with the same average annual meteorological conditions and water exchange, the ice conditions may 

 be different, owing to a phase displacement in these conditions. 



Ice abundance due to local ice is easily subjected to analysis. Actually, in the first approxi- 

 mation we may here consider that the main factors which determine ice abundance are as follows: 



1. Ice abundance and temperature in the basin at the start of winter cooling. The less ice 

 present and the higher the sea temperature, or more accurately, the greater the index of freezing, 

 the less ice abundance may be expected in the following year. 



2. Synoptic conditions in the period of cooling and ice formation. The decisive factors at 

 this period are air temperature and wind force. The speed of cooling of the sea is directly pro- 

 portional to the severity of the weather, depending on air temperature and wind force, while the 

 thickness of ice which will form with uninterrupted growth is proportional to the number of degree- 

 days of freezing. In addition, the stronger the winds the more intense is the hummocking of ice. 



It has already been pointed out that in the border seas of the Soviet sector of the arctic the thick- 

 ness of ice growth which forms during the winter exceeds 2 m only in the most northerly regions, 

 while the average thickness of hummocking ice in the same regions reaches 4 m. It is clear, 

 therefore, that the less the number of degree-days of freezing, and the less the total severity of 

 weather in periods of cooling and ice formation, and finally, the less the index of turbulence of the 

 atmosphere, the less ice abundance may be expected in the subsequent year. 



3. Synoptic conditions in the period of weakening of ice. The decisive factors in this period 

 are the temperature and humidity of the air and the force of the wind. Break-up occurs after the 

 ice has become sufficiently weakened by melting under influence of currents, tides and wind. The 

 earlier the ice weakens (due to air temperature, humidity and solar radiation), and the earlier the 

 break-up occurs (which is always hastened by the wind), the less ice abundance may be expected in 

 the sea. 



4. Synoptic conditions in the period of ice destruction. After the first movements of the ice 

 have begun, wind force and direction acquires great significance. While the most favorable wand 

 directions may be determined before the first ice movement in each individual sea, in the period of 

 ice destruction particular importance is attached not only to wind force but also to the variability of 

 its direction. Actually, the more often a wind of identical force changes its direction, the greater 

 will be the amplitude of movement of individual floes, the greater will be spring hummocking, and 

 the more broken-up the ice will be (other conditions being equal). The quantity of solar energy 

 expended in melting of ice is directly proportional to the area of open water. We recall, in addition, 

 that when warm air flows over ice, a comparatively thin cushion of cold air forms over the ice. 

 When the wind is sufficiently strong this cushion is demolished. Thus, the stronger the wind and 

 the more variable its direction in the period of ice destruction, or in other words, the greater the 

 index of atmospheric turbulence, the less ice abundance may be expected in the period of navigation, 

 provided navigation is generally feasible in the given region. 



5. Synoptic conditions in the period of navigation. In case of low ice quantity in a sea, the 

 synoptic conditions during navigation have no particular significance, but they become decisive 

 when the ice is very abundant. The wand again plays the main role here. Its force and, more 

 important, its direction, which determines the location of ice, may be favorable or unfavorable for 

 the use of sea routes within or across this area. It has already been noted that the ice abundance 

 of a sea in respect to ice of exclusively local origin is dependent, in addition to the above-listed 

 conditions, on water exchange with neighboring seas. The greater this water exchange, the greater 

 obviously will be the effect of water exchange on ice abundance. 



465 



