For each period the marigrams from the Lyttelton Harbour tide gauge were redrawn on squared graph paper. From each graph the hours of exposure at each level for the fortnight were calculated, and by multiplying by the requisite number of days the total exposure for the year at each level was worked out. These figures were divided by 8,760 (= number of hours per year) and a percentage figure was obtained. The above figures are shown graphically in Fig. 2.

Fig. 2.–Percentage exposure to air at different levels.

This graph shows the same general form as those of Colman (1933, Fig. 10, p. 459) and Evans (1947, Fig. 7, p. 281). Chapman (1943) points out that the difference in the rate of change of percentage air exposure, from level to level, is most marked in the neighbourhood of E.H.W.N. and E.L.W.N. At Taylor's Mistake the rate of change is greatest between M.L.W.N. and M.L.W.S. and between M.H.W.S. and M.H.W.N.

Wind. The prevailing winds are north-east and south-west. Practically 90% of the winds blow from these directions in approximately equal amounts. Winds are generally of a strength 1 to 3 on the Beaufort Scale. On 97 days out of 365, winds of a strength 4 to 7 are experienced. Gales are not frequent.

Wave Action. This varies from day to day and locally with the degree of shelter afforded by reefs, headlands, etc. Taylor's Mistake faces north-east, and consequently is exposed to the prevailing easterly winds. It is, however, sheltered from the southerly winds, which are generally stronger than the easterly. The action of the surf is at its maximum on exposed rocks facing north-west. On the west side of the bay the area near the beach is sheltered from direct wave action by a jutting headland. The slope of the shore also has an effect; where this is gentle surf action tends to be less violent.

The coast of Banks Peninsula is subject at times to considerable ocean swell, especially when the wind is from the easterly quarter. On the outer coasts this swell is seen to sweep along the shore instead of curving in and breaking on the rocks. The absence of shelving floors at the foot of the cliffs prevents the refraction of the waves with the dissipation of their energy on shelving beaches.

Turbidity. The bottom deposits offshore consist of loess silt and sand. The former deposit is quickly stirred up by wave action, and consequently the water inshore is often laden with fine suspended material, which must have a considerable effect on light penetration. The sandy beach in the centre of the bay is subject to considerable alteration by tidal currents and wave action, the sand level on the inshore rocks varying up to two feet after a heavy sea. The covering of the lower levels of the rocks has a limiting effect on most of the attached organisms.

Rainfall. The average annual rainfall is 24.4 inches. It is fairly evenly distributed throughout the year, the period of maximum rainfall being May-July. The driest periods are February and September-October.

Sunshine. The average annual number of hours of sunshine is 2,002 hours. The sunniest period is November-January.

Fig. 3.–Mean monthly air temperatures for Christchurch. 1864-80, 1905-45

Air Temperature. In Fig. 3, the graphs of the monthly means, mean daily maximum and minimum, mean lowest minimum and mean highest maximum, for Christchurch from 1864-80 and 1905–45 are shown. The mean annual range of 16.8° F. (9.0° C.) is not very great. However, from the point of view of the distribution of intertidal organisms the extremes of temperature are important. The difference between the highest, 95.7° F. (35.5° C.) and the lowest, 19.3° F. (–7.1° C.), temperatures recorded is 74.6° F. (41.5° C.). Variations in a single day range from 37° F. (20 5° C.) to 4° F. (2.2° C.). Local variation of temperature with the amount of shade, angle of slope and aspect is important. Temperatures on the rock surface may reach a higher temperature than the surrounding air temperature; the maximum temperature recorded on the rock surface was 84.5° F. (29.0° C.), the temperature of the air several feet away being 5.0° F. lower.