You are browsing the archive for Did You Know?.

Record Low Pressure!

4:00 pm in Did You Know?, Extreme Weather, In the News by Ted Keller

Very Deep Low 10/26/10 18z

Not since the Superstorm of 1993 has a low pressure system over the United States achieved such a low central barometric pressure!

From the National Weather Service:

“New record set today for the lowest pressure in a non-tropical storm in the mainland U.S. The massive storm system barreling across the central U.S. had a minimum central pressure of 28.24″ or 956 mb (equivalent to the minimum pressure of a Category 3 hurricane). This breaks the old record of 28.28″ (958 mb), set on …Jan. 26, 1978, during the Blizzard of 1978 (aka the Cleveland Superbomb). This is also lower than the March 1993 Superstorm (aka “The Storm of the Century”), or the “Witch of November” storm that sank the Edmund Fitzgerald in 1975, or even the Columbus Day Storm of Oct. 1962.”

Editors note: it looks like 28.21″ is the official lowest pressure of this storm.

Development of "bomb" low! Thanks NSSL!

Such intense lows at the surface of the earth are brought about in this case by a powerful jet stream wind pattern.  For lows at the surface of the earth to deepen, air must be evacuated out of the top of the low faster than it can be replaced from below.  When this is true, air molecules are removed from the column of air leading to less air pressure as measured by a barometer.  This is called hydrostatic pressure or the total weight of the air on the barometer.

Strong jet stream winds high in the atmosphere can set up a path for air to take away from the deepening low leading to a drop in pressure.

Wind at the Jet Stream Level (300mb) 12z 10/26/10

Wind and pressure are intimately related.  The deeper the low, the larger the difference in pressure or pressure gradient observed.  The stronger the pressure gradient, the faster the wind will blow.

This storm had widespread 40-50 mph wind gusts connected to it over areas of Iowa, Illinois, Minnesota and Wisconsin eariler today.  Some of the gusts near shower and thunderstorm bands reached 60-70+ mph.

Also, the storm system has spawed many tornadoes in parts of Illinois (EF2 near Peotone), Indiana and Kentucky.  At  this writing, many tornado warnings were in effect for counties in Tennesee, Kentucky and Alabama.

Link to a great write-up by the Duluth National Weather Service.

Severe Storm Front

5:55 pm in Did You Know?, The Ozarks, Weather Education by Ted Keller

Me and the Rapid Scan DOW Radar

The Severe Local Storms (SLS) Conference is held once every two years.  It’s sponsored by the American Meteorological Society (AMS) and its purpose is to bring together scientists, researchers, forecasters and anyone else involved in severe local storms. 

This meeting, held in Denver this time around, comes on the heels of a huge tornado research project.  Vortex 2 was a multimillion dollar data collection extravaganza covering two storm seasons (2009/2010) and involving hundreds of people. 

I traveled here in hopes of learning some preliminary findings from Vortex 2 and also to get a sense for how the project went and whether the goal of a better understanding of tornadoes was realized, at least at this early stage.   There are other topics of research going on as well and I wanted to learn more about these too.

The conference is made up of of sessions in which presenters are given 12 minutes to talk about their subject using Power Point and sometimes video. There are also poster sessions where the information is printed and hung up along with all the other posters.  Attendees can then just walk up to the poster that interests them and at certain times have access to the author(s) of the poster to ask questions.  Finally, there are invited speakers and special sessions to round out the conference.

As is typical when I attend these conferences, there are papers presented which are FAR beyond my understanding.  But what does make it through helps me to understand severe storms just a bit more. It’s also just fascinating!  Researchers are doing some really exciting things in the areas of remote sensing and instrumentation, computer modeling and forecasting techniques.

I’ll blog in more detail here over the next few days as I collect my thoughts and notes and as time permits.

Super Harvest Moon 2010!

3:47 pm in Astronomy, Did You Know? by Ted Keller

Tonight, a semi-rare astronomical event occurs, a full moon on the same day is the autumnal equinox.

First, the full moon will be listed at Thursday, September 23rd because it it reaches its fullest point in the wee hours of Thursday morning.  But moonrise tonight will be closer to the full moon than Thursday’s thereby making it “fullest”  These are all minor technical points.

