156 THE CLIMATE OF ST. MARY's COUNTY 



nated? The well-known isothermal charts showing the distribution 

 of temperature over the globe are constructed by entering on a chart of 

 the world the mean temperature for each station reduced to sea-level, 

 and drawing lines through points which have the same temperature. 

 From these charts it is possible to calculate the mean temperature of 

 each parallel of latitude. The mean temperature for equidistant points 

 on each parallel is ascertained, for instance, at the crossing point of 

 every 10 degrees of longitude, giving 36 values, the average of which 

 is the normal temperature for the parallel. This normal may be viewed 

 as the temperature which would result at the given parallel from an 

 equable distribution of land and water, instead of the irregular dis- 

 tribution found in nature. A comparison of the actual mean tempera- 

 tures observed with the normal for the latitude will show whether a 

 region is warmer or colder than the normal or the thermal anomaly of 

 the region. 



The parallel of 38° N. is the nearest to St. Mary's County. The mean 

 temperatures for this latitude, as found by Spitaler, and the correspond- 

 ing means for St. Mary's County are given below : 



Annual Mean January July 



Temperature. Mean. Mean. 



Parallel of 38° N 59.5° 44.2° 76.5° 



St. Mary's County 57.1° 37.1° 77.8° 



Differences — 2.4° — 7.1° + 1.3° 



The comparison shows that the annual mean temperature of St. Mary's 

 County is 2.4° lower than the temperature proper to its latitude. The 

 thermal anomaly is negative throughout most of North America (except 

 the Pacific and Gulf coasts) and the deficiency exceeds 7° in the region 

 northwest of Hudson Bay. In winter St. Mary's County is 7.1° colder 

 than the normal for its latitude, or in other words the character of the 

 year is determined by the severe winter climate of the great continental 

 interior. North of the Lakes and the St. Lawrence the thermal anomaly 

 for January is — 18°, while it is -f- 30° between Iceland and Norway. 



In summer the anomaly for St. Mary's is positive, but only slightly 

 over a degree, the difference between the July means being 1.3°. The 

 positive anomaly becomes greatest towards the interior of the continent, 



