STRONG SURFACE WINDS AT BIG DELTA , ALASKA An Example of Orographic Influence on Local Weather

The remarkably high frequency of strong surface winds in the region of Big Delta, Alaska, is studied with respect to its cause, characteristics, and local effects. During the winter, the winds are predominantly east-southeast and, unlike glacier or valley winds, are caused by a topographically induced convergence of the flow of air down the Tanana Valley which occurs at times of southeast gradient winds aloft. Strong south winds are also experienced the year round. A noteworthy characteristic of the east-southeast winds is their persistence; an extreme case is described in which gusts in excess of 40 m. p. h. endured for 7% days (January 20-28, 1952). Another characteristic of these winds is the marked diurnal variation in the frequency of their commencement, by which a strong control by atmospheric tides is inferred. An important effect of the winds is to interrupt periods of very low temperature, but sometimes to create severe " wind chill. " The paper concludes with a brief account of the forecast problem.