WMOD00317

• Contract/Permit #: 14-06-D-6999
• Project Name: CAWRP
• Title: Cascades Atmosphere Water Resources Program - Volume I
• Prepared For: US Department of the Interior Bureau of Reclaimation
• Date: 7/1/1970
• Weather Modification - Doc Type: Report

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SECTION 4

AIRBORNE OBSERVATIONS AND MEASUREMENTS

4.l Pacific Northwest Storms and Clouds
From observations and measurements made in aircraft fli~hts across the
Cascade Mountains during the winter of 1970-71 and previous winters, some
general conclusions have emerged on the structure of clouds over the Cascade

Mountains and these are summarized below.

4.1.1 Pre-frontal Conditions

In pre-frontal conditions the clouds over the Cascade Hountains are

generally layered, ranging from stratus near the ground to cirrus at about

30,000 ft. There is often a deep layer of clear air between the tops of the

orographic cloud and the base of the frontal clouds. Continuous cloud in the

vertical direction is usually found only in the immediate vicinity of a front.

Well ahead of a front the air is normally sub-saturated with respect to

water from the surface to 6,000 ft at Greenwater (Fig. 2.1). As a well-

developed front approaches the Washington coast, dry easterly winds flow

across the Cascade Mountains. These produce upslope winds on the eastern

slopes of the Cascades and in this region the air is generally saturated from

the surface up to about 6,000 ft. From about 6,000 to 12,000 ft the airflow

over the Cascades is generally from the southwest and the air is near water

saturation. As a consequence of this distribution of winds with height, solid

precipitation particles which land near the Cascade crest often originate

west of the Cascades but they may pass up to 40 miles east of the crest before

reaching the ground (see ~5.9).

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