Northern Prairie Wildlife Research Center
The Rocky Mountains located west of North Dakota act as a barrier to the prevailing westerly flow of air in the atmosphere. This barrier modifies the temperature and moisture characteristics of air masses originating in the Pacific Ocean when they flow over the mountains in ways that reinforce the continental characteristics of the climate. There are, however, no mountain barriers with respect to air mass source regions in the polar areas or the Gulf of Mexico. Air masses originating in these regions easily overflow North Dakota, sometimes with only minor changes in their basic characteristics. In every year and every season, North Dakota is visited several times by cold and dry air masses which originate in the polar regions, warm and moist air masses from tropical regions, or mild and dry air from in the northern Pacific. The usually rapid progression of these air masses over North Dakota from the different source regions results in frequent and rapid changes of weather and prevents any current weather regime from becoming monotonous.
Day length and solar zenith angle are also important factors affecting North Dakota climate. Day length ranges from less than nine hours in December to more than 16 hours in June. Noon sun angles are much higher in summer than in winter. The combination of these factors at North Dakota's location produces much more radiational energy at the earth's surface in summer than in winter, which contributes to the large seasonal temperature changes and the general north-south temperature gradient across the state.
The topography of North Dakota produces important variations in the temperature and precipitation patterns across the state. Topography affects temperature patterns because air temperature usually decreases with elevation at the rate of 2° to 4° F per 1,000 feet. Air in the lower portion of the atmosphere warms at a rate up to 6° F per 1,000 feet in descending a slope. As will be shown later, temperature variations over the state are largely a mirror-image of its general topography.
A review of the precipitation process is helpful in understanding how topography affects precipitation. For precipitation to occur, moist air in the lower layers of the atmosphere must be lifted to higher elevations where it is cooled by expansion due to lower pressures aloft. During the cooling process, moisture in the air may condense to form water droplets or ice crystals. If the lifting process continues, the water droplets or ice crystals may increase in size and eventually precipitate in the form of rain, hail or snow. The lifting process is favored and takes place most easily when the air mass temperature decreases rapidly with height. In this situation, meteorologists say that the air mass is unstable.
Additional instability results because considerable heat is released in the condensation process. Heating from condensation within a rising cloud causes the air to become lighter and more buoyant than the surrounding air. This, in turn, causes the cloud to expand rapidly within the colder and more dense environmental air. The rising air produces a chimney effect which feeds additional moist air into the system from below and sustains the process.
Topography can increase precipitation by providing the lift needed to start the process. This occurs when moist air flows uphill and the upper layers of moist air reach condensation level. When moisture in the lower layers is plentiful and the air aloft is very cold and unstable, a lifting of only a few hundred feet may be enough to start the precipitation process. On the other hand, moving air downhill inhibits the precipitation process.
Topography may also affect the precipitation process in other ways. Convection can be enhanced over higher terrain due to large temperature contrasts during strong daytime heating of the highlands. Furthermore, evaporation loss is less because rain drops fall a shorter distance from clouds over higher ground.
The patterns observed in the precipitation and temperature maps which follow are more easily understood if they are related to the topography in the state. The more significant changes in ground elevations are shown by the smoothed height contour lines in Figure 1. In general, surface elevations are highest in the southwest and lowest in the northeast. A ridge of higher elevations extends from the northwest corner of the state southeastward into Logan and McIntosh counties. The Turtle Mountains are clearly evident in the extreme north central portion of the state, while the high ground associated with the Killdeer Mountains in Dunn county also stands out. The northeast has high ground with elevations above 1,600 feet in Cavalier, Walsh, Ramsey and Towner counties.