Terrestrial microclimate: amelioration at high latitudes.

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.