Difference between the observed mean temperatures of places on the earth’s surface and the mean temperatures of their parallels of latitude has been termed as thermal anomaly. Lines drawn on a world map joining places having the same thermal anomalies are called isanomals. The map showing the thermal anomalies is known as an isonomalous map.
The northern hemisphere, where there are a larger percentage of land areas, is characterized by the largest thermal anomalies.
Since the southern hemisphere is more homogeneous because of the larger percentage of oceans, the magnitude of deviation of the observed mean temperature from the normal temperature values for the parallels of latitude is relatively smaller.
Obviously, this is due to the fact that there is a great anomaly in the distribution of land and water in both the hemispheres.
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There are two types of thermal anomalies: (a) positive anomaly, and (b) negative anomaly. As is clear from the map, thermal anomalies are controlled by the seasons. In January the positive and negative anomalies are found over the oceans and continents respectively in the northern hemisphere.
Mainly due to the presence of warm ocean currents, the greatest positive anomalies are observed in the eastern portions of middle-latitude oceans and the western margins of the adjacent continental areas.
It is a well-known fact that the North Atlantic Drift, which is deflected pole-ward as a relatively warm current, keeps the western and north-western margins of Europe warmer during the winter than the areas situated in the same latitudinal belt on the continental land mass of Eurasia.
This factor alone accounts for the highest positive anomaly, which characterizes northeastern Europe during the colder months of the year. The entire Norwegian coast is warmer for its latitude in winter.
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On the other hand, the north-eastern Siberia, because of the effect of continentality, has extreme negative thermal anomaly during the winter.
Verkhoyansk has negative thermal anomaly of -25.6° C in January, while in the same month a place situated along the Norwegian coast has a positive thermal anomaly of 25.6″ C.
To summarize, all the places situated along the west coast of Europe are warmer, and the entire land areas of the continental interior are colder for their latitudes in January.
The position regarding the distribution of thermal anomalies in July is reversed. Because of the summer-time heating, the continents in northern hemisphere have a positive thermal anomaly in July. On the contrary, the oceans have a negative anomaly in the same month. This is mainly due to the differential heating of land and water.
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An analysis of the distribution map of annual mean temperature for the world as a whole shows that the continents show negative anomalies pole-ward from latitude 40°. Equator-ward from the same latitude, continents have positive anomalies.
However, in the southern hemisphere there is preponderance of oceans which results in a more homogeneous water surface.
The distribution of temperature in this water hemisphere seems to be controlled by the amount of insolation received a fact which makes the temperature distribution relatively less complicated. That is why temperature deviations from the latitude normal are rather insignificant in the southern hemisphere.