The main idea is that tonight’s rising moon will be full and will occur on the same day as the Autumnal Equinox which for us is 10:09 pm CDT tonight (22nd).  This only lines up every twenty years or so.  It’s a Super Harvest Moon!

The Harvest Moon is the closest full moon to the equinox always.  But it’s not just a full moon, it also rises right around or after sunset.  It is these two facts which actually extended harvest time beyond sunset back in the days before practical lightning for farmers. 

The Harvest Moon usually occurs in September but can be in October occasionally, which ever full moon is closest to the fall equinox.

The Start of Fall

6:15 am in Astronomy, Did You Know? by Ted Keller

The Autumnal Equinox: it is a point in the earth’s orbit around the sun at which neither the northern or southern hemisphere is receiving more direct sunlight.

Does one hemisphere ever receive a greater or lesser share of the sun’s radiant energy?  Sure!  Because the earth has, for our purposes, a constant tilt with the plane of its orbit around the sun, an imbalance of solar energy occurs.  The tilt is 23.5 degrees.  Mercury has practically no tilt while Pluto is on its side at 119 plus degrees.

Object Axial tilt (°) Axial tilt (radians)
Mercury ~0.01 0.00
Venus 177.4 3.10
Earth 23.44 0.41
Moon 6.688 0.03
Mars 25.19 0.44
Ceres ~4 0.07
Pallas ~60 ~1
Jupiter 3.13 0.06
Saturn 26.73 0.47
Uranus 97.77 1.71
Neptune 28.32 0.49
Pluto 119.61 2.09

There are four distinctive points worth talking about.  We’ll leave the Solstice points for now and come back to them on the first day of winter.  The two equinox points occur opposite of each other in orbit.  At these points, the tilt of the earth doesn’t come into play, all points on earth are subject to the same period of time (12 hours) sunrise to sunset.  In orbit, it is the time when the earth’s axis is at right angles to the sun.

This is often referred to as the astronomical beginning of fall and is completely defined by the earth’s orbit around the sun.  It has little bearing on the actual weather occurring on or near the equinox.  Actually, meteorological has been defined as the months of September, October and November.

Another question often raised is: why don’t we actually experience the said 12 hours?  I do the math and it doesn’t work out!  It has to do with refraction and latitude and is explained well here.

Darn that Sun!

6:45 pm in Did You Know? by Ted Keller

Photo by Ted Keller

As we approach the Autumnal Equinox (Wednesday, September 22, 2010 at 10:09 pm CDT), watch your driving on east-west roads because the sun will be a real pain!

This is because the two equinoxes we have each year, the sun is rising (setting) very close to due east (west).  It is exactly east and west on the equinox day itself.

This makes driving into the sun very difficult and somewhat dangerous.  For a few moments close to a rising or setting sun, a driver would have no way to block the sun! 

Also, traffic lights become very difficult to see.  Some have extra large plates installed (the ones in the photo don’t) behind the light in order to surround it with a dark contrasting background.  If you’re waiting at a light traveling north-south this week, keep in mind that drivers traveling east-west may not be able to see a light change and may drive through the intersection even though the light is red.

Hurricane Exhaust

4:46 pm in Did You Know?, Tropics, Weather Education by Ted Keller

Air Flow in a Hurricane

As I recently informed my Introduction to Atmospheric Science students, well developed hurricanes have a feature you might not expect.

Everyone focuses (no pun intended) on the eye and eye wall and they should…it is awesome to watch and is the most powerful part of the storm.

Hurricanes spin cyclonically or counter-clockwise, a rotation initiated by the earth’s rotation. But if you look closely at the satellite loop of Danielle from Friday, the high, thin-looking clouds on the edges are actually arcing outward and have a slight anticyclonic or clockwise rotation, the opposite of what is happening below!

The reason for this behavior lies in the temperature profile of a developed hurricane. The massive amount of convection or thunderstorms in the center of these storms is releasing a tremendous amount of heat energy. This creates a warm bubble of air over the top and center of the storm. This in turn creates a relative area of high pressure which cause air to flow outward which is what is revealed in the cloud motion.  The whole system is what meteorologists call a warm core low.

AWSOM Powered