounding effect of wind shear and air pollution is more difficult to understand. Research suggests pollution hampers thunderhead formation under strong wind shear conditions. Strong wind shear and high pollution led to less rainfall. On the other hand, high pollution combined with Weak wind shear can create stronger storms and more rain.

Weather Satellites
USA Today has a series of articles relating to understanding weather satellites.

Term For The Month From Multiple Sources
Wind Profiler
A wind profiler is a piece of weather observing equipment that uses a multi-beam Doppler Radar to detect the wind speed and direction at various elevations above the ground. The NOAA Research Forecast Systems Laboratory (FSL) operates the Wind Profilers and provides the data to the NWS. The Wind Profiler network allows forecasters to track “small-scale” weather disturbances that increase the chances for severe thunderstorm development or wind shear. There is a wind profiler system located at Wood Lake (Yellow Medicine County), MN.

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Steele County SKYWARN
Owatonna, MN