Tom Niziol's Winter Weather Blog
Astronomical VS. Meteorological Winter
Posted by: Tom Niziol, 3:43 PM GMT on March 01, 2013 +4
Now that we are getting toward the end of winter, I through it might be interesting to explain the differences in how winter is defined from an astronomical and meteorological standpoint. I am pretty certain everyone is familair with the astronomical-based seasons. They are defined by the rotation of the earth around the sun and the tilt of the earth's axis. In Figure 1, I have outlined the tilt of the earth's axis in relation to the sun throughout the year. Astronomical seasons match these dates so that winter is defined for 2012-13 as the time period from December 21, 2012 through March 20, 2013.



Figure 1: The astronomical season can be explained from the point of view of the tilt of the earth on its axis. With the sun on the left hand side of the image, you can see how the sun dips well below the equator during the northern hemisphere winter, then works its way back to the equator by the spring equinox.

Meteorological winter is defined as the three coldest months throughout the winter, or December 1st through March 1st. I thought it would be interesting to show this comparison in regards to the average temperature, at a location which has four distinct seasons and is not influenced by other factors such as elevation or close proximity to larger bodies of water. So, I chose Minneapolis which undoubtedly has a significant winter season. Figure 2 is a plot of the average daily temperature from November through April.


Figure 2: Average daily temperature for Minneapolis from November 1st through April 1st.

In Figure 3a, I overlaid the astronomical winter period and you can see where it intersects the average daily temperature for Minneapolis. Rather than covering the coldest time of the winter, you can see that astronomical winter is shunted to the right a bit on the graph, so that by the time astronomical winter begins the temperature at Minneapolis is already down to 18F and conversely by the time we get to the end of astronomical winter the temperature has already gone up to 38F degrees. In Figure 3b I overlaid the meteorological winter period. Now look at where it intersects the average temperature for Minneapolis. It covers the coldest three months of the year much better, with the start and end of winter only separated by one degree.


Figure 3: Astronomical winter (top) and meteorological winter (bottom) overlaid onto the average daily temperature for Minneapolis. I highlighted the temperature at the start and end of each season to show how astronomical winter is shunted somewhat to the right of the coldest temperatures. Meteorological winter however matches the period of lowest daily temperatures very well.
Finally, just because the winter season has ended from a meteorological standpoint does not mean that we still cannot get major winter storms across the nation. In fact, below is the average monthly snowfall for Denver CO. Notice that two of their snowiest months occur on either side of meteorological winter. And don't forget, one of the biggest storms of any type on record occurred in March of 1993, so don't let your guard down too quickly or Mother Nature could bring quite a late season surprise !!


Figure 4: Average monthly snowfall for Denver over the 30 year period from 1981-2010. Note how two of their snowiest months occur outside of meteorological winter.


As always, if you have any questions or comments you can send them to me at tniziol@weather.com
Updated: 7:33 PM GMT on March 01, 2013   Permalink | A A A
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NORTHEAST SNOW STORM - THE PIVOT POINT
Posted by: Tom Niziol, 5:43 PM GMT on February 10, 2013 +3
This is a quick look at the Upton New York radar loop (courtesy College of DuPage NEXLAB) from 1800 UTC 2/8/13 through 0800 UTC 2/9/13 at approximately 15 minute intervals. The point I want to make with this animation is to show how the band from NEMO sets up across Long Island and literally pummels them for several hours. Note how the band pivots right across the island as the Low moves from south to north several miles to the east of the island during the night....
Updated: 11:22 PM GMT on February 10, 2013   Permalink | A A A
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Snow Pack Across The Sierra
Posted by: Tom Niziol, 12:25 PM GMT on February 05, 2013 +2
As many people who follow my blogs know, I absolutely love to view the high resolution imagery available from our nation's polar orbiter satellites. These satellites orbit from pole to pole at a low height, around 450 miles, compared to geo-stationary satellites which orbit around 23,000 miles above the earth. Although polar orbiters only provide a couple images a day at any one point on earth, they provide very high resolution photos. I got hooked looking at the...
Updated: 1:07 PM GMT on February 05, 2013   Permalink | A A A
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MESO-VORTICES ON THE EASTERN GREAT LAKES
Posted by: Tom Niziol, 6:11 PM GMT on February 01, 2013 +8
Recently there were some very interesting features spinning away on the Great Lakes as seen by the GEOS-East satellite. Although somewhat masked by higher thin, cirrus clouds, you can still make out the circulations or "spins" going on across three of the bodies of water which make up the Eastern Great Lakes. In Figure 1, over Georgian Bay (northernmost) you can see a rather large meso-vortex spinning away in the cloud field as it moves south along the Bay. On L...
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Great Lakes ice cover - It's that time of year again
Posted by: Tom Niziol, 9:22 PM GMT on January 10, 2013 +4
It's about that time of the year again when the Great Lakes begin to develop significant ice cover.  Of course, the buildup of ice has a lot to do with the air temperature and this year the average air temperature around the lakes has been warmer than normal.  Figure 1a shows the surface air temperature anomaly for the period November 1st, 2012 through January 5, 2013.  As you can see, the air temperature over the Great Lakes ran anywhere from 1 to 2 degrees Cent...
Categories:Winter Weather
Updated: 10:13 PM GMT on January 10, 2013   Permalink | A A A
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About tniziol
I have always enjoyed nature and in particular, I love watching the sky, there is so much to see and I always wanted to know "why" things happen.

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