The climate of Loudoun, like that of most other localities, is governed mainly by the direction of the prevailing winds, and, to a limited extent, is influenced by the county’s diversified physical features.

Though the rainfall is abundant, amounting annually to forty or fifty inches, ordinarily the air is dry and salubrious.  This ample precipitation is usually well distributed throughout the growing season and is rarely insufficient or excessive.  The summer rainfall comes largely in the form of local showers, scarcely ever attended by hail.  Loudoun streams for the most part are pure and rapid, and there appears to be no local cause to generate malaria.

In common with the rest of Virginia the climate of Loudoun corresponds very nearly with that of Cashmere and the best parts of China.  The mean annual temperature is 50 deg. to 55 deg..

Loudoun winters are not of long duration and are seldom marked by protracted severity.  Snow does not cover the ground for any considerable period and the number of bright sunny days during these seasons is unusually large.  In their extremes of cold they are less rigorous than the average winters of sections farther north or even of western localities of the same latitude.  Consequently the growing season here is much more extended than in either of those sections.  The prevailing winds in winter are from the north and west, and from these the mountains afford partial protection.

The seasons are somewhat earlier even than in the Shenandoah Valley, just over the western border of Loudoun, and the farmers here plant and harvest their crops from one week to ten days earlier than the farmers of that region.

Loudoun summers, as a rule, are long and agreeably cool, while occasional periods of extreme heat are not more oppressive than in many portions of the North.  The mountains of Loudoun have a delightful summer climate coupled with inspiring scenery, and are well known as the resort of hundreds seeking rest, recreation, or the restoration of health.  This region, owing to its low humidity, has little dew at night, and accordingly has been found especially beneficial for consumptives and those afflicted with pulmonary diseases.  The genial southwest trade winds, blowing through the long parallel valleys, impart to them and the enclosing mountains moisture borne from the far away Gulf of Mexico.

GEOLOGY.

The geology of more than half the area of Loudoun County has received thorough and intelligent treatment at the hands of Arthur Keith in his most excellent work entitled “Geology of the Catoctin Belt,” authorized and published by the United States Geological Survey.[5]

[Footnote 5:  Credit for many important disclosures and much of the detail appearing in this department is unreservedly accorded Mr. Keith and his assistants.]

Mr. Keith’s analysis covered the whole of Bull Run Mountain, the Catoctin in its course through Virginia and Maryland to its termination in southern Pennsylvania, the Blue Ridge and South Mountain for a corresponding distance, all intermediate ridges and valleys and contiguous territory lying outside this zone and paralleling the two flanking ranges.[6]