An air temperature of 14 deg.F.

or less is considered necessary to produce frost splits.
A still more recent theory is that of Busse[36] who considers the mechanical action of the wind a very important factor.

He observed: (_a_) Frost splits sometimes occur at higher temperatures than 14 deg.F.

(_b_) Most splits take place shortly before sunrise, _i.e._, at the time of lowest air and soil temperature; they are never heard to take place at noon, afternoon, or evening.

(_c_) They always occur between two roots or between the collars of two roots, (_d_) They are most frequent in old, stout-rooted, broad-crowned trees; in younger stands it is always the stoutest members that are found with frost splits, while in quite young stands they are altogether absent, (_e_) Trees on wet sites are most liable to splits, due to difference in wood structure, just as difference in wood structure makes different species vary in this regard.