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Land masses in the northern hemisphere are at their coldest in January and at their warmest in mid-July. The sea, however, lags behind; it reaches its extreme values of temperature about a month later in each case, being coldest in February and warmest in late August.

The increase in temperature which takes place from early spring to late summer shows none of the daily ups and downs that characterise the temperature of the air; the change is slow and gradual, but relatively steady.