Several Air Masses can affect this area but most behave or have been modified to behave with Continental characteristics

Summer Air Mass Associations

·         Maritime Tropic(mT)- strengthened by the sub-tropic lows

·         Continental Polar(cP)- weaker than in winter but still causes frontal wedging in summer months

·         Maritime Polar(mP)- from the west coast, but orographic lift over the rocky mountains causes most moisture to be lost before it reaches North Dakota

Winter Air Mass Associations

·         Continental Polar- as the polar highs strengthen in winter this air mass dominates

·         Continental Arctic(cA)- this can bring subzero temperatures

·         Maritime Tropic- as fronts collide cyclones can be created bring warm fronts to the north

Pidwirny, M. (2006). Original Publication "Air Masses and Frontal Transitional Zones". Fundamentals of Physical Geography, 2nd Edition. Accessed via online post 10/01/12 http://www.physicalgeography.net/fundamentals/7r.html

Frontal air mass collisions regularly take place south of North Dakota as continental polar highs and maritime tropical lows seasonally weaken and strengthen. The high latitude of Dickinson, ND makes the continental polar air mass the most influential air mass association and as a result this area usually experiences extended periods of cold fronts.  This causes the typical cold and dry winters; however severe blizzards occur periodically as low pressure systems build to the south near Colorado or to the north near Alberta, Canada (Black, 38pg).  These mid-latitude cyclones can bring occluded warm fronts creating short durations of high precipitation. 

1Example of mid-latitude cyclone developing south of North Dakota that will eventually bring a warm front, Map Courtesy of meted.ucar.edu, Accessed 11/10/12

As the sub-tropic lows of the Gulf of Mexico strengthen in summer, warm maritime tropical fronts with moist air reach far enough north to cause the summers of Dickinson to be reported as semi-humid (currentresults.com).  While the sub-tropic highs have strengthened, the oscillating rossby waves periodically bring the continental polar air mass further south.  This wedges the warmer air upward and causes brief but severe thunderstorms with heavy precipitation.

2 A 1998 winter mid-latitude cyclone caused severe storms across the United States, this diagram shows the initiation of the cyclone.  Notice how Dickinson, indicated with the yellow circle, sits in the cold front.  From http://www.cmmap.org/scienceEd/summercourse/summerCourse11/docs/KateThursdayPM.pdf Accessed 11/14/12

This diagram shows the progression of the cyclone, and shows how the warm tropical air can be skewed to the north.  Dickinson still sits in the cold front but moist tropical air is being brought into this cold area, carrying heavy amounts of precipitation. http://www.cmmap.org/scienceEd/summercourse/summerCourse11/docs/KateThursdayPM.pdf Accessed 11/14/12

                                                                                                                             

This is a broader example of a high pressure cold front sweeping across the western United States, eventually occluding with a warm front.  Dickinson is again sitting in the cold front of this occlusion.  www.weatherchannel.com   

References

·         Black, R. E. 1971. A Synoptic Climatology of Blizzards on the North-

     Central Plains of the United States. NOAA Tech. Memo. NWS CR-39.

     38 p.

Photo from http://www.ndoiljobs.com/

 

Dickinson, North Dakota Theodore Roosevelt Regional airport is the location of the weather station observed for this blog. Dickinson is in the SouthWest portion of the state and sits at an elevation of 2,011ft. The majority of this region is prairie grasslands with some intermittent farms. The 25 mile radius surrounding the city is 96% grasslands and 4% croplands(weatherspark.com, 2012). Dickinson is at a high latitude of 46.9°N and is in the longitudal center of the North American continent at 102.8°W. This northern latitude and central location creates continental polar conditions. Higher temperatures and moisture levels can occur in summer with the strengthening of the subtropic lows but this area is mostly affected by the continental polar highs. Temperatures can vary greatly throughout the seasons, as is expected in a continental environment and both drastic highs and lows can occur here.

 Temperature Averages

Temperatures given in Fahrenheit.  Graph property of Tessa Bopp, data courtesy of currentresults.com

 

In general this region is known for its mild to moderately humid summers with brief, though often severe, thunderstorms and cold, sometimes frigid, winters with problematic but fleeting snow storms. Generally precipitation occurs as light rain, light snow, or thunderstorms with the greatest average accumulation of snow occurring in December(Current Results Nexus, 2012) and the greatest amount of rainfall occurring in June(Automated Surface Observation System, 2012).  When moisture is brought into this area extreme uplift occurs and severe storms can result.  The infamous hailstorms of summer and blizzards of winter are both examples of this.

Precipitation Averages

Graph property of http://www.wunderground.com, accessed 10/01/12. Data is from Automated Surface Observation System weather station Dickinson (KDIK) and is maintained by the Federal Aviation Administration

Graph property of Tessa Bopp, data courtesy of http://www.currentresults.com/Weather/North-Dakota/average-north-dakota-weather.php as collected by Current Results Nexus

   

The Bowen Ratio of North Dakota is typically higher than the North American average. This is due to several environmental factors of SouthWest North Dakota. Some factors include lacking water in the surrounding area and the relatively high albedo of the terrain because of the dry grasslands. Little amounts of evapotranspiration occur here so the sensible heat is typically greater than latent. The latitude of this location is also relatively far north and as such the sun light received is at a greater angle and the length of time of solar radiation exposure is very short in winter.

The bowen ratio decreases some in summer with an increase in darker vegetation, an increase in moisture, and a greater amount of daylight but still continental characteristics dominate and sensible heat is typically greater than latent.



*All photos property of Tessa Bopp unless otherwise stated