THk ATR SEASONING OF WESTERN SOFTWOOD LUMBER 7 
raphy, drainage, and bodies of water, constitute the primary in- 
fluences in air drying. No matter what the yard methods, nor how 
efficiently air-seasoning practice is designed to control the drying 
conditions within the lumber pile, a warm, dry, windy climate will 
cause faster drying and lower final moisture content than will a 
cool, damp, calm climate. 
The considerable variation of geographic and climatic factors be- 
tween the several western lumber-producing regions and also between 
yards in the same region, and the influence of such variations upon 
the air-seasoning process are clearly illustrated by the data in Figures 
7 to 10 showing the effect of different weather conditions upon the 
actual drying. Even though these natural conditions are subject to 
little control, a knowledge of them and recognition of their effects on 
drying are essential to the intelligent selection of a yard site, the 
proper laying out of the drying yard, and the development of effec- 
tive piling methods. 
The aim of air-seasoning practice must necessarily be to employ 
the favorable natural elements to the greatest possible advantage and 
to minimize the effects of the unfavorable elements. Granted that 
absolute control of drying conditions is impossible, much can yet be 
done if the general principles and objectives of drying are understood 
and properly coordinated. For example, a certain amount of heat 
is transmitted to the lumber from the direct rays of the sun, which 
reach at least a part of the pile during some portion of the day. The 
outside area of the pile which receives direct sunlight as well as the 
length of the daily period during which the sun can reach the sides 
or ends of the pile can be controlled to some extent by such methods 
as varying the spacing at the sides, front, and back of the pile and 
determining the direction of these openings, 
CIRCULATION OF AIR WITHIN THE PILE 
The functions and importance of circulation in air seasoning have 
already been indicated. Circulation of air is the only drying factor 
that is subject to direct methods of control, and, in turn, it largely 
controls the effects of heat and humidity within the lumber pile. 
The movement or circulation of air in a lumber pile is of two gen- 
eral types. Horizontal circulation is dependent upon and is caused 
primarily by the local wind currents. Vertical circulation, on the 
other hand, is an individual internal movement. 
Horizontal circulation can be regulated to some extent by, yard 
layout, foundation construction, and piling methods. Arrangement 
and spacing of pile alleys, rear alleys, and the intervals between piles 
on the same alley directly affect the movement of the local air cur- 
rents. Likewise, the clearance under the pile foundations exerts an 
appreciable influence. And the actual inlet and outlet of the wind 
currents to and from the pile are greatly affected by the method of 
pile construction. 
Vertical circulation in the lumber pile is a drying factor of the 
utmost importance and should be thoroughly understood. As the 
green stock in the pile dries, the evaporation uses up heat. The air, 
thus becoming cooler and heavier, tends to drop gradually toward 
the bottom of the pile. Pile construction should therefore be de- 
