At latitudes north of the Arctic Circle the diel range of sun altitude declines progressively, reducing the frequency of temperature inversions over level, unshaded ground in summer and extending the frost-free season. North of about 70 degrees north, the potential number of inversion-free days increases rapidly with latitude; this ameliorates the microclimate close to the ground-the zone to which terrestrial organisms are increasingly restricted in higher latitudes. The relation between screen and ground-surface temperatures differs north and south of about 70 degrees north; north of that latitude there are progressively more frost-free days than would be inferred from screen temperatures, were no allowance made for this latitude-dependent change.
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